1 00:00:00,030 --> 00:00:02,400 The following content is provided under a Creative 2 00:00:02,400 --> 00:00:03,780 Commons license. 3 00:00:03,780 --> 00:00:06,020 Your support will help MIT OpenCourseWare 4 00:00:06,020 --> 00:00:10,080 continue to offer high-quality, educational resources for free. 5 00:00:10,080 --> 00:00:12,670 To make a donation or to view additional materials 6 00:00:12,670 --> 00:00:16,580 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,580 --> 00:00:17,744 at ocw.mit.edu. 8 00:00:59,380 --> 00:01:01,570 OK. 9 00:01:01,570 --> 00:01:09,600 So we have-- you can show the answer-- 57%, 1 and 3. 10 00:01:09,600 --> 00:01:13,060 So this is where we were at the end of last lecture, 11 00:01:13,060 --> 00:01:18,370 so if you didn't get this written down in your notes, 12 00:01:18,370 --> 00:01:21,424 you want to look at these definitions. 13 00:01:24,990 --> 00:01:32,470 So here in this, we have NH3 with its lone pair, 14 00:01:32,470 --> 00:01:36,280 and it is acting as the Lewis base, 15 00:01:36,280 --> 00:01:40,150 and BF3 is acting as the Lewis acid. 16 00:01:40,150 --> 00:01:43,830 So a Lewis base donates its lone pair electrons, 17 00:01:43,830 --> 00:01:46,030 and the acid accepts them. 18 00:01:46,030 --> 00:01:48,440 So what was the other definition? 19 00:01:48,440 --> 00:01:50,350 We had the Bronsted-Lowry. 20 00:01:50,350 --> 00:01:56,170 Does someone want to tell me what that definition was? 21 00:01:56,170 --> 00:02:01,440 Can we have a volunteer tell us the Bronsted-Lowry definition? 22 00:02:06,250 --> 00:02:07,260 Pull out your notes. 23 00:02:17,250 --> 00:02:17,968 There you go. 24 00:02:21,530 --> 00:02:23,600 AUDIENCE: So a Bronsted-Lowry-- 25 00:02:23,600 --> 00:02:26,170 CATHERINE DRENNAN: I'm not sure it's on. 26 00:02:26,170 --> 00:02:27,490 It is on? 27 00:02:27,490 --> 00:02:27,990 OK. 28 00:02:27,990 --> 00:02:30,520 AUDIENCE: Bronsted-Lowry base is something that 29 00:02:30,520 --> 00:02:33,040 accepts H plus or H3O plus. 30 00:02:33,040 --> 00:02:34,040 CATHERINE DRENNAN: Yeah. 31 00:02:34,040 --> 00:02:37,750 So the base accepts the hydrogen ion or the proton, 32 00:02:37,750 --> 00:02:39,720 and the acid donates. 33 00:02:39,720 --> 00:02:42,740 So is this definition now incompatible, 34 00:02:42,740 --> 00:02:44,720 or are they completely opposite, or could they 35 00:02:44,720 --> 00:02:46,650 be worked together? 36 00:02:46,650 --> 00:02:48,336 Does anyone have an opinion on that? 37 00:02:53,630 --> 00:02:55,350 So how many people think that these 38 00:02:55,350 --> 00:02:58,690 could be consistent definitions in some way? 39 00:02:58,690 --> 00:02:59,370 OK. 40 00:02:59,370 --> 00:03:00,380 Oh, quite a few people. 41 00:03:03,180 --> 00:03:04,870 One could make that argument. 42 00:03:04,870 --> 00:03:07,220 So if you know now that you can make the argument, 43 00:03:07,220 --> 00:03:08,765 does someone want to try to make it? 44 00:03:12,950 --> 00:03:15,880 We have a bag. 45 00:03:15,880 --> 00:03:17,430 All right. 46 00:03:17,430 --> 00:03:24,460 Our mic went away, but can we get up over there? 47 00:03:24,460 --> 00:03:28,790 So where did our mic runner go? 48 00:03:28,790 --> 00:03:31,290 I should have explained they were needed twice. 49 00:03:31,290 --> 00:03:31,920 All right. 50 00:03:39,450 --> 00:03:42,777 All right, who said they might try this? 51 00:03:51,436 --> 00:03:55,500 AUDIENCE: All Bronsted-Lowry bases are Lewis spaces, 52 00:03:55,500 --> 00:03:57,970 but not all Lewis bases are Bronsted-Lowry bases 53 00:03:57,970 --> 00:04:02,410 because you can act as a proxy with no hydrogen ion 54 00:04:02,410 --> 00:04:03,560 if it connects directly. 55 00:04:03,560 --> 00:04:08,140 But if a hydrogen ion bonds, an example H3O, 56 00:04:08,140 --> 00:04:10,960 to the lone pair of oxygen, then it's 57 00:04:10,960 --> 00:04:12,300 the same thing as a Lewis base. 58 00:04:19,530 --> 00:04:22,060 CATHERINE DRENNAN: OK. 59 00:04:22,060 --> 00:04:22,980 so that's right. 60 00:04:22,980 --> 00:04:26,190 The Lewis acid-Lewis base definition 61 00:04:26,190 --> 00:04:28,630 works if you don't have a hydrogen 62 00:04:28,630 --> 00:04:30,720 ion involved or a proton involved, 63 00:04:30,720 --> 00:04:33,660 but the other still applies. 64 00:04:33,660 --> 00:04:38,500 So remember that when you're accepting H plus, 65 00:04:38,500 --> 00:04:40,290 you're taking it without an electron, 66 00:04:40,290 --> 00:04:44,260 so the base is taking H plus, so it's donating its electrons 67 00:04:44,260 --> 00:04:45,500 to form the bond. 68 00:04:45,500 --> 00:04:48,700 And the acid, when it gives up H plus, 69 00:04:48,700 --> 00:04:50,540 it's keeping all of the electrons. 70 00:04:50,540 --> 00:04:54,190 It's accepting the electrons, so when the acid gives up H plus, 71 00:04:54,190 --> 00:04:57,210 it's accepting, and when the base takes H plus, 72 00:04:57,210 --> 00:04:58,870 it's using its electrons. 73 00:04:58,870 --> 00:05:06,320 So the Lewis definitions are lone-pair or electron-centric 74 00:05:06,320 --> 00:05:09,340 definitions, whereas, Bronsted-Lowry 75 00:05:09,340 --> 00:05:13,010 is thinking about the proton, but they both work together. 76 00:05:13,010 --> 00:05:15,240 So that's a good review of the definitions 77 00:05:15,240 --> 00:05:17,040 we talked about last time, and now I'll 78 00:05:17,040 --> 00:05:20,670 try to get my mic back on. 79 00:05:20,670 --> 00:05:22,190 All right. 80 00:05:22,190 --> 00:05:26,860 So today we're going to continue talking about acid base, 81 00:05:26,860 --> 00:05:29,310 and we're going to continue talking about acid base quite 82 00:05:29,310 --> 00:05:30,000 a bit. 83 00:05:30,000 --> 00:05:33,390 So we have a number more lectures on acid base. 84 00:05:33,390 --> 00:05:35,990 So we're going to start today with the relationship 85 00:05:35,990 --> 00:05:40,810 between pKw, pH, and pOH, we're going 86 00:05:40,810 --> 00:05:43,540 to talk about the strengths of acids and bases, 87 00:05:43,540 --> 00:05:46,680 and then we're going to start doing equilibrium 88 00:05:46,680 --> 00:05:48,450 acid-base problems. 89 00:05:48,450 --> 00:05:50,170 And we're going to do a bunch of those. 90 00:05:50,170 --> 00:05:53,010 And students who are doing this and learning it 91 00:05:53,010 --> 00:05:54,690 for the first time are telling me there 92 00:05:54,690 --> 00:05:57,290 is an infinite number of different kinds of problems. 93 00:05:57,290 --> 00:05:58,295 It's really not true. 94 00:05:58,295 --> 00:06:00,670 There are only five different types, and today hopefully, 95 00:06:00,670 --> 00:06:02,310 we'll get through two of them. 96 00:06:02,310 --> 00:06:05,500 So first we need to think about water 97 00:06:05,500 --> 00:06:07,830 because water is a really important solvent when 98 00:06:07,830 --> 00:06:11,150 we're talking about acid base equilibrium. 99 00:06:11,150 --> 00:06:15,280 And we mentioned last time that water 100 00:06:15,280 --> 00:06:17,490 can act as an acid and a base. 101 00:06:17,490 --> 00:06:20,070 Does anyone remember what that's called, something that 102 00:06:20,070 --> 00:06:21,520 can act as an acid and a base? 103 00:06:21,520 --> 00:06:22,765 AUDIENCE: Amphoteric. 104 00:06:22,765 --> 00:06:24,570 CATHERINE DRENNAN: Amphoteric. 105 00:06:24,570 --> 00:06:28,420 So here we can think about two waters 106 00:06:28,420 --> 00:06:32,684 going to H3O plus and OH. 107 00:06:32,684 --> 00:06:34,100 We could also write it out instead 108 00:06:34,100 --> 00:06:36,410 of just saying two waters, one water that's 109 00:06:36,410 --> 00:06:40,570 acting as the acid and another water is acting as a base. 110 00:06:40,570 --> 00:06:45,160 So the water that's acting as an acid gives up its hydrogen ion, 111 00:06:45,160 --> 00:06:46,680 or proton to the base. 112 00:06:46,680 --> 00:06:49,840 The base accepts it forming a conjugate acid, 113 00:06:49,840 --> 00:06:54,950 and this water that lost H plus becomes OH minus. 114 00:06:54,950 --> 00:07:01,170 So water plus water, H2O plus H2O, forms an acid and a base. 115 00:07:01,170 --> 00:07:04,940 So we can ask the question, then how much water 116 00:07:04,940 --> 00:07:09,600 is in a glass of water, or how much H2O is actually 117 00:07:09,600 --> 00:07:14,900 in this water container that I have here? 118 00:07:14,900 --> 00:07:16,840 So when I'm asking that question, 119 00:07:16,840 --> 00:07:21,070 how much H2O in a container of water 120 00:07:21,070 --> 00:07:24,920 and if I know the delta G0 of that process, 121 00:07:24,920 --> 00:07:28,530 I'm asking about what is the relationship 122 00:07:28,530 --> 00:07:32,180 between our products here, our ionized acid 123 00:07:32,180 --> 00:07:35,690 and our base over here, the ionized products of water, 124 00:07:35,690 --> 00:07:38,870 compared to water at equilibrium. 125 00:07:38,870 --> 00:07:42,140 Because this container, the water, is in equilibrium. 126 00:07:42,140 --> 00:07:44,770 So if I'm asking about the ratio of products 127 00:07:44,770 --> 00:07:50,130 to react as an equilibrium, what am I asking about? 128 00:07:50,130 --> 00:07:51,224 Just yell it out. 129 00:07:51,224 --> 00:07:52,140 AUDIENCE: Equilibrium. 130 00:07:52,140 --> 00:07:54,681 CATHERINE DRENNAN: I'm asking about the equilibrium constant. 131 00:07:54,681 --> 00:07:55,340 Exactly. 132 00:07:55,340 --> 00:08:00,440 I'm asking what is K. What is the equilibrium constant. 133 00:08:00,440 --> 00:08:04,320 So let's just briefly review the relationship. 134 00:08:04,320 --> 00:08:06,560 We're given here a delta G0, and we're 135 00:08:06,560 --> 00:08:10,060 asking about K. What is the relationship, how 136 00:08:10,060 --> 00:08:14,480 do you calculate K if you know delta G0? 137 00:08:14,480 --> 00:08:18,090 And we have some equations that will be given to you. 138 00:08:18,090 --> 00:08:18,590 Sorry. 139 00:08:18,590 --> 00:08:19,400 Not quite yet. 140 00:08:19,400 --> 00:08:21,450 There's going to be a clicker question coming. 141 00:08:21,450 --> 00:08:23,640 We have some equations here. 142 00:08:23,640 --> 00:08:26,780 So we have delta G0 equals minus RT natural log 143 00:08:26,780 --> 00:08:31,440 of K, which we can rearrange to solve for K. 144 00:08:31,440 --> 00:08:34,140 And let's just have a little reminder of what 145 00:08:34,140 --> 00:08:35,679 these terms are. 146 00:08:35,679 --> 00:08:37,240 So we'll put this up here. 147 00:08:37,240 --> 00:08:39,510 T is temperature, and we're going 148 00:08:39,510 --> 00:08:41,270 to be talking about room temperature. 149 00:08:41,270 --> 00:08:43,049 This isn't room temperature now. 150 00:08:43,049 --> 00:08:45,740 And in fact, almost all acid-based problems 151 00:08:45,740 --> 00:08:47,326 are going to be at room temperature. 152 00:08:47,326 --> 00:08:48,700 There might be a few differences, 153 00:08:48,700 --> 00:08:50,800 but we have a lot at room temperature. 154 00:08:50,800 --> 00:08:56,830 We have a constant, R, and so 8.314 joules 155 00:08:56,830 --> 00:09:04,470 per Kelvin per mole and delta G. So we were already told 156 00:09:04,470 --> 00:09:06,470 the value for delta G here. 157 00:09:06,470 --> 00:09:10,710 Now, thinking about this room temperature, this constant, 158 00:09:10,710 --> 00:09:12,850 and this value for delta G, do you 159 00:09:12,850 --> 00:09:16,120 expect a large or small value for K 160 00:09:16,120 --> 00:09:19,387 if you have this delta G0 value? 161 00:09:19,387 --> 00:09:20,720 And that's our clicker question. 162 00:09:51,282 --> 00:09:52,260 All right. 163 00:09:52,260 --> 00:09:54,290 Let's just do 10 more seconds. 164 00:10:08,060 --> 00:10:09,170 Yep. 165 00:10:09,170 --> 00:10:14,320 So K, you would expect to be a small value less than 1. 166 00:10:14,320 --> 00:10:16,950 And you could think about that mathematically 167 00:10:16,950 --> 00:10:20,250 from these expressions, or you could think about it 168 00:10:20,250 --> 00:10:25,960 in terms of the value of for delta G. So if we solve this 169 00:10:25,960 --> 00:10:28,540 and we put in our value for delta G0, which 170 00:10:28,540 --> 00:10:31,640 is a positive value-- so the forward direction 171 00:10:31,640 --> 00:10:35,380 of the reaction is non-spontaneous. 172 00:10:35,380 --> 00:10:39,820 So we put in our positive delta G0 and our other values here, 173 00:10:39,820 --> 00:10:44,580 and this is, again, joules per Kelvin per mole. 174 00:10:44,580 --> 00:10:47,830 And we have to make sure that we convert kilojoules to jewels 175 00:10:47,830 --> 00:10:49,780 and our units will cancel out. 176 00:10:49,780 --> 00:10:53,480 And if we do that, we get this value for natural log of K 177 00:10:53,480 --> 00:10:58,600 and this value for K of 1.0 times 10 to the minus 14th 178 00:10:58,600 --> 00:11:00,080 at room temperature. 179 00:11:00,080 --> 00:11:02,310 And you will find that you end up 180 00:11:02,310 --> 00:11:05,610 memorizing this value if you do enough acid-base problems. 181 00:11:05,610 --> 00:11:08,650 In fact, you might have it memorized already. 182 00:11:08,650 --> 00:11:12,120 So this is, in fact, a small value. 183 00:11:12,120 --> 00:11:15,130 And that indicates that only a small proportion 184 00:11:15,130 --> 00:11:17,410 of water molecules are ionized. 185 00:11:17,410 --> 00:11:22,900 About 1 molecule in every 200 million are ionized, 186 00:11:22,900 --> 00:11:25,910 so it's a very, very small value. 187 00:11:25,910 --> 00:11:29,720 So there is a lot of H2O in a glass of water. 188 00:11:29,720 --> 00:11:32,480 So in here it's mostly H2O. 189 00:11:32,480 --> 00:11:37,000 Very few molecules are ionized in that. 190 00:11:37,000 --> 00:11:38,720 Now, part of the reason why you're 191 00:11:38,720 --> 00:11:41,970 likely to have this value of K memorized 192 00:11:41,970 --> 00:11:44,790 is because it has a special name. 193 00:11:44,790 --> 00:11:50,550 This K is called Kw for water. 194 00:11:50,550 --> 00:11:52,640 So that's easy to remember. 195 00:11:52,640 --> 00:11:57,690 And Kw, the equilibrium constant for water, 196 00:11:57,690 --> 00:12:01,010 is equal to the hydronium ion concentration 197 00:12:01,010 --> 00:12:05,010 times the hydroxide ion concentration. 198 00:12:05,010 --> 00:12:07,650 And since this is an equilibrium constant, 199 00:12:07,650 --> 00:12:09,610 if you're at constant temperature at room 200 00:12:09,610 --> 00:12:14,960 temperature, this will always equal 1.0 times 10 201 00:12:14,960 --> 00:12:16,610 to the minus 14th. 202 00:12:16,610 --> 00:12:20,930 This product is always going to be this at room temperature. 203 00:12:20,930 --> 00:12:22,670 And that is why it's going to turn out 204 00:12:22,670 --> 00:12:24,360 to be important because you're going 205 00:12:24,360 --> 00:12:27,971 to use that information in solving a lot of problems. 206 00:12:27,971 --> 00:12:28,470 All right. 207 00:12:28,470 --> 00:12:31,410 So if this is an equilibrium constant, 208 00:12:31,410 --> 00:12:35,010 we want to think again about expressions for equilibrium 209 00:12:35,010 --> 00:12:38,080 and just make a note that we did not 210 00:12:38,080 --> 00:12:42,610 include water squared on the bottom of this expression. 211 00:12:42,610 --> 00:12:46,100 So Kw equals hydronium ion concentration 212 00:12:46,100 --> 00:12:48,530 times hydroxide ion concentration. 213 00:12:48,530 --> 00:12:50,530 You don't put it over water. 214 00:12:50,530 --> 00:12:53,060 And the reason for this is because this is a pretty pure 215 00:12:53,060 --> 00:12:57,060 solvent, and you don't include pure solvents 216 00:12:57,060 --> 00:12:59,090 in your equilibrium expression. 217 00:12:59,090 --> 00:13:00,930 You also don't include solids. 218 00:13:00,930 --> 00:13:04,180 So we talked about that when we were talking about solubility. 219 00:13:04,180 --> 00:13:07,760 So you have to remember that solvents like water 220 00:13:07,760 --> 00:13:09,820 is nearly pure, doesn't go in. 221 00:13:09,820 --> 00:13:12,610 Pure other liquids, pure other solids 222 00:13:12,610 --> 00:13:16,710 are not included in your equilibrium expression. 223 00:13:16,710 --> 00:13:17,570 All right. 224 00:13:17,570 --> 00:13:20,450 So we now know about water. 225 00:13:20,450 --> 00:13:23,610 We know about Kw. 226 00:13:23,610 --> 00:13:25,850 Let's do a few more definitions and then 227 00:13:25,850 --> 00:13:27,300 think about how all of these terms 228 00:13:27,300 --> 00:13:29,320 are related to each other. 229 00:13:29,320 --> 00:13:32,230 So a definition that many people already know, 230 00:13:32,230 --> 00:13:35,290 probably if you've taken any kind of science before, 231 00:13:35,290 --> 00:13:37,850 you're probably aware that pH equals 232 00:13:37,850 --> 00:13:43,040 minus log of the hydronium ion concentration of H3O plus. 233 00:13:43,040 --> 00:13:45,490 If you don't know it, it will be on your equation sheet, 234 00:13:45,490 --> 00:13:48,920 so it doesn't really matter, but there it is. 235 00:13:48,920 --> 00:13:50,730 And there's also pOH. 236 00:13:50,730 --> 00:13:55,180 pOH equals minus log of the hydroxide ion 237 00:13:55,180 --> 00:13:58,430 concentration, OH minus. 238 00:13:58,430 --> 00:14:00,900 Now, let's think about the relationships 239 00:14:00,900 --> 00:14:05,270 of pKw, pH, and pOH. 240 00:14:05,270 --> 00:14:08,930 So relationships. 241 00:14:08,930 --> 00:14:10,730 So I just told you and maybe you've 242 00:14:10,730 --> 00:14:13,400 already memorized that Kw is going 243 00:14:13,400 --> 00:14:16,280 to be equal to the hydronium ion concentration 244 00:14:16,280 --> 00:14:19,180 times the hydroxide ion concentration, 245 00:14:19,180 --> 00:14:22,660 and that's going to be equal to 1.0 times 10 to the minus 14th 246 00:14:22,660 --> 00:14:24,850 at room temperature. 247 00:14:24,850 --> 00:14:27,780 So now, if we take this expression 248 00:14:27,780 --> 00:14:31,240 and we take the log of all of these terms 249 00:14:31,240 --> 00:14:35,870 and multiply by a negative value, 250 00:14:35,870 --> 00:14:37,750 then we will get this expression. 251 00:14:37,750 --> 00:14:43,080 So minus the log of Kw is pKw. 252 00:14:43,080 --> 00:14:49,080 And minus log of the hydronium ion concentration is what? 253 00:14:49,080 --> 00:14:53,962 pH minus log of the hydroxide ion concentration is what? 254 00:14:53,962 --> 00:14:55,378 AUDIENCE: pOH. 255 00:14:55,378 --> 00:14:56,480 CATHERINE DRENNAN: pOH. 256 00:14:56,480 --> 00:15:08,210 So pKw equals pH plus pOH, and that is equal to 14.00 at room 257 00:15:08,210 --> 00:15:09,270 temperature. 258 00:15:09,270 --> 00:15:12,470 And this is a very useful expression, 259 00:15:12,470 --> 00:15:16,130 the fact that pH and pOH are equal to 14. 260 00:15:16,130 --> 00:15:19,020 If you know one of these, then the other one 261 00:15:19,020 --> 00:15:22,410 by doing a simple subtraction. 262 00:15:22,410 --> 00:15:25,810 So you will find yourself using this expression quite often 263 00:15:25,810 --> 00:15:27,870 in the problem sets. 264 00:15:27,870 --> 00:15:30,380 And so I'll say that one, the problem set that that's 265 00:15:30,380 --> 00:15:34,040 due on Friday has just a couple of acid-based problems 266 00:15:34,040 --> 00:15:34,872 at the end. 267 00:15:34,872 --> 00:15:40,940 The next problem set will be 100% acid-base problems, 268 00:15:40,940 --> 00:15:46,440 so there's a lot to look forward to. 269 00:15:46,440 --> 00:15:51,050 So let's talk about the strengths of acids and bases. 270 00:15:51,050 --> 00:15:55,230 So if you have a pH of pure water, 271 00:15:55,230 --> 00:16:00,350 it should be equal to minus log times 1.0 times 10 272 00:16:00,350 --> 00:16:05,600 to the minus 7, or the pH equals 7, which is a neutral pH. 273 00:16:05,600 --> 00:16:11,650 So if we have a scale here, pH minus 1 to pH 14, 7 274 00:16:11,650 --> 00:16:14,820 is a neutral value. 275 00:16:14,820 --> 00:16:18,270 If we're talking about things that are acidic, 276 00:16:18,270 --> 00:16:22,680 the pH of an acidic solution is less than 7, 277 00:16:22,680 --> 00:16:26,530 and so this is the acidic part over here. 278 00:16:26,530 --> 00:16:31,660 And the pH of a basic solution is greater than 7, 279 00:16:31,660 --> 00:16:35,850 so that is down in this part. 280 00:16:35,850 --> 00:16:40,800 The EPA defines waste as corrosive 281 00:16:40,800 --> 00:16:45,750 if the pH is lower than 3, so that 282 00:16:45,750 --> 00:16:52,230 would be corrosive, or higher than 12.5, also corrosive. 283 00:16:52,230 --> 00:16:56,420 So living creatures like to be more in the neutral range. 284 00:16:56,420 --> 00:17:01,510 When you get to things that are very low pH or very high pH, 285 00:17:01,510 --> 00:17:04,310 that is less good. 286 00:17:04,310 --> 00:17:08,369 So let's now check out a few pH's, and I'd 287 00:17:08,369 --> 00:17:13,240 like to have five TAs come down to help me with this. 288 00:17:13,240 --> 00:17:15,819 And I'll just show you what we're going to do. 289 00:17:15,819 --> 00:17:18,089 So we'll have some volunteers. 290 00:17:18,089 --> 00:17:20,069 We have pH paper. 291 00:17:20,069 --> 00:17:22,490 The pH paper has an indicator on. 292 00:17:22,490 --> 00:17:24,990 You put the paper in the solution 293 00:17:24,990 --> 00:17:28,099 and then try to see which thing matches best. 294 00:17:28,099 --> 00:17:32,270 So this is a very quick and dirty way to estimate pH. 295 00:17:32,270 --> 00:17:35,780 There's pH meters and indicator dyes that work better, 296 00:17:35,780 --> 00:17:38,840 but we'll have five people measure five things. 297 00:17:38,840 --> 00:17:41,280 We have ammonia, a cleaner. 298 00:17:41,280 --> 00:17:43,030 We talked about the importance of cleaning 299 00:17:43,030 --> 00:17:45,270 bathrooms last lecture. 300 00:17:45,270 --> 00:17:47,890 We have soda. 301 00:17:47,890 --> 00:17:52,840 We have vinegar, which is often used in salad dressings. 302 00:17:52,840 --> 00:17:55,790 Before I came here I just went to a random sink 303 00:17:55,790 --> 00:17:59,520 and got some MIT water to measure. 304 00:17:59,520 --> 00:18:02,660 And then this, this is special. 305 00:18:02,660 --> 00:18:06,710 This is a prescription medicine. 306 00:18:06,710 --> 00:18:12,500 So this is a solution of iron 2 sulfate. 307 00:18:12,500 --> 00:18:16,710 And this is prescribed if have an iron deficiency. 308 00:18:16,710 --> 00:18:19,600 An iron deficiency is pretty bad, and a lot of kids 309 00:18:19,600 --> 00:18:20,810 have iron deficiencies. 310 00:18:20,810 --> 00:18:22,680 It's really common in infants. 311 00:18:22,680 --> 00:18:25,460 So my daughter had an iron deficiency, 312 00:18:25,460 --> 00:18:27,170 and this was her medicine. 313 00:18:27,170 --> 00:18:32,260 In fact, this box is completely full of this medicine. 314 00:18:32,260 --> 00:18:36,360 So kids cannot swallow pills very easily. 315 00:18:36,360 --> 00:18:38,940 How many of you actually cannot swallow a pill? 316 00:18:38,940 --> 00:18:41,340 A lot of adults are pill challenged. 317 00:18:41,340 --> 00:18:43,400 How many are happy swallowing pills? 318 00:18:43,400 --> 00:18:44,820 Let's do a more positive thing. 319 00:18:44,820 --> 00:18:46,820 OK. 320 00:18:46,820 --> 00:18:51,620 So swallowing pills is not everyone's favorite. 321 00:18:51,620 --> 00:18:54,970 When you're a kid, it's really impossible 322 00:18:54,970 --> 00:18:57,930 to tell someone to put something in their throat 323 00:18:57,930 --> 00:19:00,400 and go back and try to get it down. 324 00:19:00,400 --> 00:19:02,140 They choke, spit up, it's awful. 325 00:19:02,140 --> 00:19:05,440 So kids have to take medicine in solution. 326 00:19:05,440 --> 00:19:10,330 So this medicine is 300 milligrams in 5 milliliters. 327 00:19:10,330 --> 00:19:12,620 It's very concentrated, so my daughter 328 00:19:12,620 --> 00:19:15,010 had a pretty severe iron deficiency. 329 00:19:15,010 --> 00:19:19,030 So when she tried taking it, and I tasted it, 330 00:19:19,030 --> 00:19:21,276 it was horrific beyond belief. 331 00:19:21,276 --> 00:19:23,150 And after she had taken it a couple of times, 332 00:19:23,150 --> 00:19:26,900 she started to have sores on her tongue and in her mouth. 333 00:19:26,900 --> 00:19:29,950 And I as a chemist said, let's measure the pH. 334 00:19:29,950 --> 00:19:34,230 So you will measure the pH of this prescription medicine 335 00:19:34,230 --> 00:19:38,850 for a four-year-old and tell me whether the EPA would prove it 336 00:19:38,850 --> 00:19:39,380 or not. 337 00:19:39,380 --> 00:19:42,300 Anyway, so here are the TAs, and they're going to come around. 338 00:19:42,300 --> 00:19:45,777 Raise your hand if you're willing to measure a pH. 339 00:19:45,777 --> 00:19:46,860 TAs, just grab a solution. 340 00:19:46,860 --> 00:19:50,830 I should have had you pour it, but OK, we're doing it. 341 00:19:50,830 --> 00:19:57,300 And we'll go around, and we'll measure, and tell me 342 00:19:57,300 --> 00:19:59,288 what you find, and I'll write it on the board. 343 00:20:07,866 --> 00:20:08,824 Yeah, OK. 344 00:20:15,600 --> 00:20:16,100 All right. 345 00:20:16,100 --> 00:20:17,105 So we have pH. 346 00:20:53,510 --> 00:20:54,010 All right. 347 00:20:54,010 --> 00:20:54,898 Do we have an answer? 348 00:21:01,012 --> 00:21:03,532 Did we get one? 349 00:21:03,532 --> 00:21:04,240 What do you have? 350 00:21:04,240 --> 00:21:04,989 AUDIENCE: Ammonia. 351 00:21:04,989 --> 00:21:06,610 CATHERINE DRENNAN: Ammonia is 12. 352 00:21:15,400 --> 00:21:17,840 So almost corrosive, but not quite. 353 00:21:17,840 --> 00:21:20,210 You can still have clean, but if you 354 00:21:20,210 --> 00:21:24,920 want an excuse not to clean, you're like, sorry, too close. 355 00:21:24,920 --> 00:21:27,210 Too Corrosive Do we have another one? 356 00:21:27,210 --> 00:21:29,190 AUDIENCE: MIT water is 7. 357 00:21:29,190 --> 00:21:30,729 CATHERINE DRENNAN: MIT water is 7? 358 00:21:30,729 --> 00:21:31,770 AUDIENCE: MIT water is 7. 359 00:21:31,770 --> 00:21:32,920 CATHERINE DRENNAN: Awesome. 360 00:21:37,160 --> 00:21:40,290 So often, actually, water that you get from a tap 361 00:21:40,290 --> 00:21:45,370 is not 7 because often there are ions that are dissolved in it. 362 00:21:45,370 --> 00:21:48,890 But a little bit around 7, a little bit of ions 363 00:21:48,890 --> 00:21:49,700 never hurt anyone. 364 00:21:49,700 --> 00:21:51,725 We have lots of ions in our body. 365 00:21:51,725 --> 00:21:52,225 Soda? 366 00:21:52,225 --> 00:21:53,680 AUDIENCE: 3 and 1/2. 367 00:21:53,680 --> 00:21:55,568 CATHERINE DRENNAN: Soda at 3 and 1/2. 368 00:22:01,020 --> 00:22:04,450 So soda is definitely getting into our corrosive range. 369 00:22:04,450 --> 00:22:05,955 AUDIENCE: Vinegar at 2. 370 00:22:05,955 --> 00:22:07,920 CATHERINE DRENNAN: And we have vinegar at 2. 371 00:22:13,030 --> 00:22:14,630 And that is corrosive. 372 00:22:14,630 --> 00:22:19,190 People often use pure vinegar to clean out a coffee pot 373 00:22:19,190 --> 00:22:20,980 because it is pretty corrosive. 374 00:22:20,980 --> 00:22:23,570 It really does a good job of cleaning it. 375 00:22:23,570 --> 00:22:25,290 When you're having it in salad dressing, 376 00:22:25,290 --> 00:22:27,250 you don't want to have it pure vinegar. 377 00:22:27,250 --> 00:22:29,176 That would not be good to drink. 378 00:22:29,176 --> 00:22:30,175 And what about medicine? 379 00:22:30,175 --> 00:22:32,680 AUDIENCE: Medicine has a pH of 2. 380 00:22:32,680 --> 00:22:33,780 CATHERINE DRENNAN: 2. 381 00:22:33,780 --> 00:22:38,350 So we also have in the corrosive category 382 00:22:38,350 --> 00:22:41,480 the medicine that my four-year-old daughter 383 00:22:41,480 --> 00:22:44,050 was asked to take. 384 00:22:44,050 --> 00:22:50,760 So when she started getting these sores in her mouth 385 00:22:50,760 --> 00:22:55,530 from taking her medicine, I brought some pH paper home 386 00:22:55,530 --> 00:22:57,170 and measured the pH and discovered 387 00:22:57,170 --> 00:22:59,310 that what she was taking was corrosive. 388 00:22:59,310 --> 00:23:01,990 So I went to the doctor and said, can you 389 00:23:01,990 --> 00:23:03,820 prescribe something different. 390 00:23:03,820 --> 00:23:05,690 This medicine is corrosive. 391 00:23:05,690 --> 00:23:08,780 And the doctor looked at me and it's like the pH, 392 00:23:08,780 --> 00:23:12,470 the pH is like 2 or lower. 393 00:23:12,470 --> 00:23:15,775 And she looked at me, and she's like, what do you mean pH? 394 00:23:21,200 --> 00:23:24,390 This is why I spend many lectures 395 00:23:24,390 --> 00:23:27,510 on pH in this class and acids and bases. 396 00:23:27,510 --> 00:23:31,200 Some of you will be doctors, and some of you 397 00:23:31,200 --> 00:23:35,360 will not encourage parents to buy this much corrosive thing 398 00:23:35,360 --> 00:23:38,980 to feed their child when they're iron deficient. 399 00:23:38,980 --> 00:23:40,540 So this is what I'm talking about. 400 00:23:40,540 --> 00:23:43,040 And for those of you who are not going to be doctors, 401 00:23:43,040 --> 00:23:44,730 many of you will probably have children 402 00:23:44,730 --> 00:23:45,870 with an iron deficiency. 403 00:23:45,870 --> 00:23:47,200 It's super common. 404 00:23:47,200 --> 00:23:50,520 So you will know that you need to measure 405 00:23:50,520 --> 00:23:53,810 the pH of the solution before giving it to your child. 406 00:23:53,810 --> 00:23:55,220 So what did I do? 407 00:23:55,220 --> 00:24:00,010 So what I did was I bought adult pills, same stuff, 408 00:24:00,010 --> 00:24:04,040 iron sulfate, and I made sure that this 409 00:24:04,040 --> 00:24:06,380 told me the number of milligrams that were in there. 410 00:24:06,380 --> 00:24:08,160 I looked at what was prescribed. 411 00:24:08,160 --> 00:24:11,330 So I got the pills with the correct number of milligrams, 412 00:24:11,330 --> 00:24:14,840 and then I crushed them, and that stuff also 413 00:24:14,840 --> 00:24:18,510 tastes really nasty, but a little tip for the future, 414 00:24:18,510 --> 00:24:21,680 there is one and only one that I discovered, 415 00:24:21,680 --> 00:24:26,370 taste that can cover the taste of iron 2 sulfate, 416 00:24:26,370 --> 00:24:28,056 and that is Nutella. 417 00:24:31,660 --> 00:24:35,290 So don't give your child corrosive medicine. 418 00:24:35,290 --> 00:24:39,990 Give them crushed pills in Nutella. 419 00:24:39,990 --> 00:24:42,080 pH is important. 420 00:24:42,080 --> 00:24:46,010 pH is important. 421 00:24:46,010 --> 00:24:49,190 So let's talk about the strengths of acids. 422 00:24:53,770 --> 00:24:57,520 And you might want to reconsider huge soda consumption too. 423 00:24:57,520 --> 00:25:00,360 I don't really know, but that's something 424 00:25:00,360 --> 00:25:02,090 you might want to think about. 425 00:25:02,090 --> 00:25:03,082 Strength of acids. 426 00:25:06,270 --> 00:25:13,050 So here we have some acetic acid, CH3COOH aqueous, 427 00:25:13,050 --> 00:25:17,260 dissolved in our solvent, which is liquid water. 428 00:25:17,260 --> 00:25:20,140 And this is an acid, so it will give up 429 00:25:20,140 --> 00:25:24,400 a hydrogen ion or a proton to the water, which acts as a base 430 00:25:24,400 --> 00:25:30,630 and forms our hydronium ions, H3O plus, and also 431 00:25:30,630 --> 00:25:32,710 the conjugate base of the acid. 432 00:25:32,710 --> 00:25:40,520 So it's the acid missing H plus, CH3C00 minus. 433 00:25:40,520 --> 00:25:44,580 So if we're talking about the strength of the acid, what 434 00:25:44,580 --> 00:25:48,050 we really want to know is how much of that acid 435 00:25:48,050 --> 00:25:51,230 forms ions, how much of it ionizes. 436 00:25:51,230 --> 00:25:53,310 That is going to determine how strong it 437 00:25:53,310 --> 00:25:54,980 is because the amount that ionizes 438 00:25:54,980 --> 00:25:59,630 is equal to the amount of the hydronium ions you get, 439 00:25:59,630 --> 00:26:01,850 and pH equals minus the concentration 440 00:26:01,850 --> 00:26:03,040 of hydronium ions. 441 00:26:03,040 --> 00:26:07,890 So the pH depends on the extent to which it ionizes. 442 00:26:07,890 --> 00:26:10,960 And again, we're talking about at equilibrium, 443 00:26:10,960 --> 00:26:14,870 so we're talking about a equilibrium constant, 444 00:26:14,870 --> 00:26:17,220 and this one also gets a fancy name. 445 00:26:17,220 --> 00:26:19,830 In acids and bases, all the equilibrium 446 00:26:19,830 --> 00:26:21,750 constants get their own little names, 447 00:26:21,750 --> 00:26:25,480 and this is the acid ionization constant, Ka. 448 00:26:25,480 --> 00:26:29,440 So it's the equilibrium constant for an acid, Ka. 449 00:26:29,440 --> 00:26:32,190 So that's at least very easy to remember. 450 00:26:32,190 --> 00:26:35,620 So now based on what you know about equilibrium constants, 451 00:26:35,620 --> 00:26:39,545 why don't you tell me the answer to this? 452 00:27:19,583 --> 00:27:20,666 All right 10 more seconds. 453 00:27:35,850 --> 00:27:37,920 All right. 454 00:27:37,920 --> 00:27:40,140 So this one is wrong. 455 00:27:40,140 --> 00:27:42,920 It's always products over reactants for an equilibrium 456 00:27:42,920 --> 00:27:43,850 expression. 457 00:27:43,850 --> 00:27:46,400 Water is a solvent and is nearly pure, 458 00:27:46,400 --> 00:27:48,220 so it should not be there. 459 00:27:48,220 --> 00:27:51,480 This should be in the expression because it's aqueous. 460 00:27:51,480 --> 00:27:54,970 If it were solid, it wouldn't be included, but it's aqueous, 461 00:27:54,970 --> 00:27:57,440 so its concentration is going to change, 462 00:27:57,440 --> 00:27:59,800 so it is in our expression. 463 00:27:59,800 --> 00:28:02,320 See answer to 3 for that one. 464 00:28:02,320 --> 00:28:05,610 Enough information, and it's not correct. 465 00:28:05,610 --> 00:28:09,070 So we can write the expression for Ka. 466 00:28:09,070 --> 00:28:12,520 So that's equal to the products, our hydronium ion 467 00:28:12,520 --> 00:28:18,190 concentration, our conjugate base over are conjugate acid. 468 00:28:18,190 --> 00:28:22,360 And so we can look at what the value is for this expression, 469 00:28:22,360 --> 00:28:24,190 and we can look it up. 470 00:28:24,190 --> 00:28:27,540 There's lots and lots of tables of these in your book. 471 00:28:27,540 --> 00:28:33,430 And it is 1.76 times 10 to the minus 5 or minus 3. 472 00:28:33,430 --> 00:28:36,620 I don't have my glasses, whatever it says there. 473 00:28:36,620 --> 00:28:41,690 I think it's 5 at room temperature, which is small. 474 00:28:41,690 --> 00:28:45,690 So when Ka is small, it means that very little of it 475 00:28:45,690 --> 00:28:48,100 is ionizing. 476 00:28:48,100 --> 00:28:53,890 And so only a small number of our acetic acid molecules 477 00:28:53,890 --> 00:28:56,820 are donating their proton when they're dissolved in water. 478 00:28:56,820 --> 00:28:59,080 So that's the definition of a weak acid. 479 00:28:59,080 --> 00:29:04,360 When it has a very small Ka, it's not ionizing very much. 480 00:29:04,360 --> 00:29:06,860 Now, in doing these units, a lot of people 481 00:29:06,860 --> 00:29:11,840 get hung up on the names of the different acids 482 00:29:11,840 --> 00:29:13,980 and one of the first steps in solving 483 00:29:13,980 --> 00:29:17,560 a problem is to write the equilibrium expression out, 484 00:29:17,560 --> 00:29:19,290 but people get hung up with that. 485 00:29:19,290 --> 00:29:21,240 So don't get hung up with that. 486 00:29:21,240 --> 00:29:24,940 You can use generic expressions for acids and water. 487 00:29:24,940 --> 00:29:27,660 If you don't want to write out the whole name of the acid, 488 00:29:27,660 --> 00:29:31,330 you can always just say HA, aqueous, plus the solvent, 489 00:29:31,330 --> 00:29:36,880 water, goes to hydronium ions plus A minus, 490 00:29:36,880 --> 00:29:39,130 the conjugate of the weak acid. 491 00:29:39,130 --> 00:29:43,940 So this is an acid in water, and the acid is HA here. 492 00:29:43,940 --> 00:29:47,740 And an acid expression should be forming hydronium ions. 493 00:29:47,740 --> 00:29:49,920 If it's acidic, you should have acidic pH, 494 00:29:49,920 --> 00:29:53,770 and so you need to have some H3O plus around. 495 00:29:53,770 --> 00:29:55,970 You can also write this expression 496 00:29:55,970 --> 00:29:59,110 as BH plus plus water. 497 00:29:59,110 --> 00:30:06,000 BH plus can give up the H plus and become B and also generate 498 00:30:06,000 --> 00:30:07,610 hydronium ions. 499 00:30:07,610 --> 00:30:11,175 So here are the acid is BHA. 500 00:30:11,175 --> 00:30:14,620 And often when you're looking at these kinds of problems, 501 00:30:14,620 --> 00:30:16,840 a weak acid will be HA, but sometimes you 502 00:30:16,840 --> 00:30:20,770 have a problem involving the conjugate acid of a weak base, 503 00:30:20,770 --> 00:30:24,110 and that's often expressed as BH plus. 504 00:30:24,110 --> 00:30:30,050 So both of these expressions are valid for an acid in water. 505 00:30:30,050 --> 00:30:34,790 So now let's think about strong acids versus weaker acids, 506 00:30:34,790 --> 00:30:36,970 and here's our definition in this class. 507 00:30:36,970 --> 00:30:41,920 A strong acid has a Ka greater than 1, a lot of strong acids 508 00:30:41,920 --> 00:30:45,480 have a Ka really, really, really greater than 1, 509 00:30:45,480 --> 00:30:48,280 and the acid ionizes almost completely. 510 00:30:48,280 --> 00:30:51,100 So if you say it's a strong acid in water, 511 00:30:51,100 --> 00:30:53,890 whatever concentration of that acid you put in 512 00:30:53,890 --> 00:30:56,170 is going to be equal to the concentration of hydronium 513 00:30:56,170 --> 00:30:57,060 ions. 514 00:30:57,060 --> 00:30:58,890 It basically goes straight. 515 00:30:58,890 --> 00:31:02,980 You form lots and lots and lots of products at equilibrium. 516 00:31:02,980 --> 00:31:06,650 A weak acid has a Ka of less than 1, 517 00:31:06,650 --> 00:31:10,030 and a weak acid does not for many ionized species 518 00:31:10,030 --> 00:31:11,130 in solution. 519 00:31:11,130 --> 00:31:14,765 Not very much H3O plus is formed. 520 00:31:18,450 --> 00:31:23,960 So what about pKa? 521 00:31:23,960 --> 00:31:28,040 pKa's are really important in organic chemistry, in biology, 522 00:31:28,040 --> 00:31:29,700 in a lot of areas. 523 00:31:29,700 --> 00:31:35,470 And every year the organic chemistry faculty in 512 524 00:31:35,470 --> 00:31:40,114 talk about pKa's in organic, and the students there say, no, 525 00:31:40,114 --> 00:31:41,280 we never learned about that. 526 00:31:41,280 --> 00:31:43,990 They're like, you took freshman chem, right? 527 00:31:43,990 --> 00:31:46,540 It's like a GIR or you pass the test. 528 00:31:46,540 --> 00:31:49,480 You should know about pKa's, and they're like, no. 529 00:31:49,480 --> 00:31:53,870 So they contact me, and I'm like they did not take 5.111 if they 530 00:31:53,870 --> 00:31:55,770 do not know what a pKa is. 531 00:31:55,770 --> 00:31:59,280 There are other courses that sometimes people decide to take 532 00:31:59,280 --> 00:32:04,770 that are not 5.111, but in 5.111, you know about pKa's. 533 00:32:04,770 --> 00:32:08,310 So what I want you to do is later in life 534 00:32:08,310 --> 00:32:10,670 when you're in a class and pKa's come up 535 00:32:10,670 --> 00:32:13,210 and everyone else in the class is like, I don't know. 536 00:32:13,210 --> 00:32:17,080 You're like I took 5.111. 537 00:32:17,080 --> 00:32:18,350 I can answer that. 538 00:32:18,350 --> 00:32:20,990 That makes me very happy. 539 00:32:20,990 --> 00:32:23,000 So pKa's. 540 00:32:23,000 --> 00:32:27,290 so pKa is minus the log of the Ka. 541 00:32:27,290 --> 00:32:29,320 That's easy to remember. 542 00:32:29,320 --> 00:32:32,970 We already talked about the relationship of Ka 543 00:32:32,970 --> 00:32:35,980 with strong acids or weak acids. 544 00:32:35,980 --> 00:32:40,060 The lower the value of the Ka the higher 545 00:32:40,060 --> 00:32:41,600 the value of the pKa. 546 00:32:41,600 --> 00:32:44,000 That's just out of that expression. 547 00:32:44,000 --> 00:32:50,990 So the higher the pKa means what about the acid? 548 00:32:50,990 --> 00:32:54,330 Think about what you know about Ka to answer this question. 549 00:33:14,230 --> 00:33:14,970 All right. 550 00:33:14,970 --> 00:33:15,815 10 more seconds. 551 00:33:30,410 --> 00:33:32,580 All right. 552 00:33:32,580 --> 00:33:35,300 So that is correct. 553 00:33:35,300 --> 00:33:37,130 So we have a weaker acid. 554 00:33:37,130 --> 00:33:43,270 So if we have a low Ka, that means 555 00:33:43,270 --> 00:33:48,250 that it is a weak acid, and then the higher value of Ka. 556 00:33:48,250 --> 00:33:51,220 So the higher the Ka, that's going 557 00:33:51,220 --> 00:33:57,130 to mean the lower the value or the weaker of the acid. 558 00:33:57,130 --> 00:34:00,870 So just remember the relationship from the equation, 559 00:34:00,870 --> 00:34:03,090 and think about it, and you can think about the value 560 00:34:03,090 --> 00:34:05,560 of Ka, products over reactants. 561 00:34:05,560 --> 00:34:07,190 You might have a lot of products. 562 00:34:07,190 --> 00:34:10,489 That means a lot of ionization and a stronger acid 563 00:34:10,489 --> 00:34:15,510 and fewer products, that means it's weaker. 564 00:34:15,510 --> 00:34:18,320 So let's look at some tables, and I just 565 00:34:18,320 --> 00:34:21,969 put the values that are with the arrows in your notes. 566 00:34:21,969 --> 00:34:23,980 I didn't put the entire table. 567 00:34:23,980 --> 00:34:27,870 The tables are in the book, and this is page one of the table, 568 00:34:27,870 --> 00:34:29,860 so there's a lot of values here. 569 00:34:29,860 --> 00:34:31,420 So this has the acid. 570 00:34:31,420 --> 00:34:34,760 It tells you what the HA term is, what the A minus, 571 00:34:34,760 --> 00:34:37,679 so what the weak acid is, what the conjugate base is. 572 00:34:37,679 --> 00:34:40,139 It gives you the Ka and the pKa. 573 00:34:40,139 --> 00:34:41,739 And so you can see the relationship 574 00:34:41,739 --> 00:34:44,690 between Ka and pKa. 575 00:34:44,690 --> 00:34:49,300 So up here, HI is at the top of this table. 576 00:34:49,300 --> 00:34:52,370 So that probably means it's the strongest or the weakest. 577 00:34:52,370 --> 00:34:53,270 Which do you think? 578 00:34:53,270 --> 00:34:55,955 Is this the strongest or the weakest? 579 00:34:55,955 --> 00:34:56,830 You just yell it out. 580 00:34:56,830 --> 00:34:57,746 AUDIENCE: Strongest. 581 00:34:57,746 --> 00:35:00,040 CATHERINE DRENNAN: It is the strongest. 582 00:35:00,040 --> 00:35:00,730 Right. 583 00:35:00,730 --> 00:35:04,200 So it has Ka value that is much, much, much, much, much, much, 584 00:35:04,200 --> 00:35:07,570 much, much, much, much, much greater than 1. 585 00:35:07,570 --> 00:35:12,510 And it has a very, very, very negative tiny, tiny pKa value. 586 00:35:12,510 --> 00:35:15,180 So this is a super strong acid. 587 00:35:15,180 --> 00:35:16,851 In fact, do not use that. 588 00:35:16,851 --> 00:35:19,350 There's no reason really you would ever want to be using it. 589 00:35:19,350 --> 00:35:21,540 It is not a good thing to play with. 590 00:35:21,540 --> 00:35:22,040 All right. 591 00:35:22,040 --> 00:35:28,000 So HCl is used more often in pH-ing things. 592 00:35:28,000 --> 00:35:30,740 Is this a strong or weak acid? 593 00:35:30,740 --> 00:35:32,286 AUDIENCE: Strong. 594 00:35:32,286 --> 00:35:34,150 CATHERINE DRENNAN: It's also strong. 595 00:35:34,150 --> 00:35:38,500 It's K or Ka is also greater than 1, 596 00:35:38,500 --> 00:35:40,390 but not as strong as this. 597 00:35:40,390 --> 00:35:43,460 We have 10 to the seventh, and that one, gosh, 598 00:35:43,460 --> 00:35:45,790 I should bring my glasses, 10 to the 11th. 599 00:35:45,790 --> 00:35:46,860 That's super strong. 600 00:35:46,860 --> 00:35:48,070 That's still pretty strong. 601 00:35:48,070 --> 00:35:48,570 All right. 602 00:35:48,570 --> 00:35:50,880 So let's look down here now. 603 00:35:50,880 --> 00:35:54,782 Tell me, is this a strong or weak acid? 604 00:35:54,782 --> 00:35:56,090 AUDIENCE: Weak. 605 00:35:56,090 --> 00:35:58,920 CATHERINE DRENNAN: That is weak acid. 606 00:35:58,920 --> 00:36:06,140 The Ka is less than 1 here, and I think that's a minus 2. 607 00:36:06,140 --> 00:36:09,650 And the pKa value, now we're out of the negative number, 608 00:36:09,650 --> 00:36:11,630 so it's on the bigger side. 609 00:36:11,630 --> 00:36:14,710 And down here, is this strong or weak? 610 00:36:14,710 --> 00:36:16,000 AUDIENCE: [INAUDIBLE]. 611 00:36:16,000 --> 00:36:17,550 CATHERINE DRENNAN: Also weak. 612 00:36:17,550 --> 00:36:19,540 And it's even weaker than this one. 613 00:36:19,540 --> 00:36:25,650 It's like 10 to the minus 4, and pKa value is 3.75. 614 00:36:25,650 --> 00:36:28,800 So you see you can look at the Ka values 615 00:36:28,800 --> 00:36:31,440 and think about whether it's a strong or weak acid. 616 00:36:31,440 --> 00:36:33,940 You can also look at the pKa values. 617 00:36:33,940 --> 00:36:36,980 This is really, really small negative value. 618 00:36:36,980 --> 00:36:38,960 This is a bigger value down here. 619 00:36:38,960 --> 00:36:41,320 And this is not been the weakest acid. 620 00:36:41,320 --> 00:36:46,470 There's a whole other page of tables for acids, lots and lots 621 00:36:46,470 --> 00:36:48,220 of acids. 622 00:36:48,220 --> 00:36:50,510 Less bases. 623 00:36:50,510 --> 00:36:55,600 There are a few, and you will see this space quite often, 624 00:36:55,600 --> 00:36:57,640 NH3. 625 00:36:57,640 --> 00:37:00,010 So we have a base in water. 626 00:37:00,010 --> 00:37:03,730 The base accepts a hydrogen ion or proton from the water, 627 00:37:03,730 --> 00:37:06,030 forming NH4 plus. 628 00:37:06,030 --> 00:37:10,120 And then after water loses its hydrogen ion, 629 00:37:10,120 --> 00:37:13,120 it forms OH minus. 630 00:37:13,120 --> 00:37:17,260 So the equilibrium expression or the K 631 00:37:17,260 --> 00:37:23,560 for this base in water problem is called the base ionization 632 00:37:23,560 --> 00:37:26,040 constant, or Kb. 633 00:37:26,040 --> 00:37:29,560 And we can write that expression here. 634 00:37:29,560 --> 00:37:32,060 We have the ammonium ion concentration 635 00:37:32,060 --> 00:37:35,040 times the hydroxide ion concentration 636 00:37:35,040 --> 00:37:37,740 over the concentration of ammonia. 637 00:37:37,740 --> 00:37:40,220 Again, water is not in the expression. 638 00:37:40,220 --> 00:37:43,130 It stays pretty much pure throughout this entire thing. 639 00:37:43,130 --> 00:37:44,500 It's the solvent. 640 00:37:44,500 --> 00:37:47,480 And so this is our expression for Kb. 641 00:37:47,480 --> 00:37:50,570 And if you're writing an expression for Kb, 642 00:37:50,570 --> 00:37:53,040 you should always make sure, check yourself 643 00:37:53,040 --> 00:37:56,380 that you're putting hydroxide ion concentration in there. 644 00:37:56,380 --> 00:38:00,270 If it is a base in water, it should be forming hydroxide ion 645 00:38:00,270 --> 00:38:01,410 concentration. 646 00:38:01,410 --> 00:38:04,180 If you start writing a Kb and you have hydronium ion 647 00:38:04,180 --> 00:38:07,310 concentration in there, you want to stop and rethink 648 00:38:07,310 --> 00:38:09,320 what you're doing. 649 00:38:09,320 --> 00:38:12,690 So in this case, we also have a weak base. 650 00:38:12,690 --> 00:38:18,670 1.8 times 10 to the minus 5 is a small number for Kb. 651 00:38:18,670 --> 00:38:21,520 It's a small equilibrium number, so that means 652 00:38:21,520 --> 00:38:23,130 you're not a lot of products. 653 00:38:23,130 --> 00:38:27,360 Not a lot ionized when you put this weak base in water. 654 00:38:27,360 --> 00:38:30,730 So only a tiny amount of NH3 ionizes 655 00:38:30,730 --> 00:38:35,150 to NH4 plus and OH in solution, so this is what 656 00:38:35,150 --> 00:38:39,440 they call moderately weak base. 657 00:38:39,440 --> 00:38:45,170 So as we saw before, you can have generic expressions 658 00:38:45,170 --> 00:38:46,800 for a base in water. 659 00:38:46,800 --> 00:38:51,440 We can write B aqueous plus water goes to BH plus. 660 00:38:51,440 --> 00:38:54,810 The base accepts a hydrogen ion or proton, 661 00:38:54,810 --> 00:38:58,550 and the water loses one forming OH minus. 662 00:38:58,550 --> 00:39:00,480 We can also write the expression that 663 00:39:00,480 --> 00:39:05,690 often exists from the conjugate of a weak acid, 664 00:39:05,690 --> 00:39:07,170 and this is a weak base. 665 00:39:07,170 --> 00:39:11,620 So A minus plus water goes to HA plus hydroxide ion 666 00:39:11,620 --> 00:39:13,050 concentration. 667 00:39:13,050 --> 00:39:15,690 So either of these are generic expressions 668 00:39:15,690 --> 00:39:20,820 that you can write for a base in water. 669 00:39:20,820 --> 00:39:24,230 So in terms of the definitions, it's the same. 670 00:39:24,230 --> 00:39:28,140 You would say a strong base is something that ionizes 671 00:39:28,140 --> 00:39:31,720 almost completely to give OH. 672 00:39:31,720 --> 00:39:34,640 There aren't a lot of examples of strong bases. 673 00:39:34,640 --> 00:39:36,800 Most of the ones you'll see in your class 674 00:39:36,800 --> 00:39:40,030 are things like sodium hydroxide. 675 00:39:40,030 --> 00:39:41,480 Yes, that is hydroxide. 676 00:39:41,480 --> 00:39:44,480 That's a strong base, and if you put it in water, 677 00:39:44,480 --> 00:39:46,830 you should definitely form a lot of hydroxide. 678 00:39:46,830 --> 00:39:50,100 So they're not a whole lot of examples there. 679 00:39:50,100 --> 00:39:53,250 So they're not all those tables. 680 00:39:53,250 --> 00:39:55,240 It's not like the acids. 681 00:39:55,240 --> 00:39:58,100 But there are some terms that you still need to know. 682 00:39:58,100 --> 00:40:04,600 So pKb equals minus log of the Kb. 683 00:40:04,600 --> 00:40:09,730 And again, the larger the value for Kb, the stronger the base. 684 00:40:09,730 --> 00:40:14,100 Again, it tells you you have a lot that have ionized. 685 00:40:14,100 --> 00:40:18,090 And because of this relationship of this equation, the larger 686 00:40:18,090 --> 00:40:22,677 the pKb the weaker the base. 687 00:40:22,677 --> 00:40:24,260 Because if you have a big number here, 688 00:40:24,260 --> 00:40:27,750 you're going to have a small number there. 689 00:40:27,750 --> 00:40:33,050 So now, let's talk about conjugate acids 690 00:40:33,050 --> 00:40:34,690 and their bases. 691 00:40:34,690 --> 00:40:37,370 This is super important for buffers 692 00:40:37,370 --> 00:40:42,060 that, hopefully, we'll get to in class on Friday. 693 00:40:42,060 --> 00:40:47,110 So the stronger the acid, the weaker its conjugate base. 694 00:40:47,110 --> 00:40:51,600 The stronger the base, the weaker its conjugate acid. 695 00:40:51,600 --> 00:40:54,150 And here we have HCl, which we determined 696 00:40:54,150 --> 00:40:58,910 was a strong acid a little while ago, giving up its hydrogen 697 00:40:58,910 --> 00:41:06,450 ion to water, forming hydronium ion concentration and Cl minus. 698 00:41:06,450 --> 00:41:09,320 Cl minus is not a very good conjugate base. 699 00:41:09,320 --> 00:41:10,890 It's not a good base. 700 00:41:10,890 --> 00:41:14,460 So if we look at this table when we have a strong acid, 701 00:41:14,460 --> 00:41:18,080 its conjugate it is going to be ineffective as a base. 702 00:41:18,080 --> 00:41:20,930 You're basically not pushing that equilibrium back 703 00:41:20,930 --> 00:41:22,710 at all, the other direction. 704 00:41:22,710 --> 00:41:26,470 It's completely ionized, and it stays that way. 705 00:41:26,470 --> 00:41:29,840 But if you have a moderately weak acid, 706 00:41:29,840 --> 00:41:33,700 you're going to have a very weak base, a very weak acid, 707 00:41:33,700 --> 00:41:37,770 you'll have a moderately weak conjugate base, a strong base 708 00:41:37,770 --> 00:41:40,910 and you'll have something that's ineffective as an acid. 709 00:41:40,910 --> 00:41:44,970 So they are inversely related to each other. 710 00:41:44,970 --> 00:41:48,610 So let's think about why this would be true. 711 00:41:48,610 --> 00:41:57,070 And this relationship holds Ka times Kb equals Kw. 712 00:41:57,070 --> 00:42:01,930 And we know that Kw is 1.0 times 10 to the minus 713 00:42:01,930 --> 00:42:04,590 14th at room temperature. 714 00:42:04,590 --> 00:42:09,920 And if we put logs and then minus logs by everything, 715 00:42:09,920 --> 00:42:15,330 we can derive this expression, which is the pKa plus the pKb 716 00:42:15,330 --> 00:42:20,540 equals the pKw equals 14.00. 717 00:42:20,540 --> 00:42:25,200 So you can't have an acid and its conjugate base 718 00:42:25,200 --> 00:42:26,630 both be strong. 719 00:42:26,630 --> 00:42:30,950 You can't have a base in its conjugate acid both be strong. 720 00:42:30,950 --> 00:42:34,480 The pK's need to add up to 14. 721 00:42:34,480 --> 00:42:37,540 So if you have something that's a good acid, 722 00:42:37,540 --> 00:42:42,370 then its conjugate base is not going to be particularly good. 723 00:42:42,370 --> 00:42:46,840 The pKa and the pKb must add up to 14. 724 00:42:46,840 --> 00:42:48,930 If you have something that's a good base, 725 00:42:48,930 --> 00:42:51,740 its conjugate acid is not going to be that good. 726 00:42:51,740 --> 00:42:57,520 So these are connected to each other. 727 00:42:57,520 --> 00:43:00,770 So we can think about, then, the strong acids 728 00:43:00,770 --> 00:43:03,880 and the strong bases again. 729 00:43:03,880 --> 00:43:06,680 So we talk about equilibrium, but really this 730 00:43:06,680 --> 00:43:09,650 is pushing the equilibrium pretty much to completion. 731 00:43:09,650 --> 00:43:13,530 If it's a strong acid in water, however much strong acid 732 00:43:13,530 --> 00:43:17,650 you put in is how much hydronium ion concentration you get out. 733 00:43:17,650 --> 00:43:19,990 A strong base is going to give you 734 00:43:19,990 --> 00:43:23,360 that amount of hydroxide ion concentration. 735 00:43:23,360 --> 00:43:27,150 Strong acids and bases push the equilibrium pretty much 736 00:43:27,150 --> 00:43:29,270 completely toward ionization. 737 00:43:29,270 --> 00:43:32,350 We have these numbers of 10 to the 11th, 10 to the seventh. 738 00:43:32,350 --> 00:43:34,950 These are hugely over here. 739 00:43:34,950 --> 00:43:39,060 We're forming almost ionizing completely. 740 00:43:39,060 --> 00:43:43,620 Now, for weak acids and bases, it's very different. 741 00:43:43,620 --> 00:43:47,820 Here the equilibrium you are going back and forth. 742 00:43:47,820 --> 00:43:50,150 You have this dynamic equilibrium. 743 00:43:50,150 --> 00:43:55,810 The acid in water is forming hydronium ions and an A minus, 744 00:43:55,810 --> 00:44:00,160 but a minus is also a somewhat good, weak base 745 00:44:00,160 --> 00:44:03,360 pushing the equilibrium back the other way. 746 00:44:03,360 --> 00:44:08,330 So in this case, we have a back and forth between the forward 747 00:44:08,330 --> 00:44:10,300 and the reverse. 748 00:44:10,300 --> 00:44:12,385 So if you have very weak, you have moderate, 749 00:44:12,385 --> 00:44:14,720 you have moderate, you have very weak, 750 00:44:14,720 --> 00:44:17,920 and this is what's important for forming buffers, 751 00:44:17,920 --> 00:44:21,650 and buffers are really important. 752 00:44:21,650 --> 00:44:26,570 Let's see if we can get to one of these types of problems 753 00:44:26,570 --> 00:44:31,250 because they're five we need to get to in this unit. 754 00:44:31,250 --> 00:44:33,630 And there are only five. 755 00:44:33,630 --> 00:44:35,850 And people will say salt and water is another, 756 00:44:35,850 --> 00:44:37,490 but salt and water just breaks down 757 00:44:37,490 --> 00:44:39,490 into weak acid and weak bases. 758 00:44:39,490 --> 00:44:42,580 So everyone can learn how to do five types of problems, 759 00:44:42,580 --> 00:44:45,270 so this is very doable. 760 00:44:45,270 --> 00:44:45,770 All right. 761 00:44:45,770 --> 00:44:48,560 So let's just look at equilibrium of weak acids 762 00:44:48,560 --> 00:44:50,790 and we'll save weak bases for last time. 763 00:44:50,790 --> 00:44:55,480 So a weak acid is vitamin C, and I brought some vitamin C here. 764 00:44:55,480 --> 00:44:59,420 I tried to do a demo once and discovered that vitamin C, 765 00:44:59,420 --> 00:45:01,510 it's really coated well, so it does not 766 00:45:01,510 --> 00:45:03,980 dissolve until it hits the acid of your stomach, 767 00:45:03,980 --> 00:45:05,820 so I could not dissolve it. 768 00:45:05,820 --> 00:45:08,490 So I'll just hold this up right here. 769 00:45:08,490 --> 00:45:12,980 But if we had a 500 milligram tablet, which 770 00:45:12,980 --> 00:45:17,610 I think is what this is and if I had 100 mils of water 771 00:45:17,610 --> 00:45:19,360 and if this wasn't coated so there's 772 00:45:19,360 --> 00:45:23,710 no way you can dissolve it, then I could calculate the pH. 773 00:45:23,710 --> 00:45:25,070 So how am I going to do this? 774 00:45:25,070 --> 00:45:29,280 I'm given the Ka, I'm told the number of milligrams, 775 00:45:29,280 --> 00:45:31,370 and I'm told the volume of water. 776 00:45:31,370 --> 00:45:35,110 So the first thing you want to do is calculate molarity. 777 00:45:35,110 --> 00:45:39,970 So you want convert grams to moles or milligrams 778 00:45:39,970 --> 00:45:41,840 to grams to moles. 779 00:45:41,840 --> 00:45:45,389 And then you want to use your volume, and you have 100 mils, 780 00:45:45,389 --> 00:45:46,680 but we're going to do molarity. 781 00:45:46,680 --> 00:45:49,600 So we want to convert milliliters to liters. 782 00:45:49,600 --> 00:45:56,680 And we can calculate 0.0284 molar moles per liter. 783 00:45:56,680 --> 00:45:59,560 Then we can write our expression, and if you want to, 784 00:45:59,560 --> 00:46:02,290 you can just write HA here and A minus. 785 00:46:02,290 --> 00:46:04,620 You don't have to write out the whole thing 786 00:46:04,620 --> 00:46:07,710 if you don't want to. 787 00:46:07,710 --> 00:46:08,690 All right. 788 00:46:08,690 --> 00:46:10,990 So here is the expression again. 789 00:46:10,990 --> 00:46:13,960 Now, just as we saw with chemical equilibrium, 790 00:46:13,960 --> 00:46:17,970 we can write this table talking about the initial molarity. 791 00:46:17,970 --> 00:46:22,550 So we have a weak acid we have no hydronium ion concentration. 792 00:46:22,550 --> 00:46:26,110 We have no weak conjugate base over here, 793 00:46:26,110 --> 00:46:29,690 so the change then is going to be the constant, the molarity 794 00:46:29,690 --> 00:46:31,500 that we had, and these are change 795 00:46:31,500 --> 00:46:34,580 in molarities, so don't put moles or other things here, 796 00:46:34,580 --> 00:46:39,100 molarity here, minus x, that's what you have in equilibrium, 797 00:46:39,100 --> 00:46:42,880 plus x plus x. 798 00:46:42,880 --> 00:46:46,510 So we can write our expression now for Ka. 799 00:46:46,510 --> 00:46:48,910 We're given the value Ka, and we know 800 00:46:48,910 --> 00:46:50,730 how to write an expression. 801 00:46:50,730 --> 00:46:53,000 We have our hydronium ion concentration, 802 00:46:53,000 --> 00:46:57,180 our conjugate base over here over our conjugate acid. 803 00:46:57,180 --> 00:47:04,170 This is equal to x squared 0.0284 minus x. 804 00:47:04,170 --> 00:47:07,220 Now, if you want to with these problems, 805 00:47:07,220 --> 00:47:09,690 you can make an assumption that x 806 00:47:09,690 --> 00:47:12,450 is going to be small and then not have 807 00:47:12,450 --> 00:47:14,710 to use the quadratic equation, but you do 808 00:47:14,710 --> 00:47:16,460 need to check your assumption. 809 00:47:16,460 --> 00:47:18,870 But I'll show you how to do it with an assumption, 810 00:47:18,870 --> 00:47:22,620 so we're going to assume x is small, and this value, the x, 811 00:47:22,620 --> 00:47:23,990 will drop out. 812 00:47:23,990 --> 00:47:26,080 So we can rewrite this expression 813 00:47:26,080 --> 00:47:31,660 as x squared over just the concentration we started with. 814 00:47:31,660 --> 00:47:35,330 And then we can solve for x. 815 00:47:35,330 --> 00:47:40,170 And x, in that case, is point 0.00151, but really 816 00:47:40,170 --> 00:47:41,750 two significant figures. 817 00:47:41,750 --> 00:47:44,350 Our Ka is just 2. 818 00:47:44,350 --> 00:47:46,590 Now, we can check the assumption. 819 00:47:46,590 --> 00:47:48,350 And we're going to check the assumption 820 00:47:48,350 --> 00:47:50,730 that this is, in fact, small. 821 00:47:50,730 --> 00:47:56,950 And if it's less than 5%, then we can use that. 822 00:47:56,950 --> 00:48:00,320 So here this is the x we calculated. 823 00:48:00,320 --> 00:48:03,470 Here is the amount times 100% gives us 824 00:48:03,470 --> 00:48:07,260 5.3%, which is more than 5. 825 00:48:07,260 --> 00:48:09,800 5 is the magic number for the course, 826 00:48:09,800 --> 00:48:12,499 and so then we have to use the quadratic equation. 827 00:48:12,499 --> 00:48:14,290 Now, a lot of you probably have calculators 828 00:48:14,290 --> 00:48:16,700 that can work with quadratic equation, 829 00:48:16,700 --> 00:48:18,240 so you don't really care, but if you 830 00:48:18,240 --> 00:48:20,860 want to check the assumption, you can, and sometimes 831 00:48:20,860 --> 00:48:22,700 we'll need to check it. 832 00:48:22,700 --> 00:48:25,190 So this value, this percent here, 833 00:48:25,190 --> 00:48:28,210 is sometimes called percent ionization 834 00:48:28,210 --> 00:48:31,170 because it's x over the amount you started with. 835 00:48:31,170 --> 00:48:35,090 It's the percent that ionized or percent deprotonated. 836 00:48:35,090 --> 00:48:37,240 If this is an acid, it's the amount 837 00:48:37,240 --> 00:48:39,660 of hydronium, the amount that deprotonated 838 00:48:39,660 --> 00:48:41,309 over what you started with. 839 00:48:41,309 --> 00:48:43,600 So sometimes you're asked to do this even if you're not 840 00:48:43,600 --> 00:48:46,440 checking any assumptions. 841 00:48:46,440 --> 00:48:46,950 All right. 842 00:48:46,950 --> 00:48:49,560 So we got to end with if a quicker question. 843 00:48:49,560 --> 00:48:52,540 We get here this is really two significant figures. 844 00:48:52,540 --> 00:48:55,430 So why don't you tell me how many significant figures 845 00:48:55,430 --> 00:48:57,445 are in your answer for pH. 846 00:49:20,830 --> 00:49:21,330 All right. 847 00:49:21,330 --> 00:49:22,120 10 more seconds. 848 00:49:42,890 --> 00:49:47,900 So let's take a look at the answer here. 849 00:49:47,900 --> 00:49:50,360 So there's the answer. 850 00:49:50,360 --> 00:49:52,930 So this had two significant figures, 851 00:49:52,930 --> 00:49:55,190 which means you get two significant figures 852 00:49:55,190 --> 00:49:56,912 after the decimal point. 853 00:49:56,912 --> 00:49:58,870 So maybe we'll have one of these in the clicker 854 00:49:58,870 --> 00:50:05,250 competition on Friday, and we'll do our weak bases next time. 855 00:50:05,250 --> 00:50:08,716 So again, we've been studying acid base. 856 00:50:11,670 --> 00:50:17,940 And there are only five types of acid-base problems, weak acid 857 00:50:17,940 --> 00:50:22,510 in water, which we already did, check, weak basin water, which 858 00:50:22,510 --> 00:50:24,980 we're going to do right now, and as soon as we're 859 00:50:24,980 --> 00:50:27,580 done doing week base in water, I'm 860 00:50:27,580 --> 00:50:30,470 going to explain to you how salt and water are just 861 00:50:30,470 --> 00:50:33,300 weak acid in water or weak base in water problems. 862 00:50:33,300 --> 00:50:35,800 So you already know how to do them as soon as they teach you 863 00:50:35,800 --> 00:50:37,680 about weak bases. 864 00:50:37,680 --> 00:50:40,820 And then we're going to move on and do buffers. 865 00:50:40,820 --> 00:50:42,510 And then next week we're going to do 866 00:50:42,510 --> 00:50:48,090 strong acids and strong bases, and then you'll have all five. 867 00:50:48,090 --> 00:50:52,820 You already have enough that after today's lecture, 868 00:50:52,820 --> 00:50:55,040 you can do, I think, all the problems 869 00:50:55,040 --> 00:50:57,720 set 7 except for the last two questions, 870 00:50:57,720 --> 00:51:01,030 I believe, or at least a large fraction of problem set 7. 871 00:51:01,030 --> 00:51:07,350 I will warn you that acid-base problems take a lot of writing 872 00:51:07,350 --> 00:51:09,290 and a lot of time to work. 873 00:51:09,290 --> 00:51:14,310 One problem often has five or six parts to it, 874 00:51:14,310 --> 00:51:16,970 and they're all time-consuming part pretty much. 875 00:51:16,970 --> 00:51:20,450 So don't leave problem set 7 to the last minute. 876 00:51:20,450 --> 00:51:23,716 That would be a mistake on this particular problem set. 877 00:51:27,460 --> 00:51:29,720 Weak bases. 878 00:51:29,720 --> 00:51:33,910 Acid-base type problem 2. 879 00:51:33,910 --> 00:51:41,370 So in this example, we have ammonia, NH3, in water, 880 00:51:41,370 --> 00:51:44,880 and we measured the pH of some solutions 881 00:51:44,880 --> 00:51:47,100 before and saw they were basic. 882 00:51:47,100 --> 00:51:49,800 We had a pH of, I think, 12 last time. 883 00:51:49,800 --> 00:51:53,780 And so this conjugate base goes to a conjugate acid. 884 00:51:53,780 --> 00:51:57,550 So the base accepts a hydrogen ion or proton from water, 885 00:51:57,550 --> 00:52:00,470 becomes NH4 plus, and the water that 886 00:52:00,470 --> 00:52:03,880 lost its hydrogen ion becomes OH minus, 887 00:52:03,880 --> 00:52:06,220 and that is a base in water problem 888 00:52:06,220 --> 00:52:09,690 because you're forming hydroxide ions. 889 00:52:09,690 --> 00:52:13,730 For a base in water, you're talking about the equilibrium 890 00:52:13,730 --> 00:52:16,720 constant for that base in water, which has a special name 891 00:52:16,720 --> 00:52:19,170 KB, B for base. 892 00:52:19,170 --> 00:52:22,520 And it's important to remember the A's and B's 893 00:52:22,520 --> 00:52:25,310 on the equilibrium constant because it's always 894 00:52:25,310 --> 00:52:27,220 checking your work. 895 00:52:27,220 --> 00:52:28,830 Is this a base in water problem. 896 00:52:28,830 --> 00:52:32,380 People sometimes try to apply Ka's when they should be doing 897 00:52:32,380 --> 00:52:33,020 Kb's. 898 00:52:33,020 --> 00:52:35,670 So pay attention to basin water. 899 00:52:35,670 --> 00:52:37,930 We're thinking about Kb. 900 00:52:37,930 --> 00:52:41,170 So here if we're going to calculate the pH of a solution, 901 00:52:41,170 --> 00:52:45,310 we have molarity of 0.15, and room temperature, 902 00:52:45,310 --> 00:52:47,500 pretty much everything is at room temperature. 903 00:52:47,500 --> 00:52:49,160 So whenever you're asked to calculate 904 00:52:49,160 --> 00:52:51,460 the pH of a weak base in water, you 905 00:52:51,460 --> 00:52:54,580 want to think about at equilibrium, what's 906 00:52:54,580 --> 00:52:57,610 the condition now, what will be the condition, what's 907 00:52:57,610 --> 00:53:00,790 the change, and what will be the condition at equilibrium. 908 00:53:00,790 --> 00:53:03,050 And so you can make this table. 909 00:53:03,050 --> 00:53:04,820 You can write out the expression. 910 00:53:04,820 --> 00:53:06,500 Again, you can forget about water. 911 00:53:06,500 --> 00:53:07,730 Water is our solvent. 912 00:53:07,730 --> 00:53:10,270 It's not going to be showing up. 913 00:53:10,270 --> 00:53:14,020 And then put in our initial molarity, 0.15. 914 00:53:14,020 --> 00:53:17,030 And we put zeros in the other categories. 915 00:53:17,030 --> 00:53:20,313 And why don't you try the rest? 916 00:53:44,930 --> 00:53:46,728 Is it slowing down? 917 00:53:46,728 --> 00:53:49,352 SAM: [INAUDIBLE] answers are really quick. 918 00:53:49,352 --> 00:53:50,560 CATHERINE DRENNAN: All right. 919 00:53:50,560 --> 00:53:51,835 Let's just do 10 more seconds. 920 00:54:05,981 --> 00:54:06,480 Yup. 921 00:54:06,480 --> 00:54:07,970 Excellent. 922 00:54:07,970 --> 00:54:11,990 Sam told me everyone responded really quickly. 923 00:54:11,990 --> 00:54:14,230 So that is, in fact, what you're going to do. 924 00:54:14,230 --> 00:54:16,890 We'll put that up here as well. 925 00:54:16,890 --> 00:54:21,190 So we lose some of this minus x. 926 00:54:21,190 --> 00:54:24,540 So at equilibrium, we have point 1.5 minus x. 927 00:54:24,540 --> 00:54:30,560 And we get plus x here and plus x there. 928 00:54:30,560 --> 00:54:35,570 Now, we can write our Kb for our base in water, its products, 929 00:54:35,570 --> 00:54:40,150 the conjugate acid, NH4 plus, times the concentration 930 00:54:40,150 --> 00:54:43,190 of hydroxide ions over NH3. 931 00:54:43,190 --> 00:54:44,710 Again, water is the solvent. 932 00:54:44,710 --> 00:54:46,950 It's not in the equation. 933 00:54:46,950 --> 00:54:50,380 And then we can write it out-- this is x. 934 00:54:50,380 --> 00:54:53,240 That's x, so we have x squared. 935 00:54:53,240 --> 00:54:56,740 And then the change in the amount of the weak base, 936 00:54:56,740 --> 00:55:00,880 when it's in water, is 0.15 minus x. 937 00:55:00,880 --> 00:55:05,350 So at this point, you can either use the quadratic equation, 938 00:55:05,350 --> 00:55:08,830 or you can make an assumption that x is going 939 00:55:08,830 --> 00:55:11,370 to be small compared to 0.5. 940 00:55:11,370 --> 00:55:13,650 We don't want to make the assumption up here. 941 00:55:13,650 --> 00:55:17,060 We want to get to this point before we try the assumption, 942 00:55:17,060 --> 00:55:19,770 and you always need to check your assumptions 943 00:55:19,770 --> 00:55:22,900 to make sure they are correct. 944 00:55:22,900 --> 00:55:27,040 But if we use the assumption here just to simplify the math. 945 00:55:27,040 --> 00:55:29,690 And so the assumption is that this x is small, 946 00:55:29,690 --> 00:55:33,530 so we can drop that x minus x out, 947 00:55:33,530 --> 00:55:41,270 and then we can calculate that x is 0.00164. 948 00:55:41,270 --> 00:55:44,870 And then we can check the assumption, 949 00:55:44,870 --> 00:55:46,847 and that is another clicker question. 950 00:56:35,011 --> 00:56:35,510 All right. 951 00:56:35,510 --> 00:56:36,450 10 more seconds. 952 00:56:52,200 --> 00:56:53,950 All right. 953 00:56:53,950 --> 00:56:56,010 It is number 1. 954 00:56:56,010 --> 00:56:58,760 So here you're checking the assumption and the way 955 00:56:58,760 --> 00:57:02,500 that you check the assumption is that you put x that you've 956 00:57:02,500 --> 00:57:07,410 calculated by simplifying it, divide it by the number 957 00:57:07,410 --> 00:57:11,650 that you're asking is it smaller than, and then times 100 958 00:57:11,650 --> 00:57:14,140 because our rule is 5%. 959 00:57:14,140 --> 00:57:18,460 So if this value is greater than 5% of this value, 960 00:57:18,460 --> 00:57:20,300 you need to use the quadratic. 961 00:57:20,300 --> 00:57:23,160 If it's less than 5%, then you can go ahead 962 00:57:23,160 --> 00:57:24,790 and use your assumption. 963 00:57:24,790 --> 00:57:26,980 And so this is the assumption we're talking about, 964 00:57:26,980 --> 00:57:29,840 that this number is under 5%. 965 00:57:29,840 --> 00:57:37,250 And for weak acids and bases, it very often is less than 5% 966 00:57:37,250 --> 00:57:40,450 because they're weak, so there's not a lot of ionization. 967 00:57:40,450 --> 00:57:45,940 And this is often referred to as percentage ionization as well. 968 00:57:45,940 --> 00:57:46,440 All right. 969 00:57:46,440 --> 00:57:50,130 So our checking assumption, again, 970 00:57:50,130 --> 00:57:58,960 that x 0.00164 divided by 0.15 times 100% is 1%, 971 00:57:58,960 --> 00:58:04,870 1.1, that's less than 5, so the assumption is OK. 972 00:58:04,870 --> 00:58:09,670 And then we can calculate pOH because this 973 00:58:09,670 --> 00:58:13,350 is a weak base in water, so the x that we calculated 974 00:58:13,350 --> 00:58:17,140 is the amount of hydroxide ion in solution. 975 00:58:17,140 --> 00:58:22,050 So we calculate pOH, and we get 2.79. 976 00:58:22,050 --> 00:58:24,000 And that's two significant figures, 977 00:58:24,000 --> 00:58:26,070 everything we had after the decimal 978 00:58:26,070 --> 00:58:29,610 because everything that we had was two significant figures. 979 00:58:29,610 --> 00:58:31,780 And so this is two significant figures, 980 00:58:31,780 --> 00:58:34,660 two significant figures after the decimal point. 981 00:58:34,660 --> 00:58:36,720 And then you need to calculate pH 982 00:58:36,720 --> 00:58:38,550 because the problem asked for pH. 983 00:58:38,550 --> 00:58:40,750 So this is, again, at room temperature. 984 00:58:40,750 --> 00:58:47,490 So we can use 14 minus 2.79 is 11.21. 985 00:58:47,490 --> 00:58:49,420 That's our pH. 986 00:58:49,420 --> 00:58:51,640 And don't make the mistake of stopping here 987 00:58:51,640 --> 00:58:56,690 forgetting that x is hydroxide instead of hydronium ion 988 00:58:56,690 --> 00:59:02,630 and tell me that the pH of my weak base in solution is pH 2. 989 00:59:02,630 --> 00:59:05,977 So one of the things that's really good about this unit 990 00:59:05,977 --> 00:59:08,060 is that there are some good checks that you've got 991 00:59:08,060 --> 00:59:09,830 the right answer at the end. 992 00:59:09,830 --> 00:59:14,110 If you're talking about a base in solution, 993 00:59:14,110 --> 00:59:16,360 you should have a pH above what value? 994 00:59:16,360 --> 00:59:17,306 AUDIENCE: 7. 995 00:59:17,306 --> 00:59:18,690 CATHERINE DRENNAN: About 7. 996 00:59:18,690 --> 00:59:20,980 So it should be on the basic side of neutral. 997 00:59:20,980 --> 00:59:23,890 pH 7 is neutral, so it should be above 7. 998 00:59:23,890 --> 00:59:26,600 And so if this was your answer for pH, 999 00:59:26,600 --> 00:59:28,570 that wouldn't make a lot of sense. 1000 00:59:28,570 --> 00:59:30,050 So always check at the end. 1001 00:59:30,050 --> 00:59:33,430 And it's fine if you get to an exam and you get to the end 1002 00:59:33,430 --> 00:59:35,460 and you did something wrong and you 1003 00:59:35,460 --> 00:59:38,290 realize that your answer doesn't make any sense, if you write, 1004 00:59:38,290 --> 00:59:40,350 this answer doesn't make any sense, 1005 00:59:40,350 --> 00:59:42,730 but I don't have time to figure out what I gave wrong, 1006 00:59:42,730 --> 00:59:46,970 you will get points for recognizing that that is not 1007 00:59:46,970 --> 00:59:48,380 a valid answer. 1008 00:59:48,380 --> 00:59:50,520 Because it's a weak acid problem and you 1009 00:59:50,520 --> 00:59:52,850 have a basic pH or vice versa. 1010 00:59:52,850 --> 00:59:54,005 So keep that in mind. 1011 00:59:54,005 --> 00:59:55,630 There's lots of partial credit. 1012 00:59:55,630 --> 00:59:58,800 Grading acid-base problems on exams 1013 00:59:58,800 --> 01:00:04,770 is a whole new fun adventure that the TAs have no idea what 1014 01:00:04,770 --> 01:00:06,090 is going to be happening. 1015 01:00:06,090 --> 01:00:09,020 But I'll be right with you there for 10 hours of grading, 1016 01:00:09,020 --> 01:00:10,300 and we'll have a good time. 1017 01:00:10,300 --> 01:00:12,520 Write neatly, please.