1 00:00:00,000 --> 00:00:00,104 The following content is provided under a Creative 2 00:00:00,104 --> 00:00:00,143 Commons license. 3 00:00:00,143 --> 00:00:00,247 Your support will help MIT OpenCourseWare continue to 4 00:00:00,247 --> 00:00:00,351 offer high quality educational resources for free. 5 00:00:00,351 --> 00:00:00,468 To make a donation or view additional materials from 6 00:00:00,468 --> 00:00:00,572 hundreds of MIT courses, visit MIT OpenCourseWare at 7 00:00:00,572 --> 00:00:00,600 ocw.mit.edu. 8 00:00:00,600 --> 00:00:21,800 PROFESSOR: -- 9 00:00:21,800 --> 00:00:23,760 All the things would be relevant, so here are some 10 00:00:23,760 --> 00:00:25,870 examples from a couple of lectures ago. 11 00:00:25,870 --> 00:00:29,035 And so here we're thinking about what happens when we add 12 00:00:29,035 --> 00:00:30,330 an inert gas. 13 00:00:30,330 --> 00:00:33,040 And remember, it's all about the partial pressure. 14 00:00:33,040 --> 00:00:35,290 So, you always have to ask yourself did the partial 15 00:00:35,290 --> 00:00:38,290 pressure change, and partial pressure's going to change if 16 00:00:38,290 --> 00:00:39,740 there's a change in volume. 17 00:00:39,740 --> 00:00:44,115 So, the secret in this problem is realizing that if inert gas 18 00:00:44,115 --> 00:00:48,060 is added, and the total pressure is kept constant, 19 00:00:48,060 --> 00:00:51,980 what had to have happened? 20 00:00:51,980 --> 00:00:55,660 Yup, the volume would have had to increase, and so the system 21 00:00:55,660 --> 00:00:58,950 is put at stress, and it responds in a way to minimize 22 00:00:58,950 --> 00:00:59,880 the stress. 23 00:00:59,880 --> 00:01:03,330 So it's going to respond in a way to go from fewer numbers 24 00:01:03,330 --> 00:01:05,700 of molecules to more molecules. 25 00:01:05,700 --> 00:01:08,070 So, on one side of the equation reactants there are 26 00:01:08,070 --> 00:01:11,990 three, and on products there are two, so it's going to 27 00:01:11,990 --> 00:01:14,930 shift toward reactants. 28 00:01:14,930 --> 00:01:15,350 All right. 29 00:01:15,350 --> 00:01:19,420 So today we're going to have another clicker competition, 30 00:01:19,420 --> 00:01:23,680 and because it is Halloween, this is the prize for the 31 00:01:23,680 --> 00:01:27,450 recitation that has the most correct answers. 32 00:01:27,450 --> 00:01:39,800 So, let's see how we do today. 33 00:01:39,800 --> 00:01:42,830 All right. 34 00:01:42,830 --> 00:01:46,890 So we're going to continue where we left off on 35 00:01:46,890 --> 00:01:50,570 Wednesday, and so these are your notes -- 36 00:01:50,570 --> 00:01:53,020 I've added them to today's handout and they're also in 37 00:01:53,020 --> 00:01:55,870 the handout from the last class. 38 00:01:55,870 --> 00:01:59,220 And I want to make a note that it's a good idea to start this 39 00:01:59,220 --> 00:02:01,710 problem-set early. 40 00:02:01,710 --> 00:02:03,930 You don't know everything you need to know to do the 41 00:02:03,930 --> 00:02:06,700 problem-set, but you do know a number of them, so there's 42 00:02:06,700 --> 00:02:08,340 some questions on thermodynamics and 43 00:02:08,340 --> 00:02:11,710 equilibrium, and Le Chatelier's principle, you can 44 00:02:11,710 --> 00:02:15,740 do all those problems. So next week -- today we're going to 45 00:02:15,740 --> 00:02:21,240 talk about bases and buffers, and then we're going to move 46 00:02:21,240 --> 00:02:24,120 into acid-base titrations on Monday. 47 00:02:24,120 --> 00:02:29,530 And so, the problem-set looks like it's not that long, it's 48 00:02:29,530 --> 00:02:32,910 not that many questions, but the acid-base titrations have 49 00:02:32,910 --> 00:02:36,830 many parts, and each part is actually quite long. 50 00:02:36,830 --> 00:02:39,720 So it's a very deceptive problem-set. 51 00:02:39,720 --> 00:02:44,090 So don't be fooled by the total number of questions. 52 00:02:44,090 --> 00:02:47,460 All right, so we were talking about acid and water and base 53 00:02:47,460 --> 00:02:50,700 and water and p h, and so we're going to continue with 54 00:02:50,700 --> 00:02:53,150 base and water right now. 55 00:02:53,150 --> 00:02:58,740 So here we have a base in water, and so in this case, 56 00:02:58,740 --> 00:03:00,810 the water is acting as an acid. 57 00:03:00,810 --> 00:03:05,590 It's giving up a hydrogen ion or proton to the n h 3, 58 00:03:05,590 --> 00:03:08,070 causing it to form its conjugate, n h 4 plus, ammonia 59 00:03:08,070 --> 00:03:12,550 ion, and also hydroxide ion. 60 00:03:12,550 --> 00:03:18,460 So here we have a base in water, and when we're talking 61 00:03:18,460 --> 00:03:22,650 about a base in water, we're going to talk about base 62 00:03:22,650 --> 00:03:25,870 ionization constant, or k b. 63 00:03:25,870 --> 00:03:28,960 So at the end of last class we talked about the acid 64 00:03:28,960 --> 00:03:32,650 ionization constant, or k a, and when you're talking about 65 00:03:32,650 --> 00:03:35,750 bases, you're going to talk about k b's. 66 00:03:35,750 --> 00:03:40,220 So, k b, it's an equilibrium constant for this reaction of 67 00:03:40,220 --> 00:03:45,010 a base in water, and so it'll be equal to the products in h 68 00:03:45,010 --> 00:03:50,890 4 plus and hydroxide ion over the reactant, n h 3. 69 00:03:50,890 --> 00:03:54,040 The water is the solvent here, and since this is all pretty 70 00:03:54,040 --> 00:03:56,985 dilute, it's mostly pure and its concentration isn't going 71 00:03:56,985 --> 00:03:58,930 to change, so it's not included in 72 00:03:58,930 --> 00:04:01,270 that equilibrium constant. 73 00:04:01,270 --> 00:04:05,240 So we have a k b when we're talking about a base in water. 74 00:04:05,240 --> 00:04:07,910 And we were talking about a base in water, the equation 75 00:04:07,910 --> 00:04:09,340 should work that you have hydroxide ions 76 00:04:09,340 --> 00:04:13,090 on one side of it. 77 00:04:13,090 --> 00:04:15,640 So here, the k b is 1 . 78 00:04:15,640 --> 00:04:20,780 8 times 10 to the minus 5 at 25 degrees, so that's a fairly 79 00:04:20,780 --> 00:04:22,810 small number. 80 00:04:22,810 --> 00:04:27,480 And so the small value tells us that only a little bit of 81 00:04:27,480 --> 00:04:30,940 the n h 3 is going to ionize when it's in solution. 82 00:04:30,940 --> 00:04:34,580 So only a little bit it's going to form n h 4 plus and 83 00:04:34,580 --> 00:04:36,490 hydroxide ions. 84 00:04:36,490 --> 00:04:39,240 So that's what that small number tells us. 85 00:04:39,240 --> 00:04:42,650 So that tells us that it's going to be a weak base. 86 00:04:42,650 --> 00:04:45,470 So a strong base is something that's going to react pretty 87 00:04:45,470 --> 00:04:50,210 much completely to go to hydroxide ion concentration. 88 00:04:50,210 --> 00:04:54,980 A weak base only ionizes a little bit in water. 89 00:04:54,980 --> 00:04:58,060 And you can tell about whether something's strong or weak by 90 00:04:58,060 --> 00:05:01,750 its k b value, or if it's an acid, it's k a 91 00:05:01,750 --> 00:05:05,700 value or p k a value. 92 00:05:05,700 --> 00:05:10,080 So here are some general ways to write these equations. 93 00:05:10,080 --> 00:05:13,830 We have a base in water and so the water's going to act as 94 00:05:13,830 --> 00:05:17,200 the acid, base is going to accept that proton or hydrogen 95 00:05:17,200 --> 00:05:20,790 ion forming a base, h plus, and hydroxide ion. 96 00:05:20,790 --> 00:05:24,180 So the base is just written as b. 97 00:05:24,180 --> 00:05:27,540 You could also write the base as a minus, something a minus 98 00:05:27,540 --> 00:05:31,480 in water going to h a plus hydroxide ion. 99 00:05:31,480 --> 00:05:33,960 So sometimes you might see that when you're talking about 100 00:05:33,960 --> 00:05:36,290 a conjugate base of a weak acid. 101 00:05:36,290 --> 00:05:38,980 So these are two expressions that you'll see that are 102 00:05:38,980 --> 00:05:42,280 fairly generic that expresses what happens when you have 103 00:05:42,280 --> 00:05:43,490 bases in water. 104 00:05:43,490 --> 00:05:45,740 Now remember, you know it's a base in water, you better have 105 00:05:45,740 --> 00:05:49,280 hydroxide ions on the other side, because a base in water 106 00:05:49,280 --> 00:05:53,080 is going to be forming hydroxide ions, and acid in 107 00:05:53,080 --> 00:05:54,750 water would be forming hydronium ions. 108 00:05:54,750 --> 00:06:01,530 So, a strong base, again, almost completely ionizes to o 109 00:06:01,530 --> 00:06:06,000 h when in water, and here we can know what's strong or weak 110 00:06:06,000 --> 00:06:10,060 by the k b, so the larger the k b the stronger the base. 111 00:06:10,060 --> 00:06:14,930 And like there is the p k a, there's also a term p k b. 112 00:06:14,930 --> 00:06:18,070 P k b is minus log of the k b. 113 00:06:18,070 --> 00:06:21,060 And the larger the p k b, the weaker the base. 114 00:06:21,060 --> 00:06:23,530 Now you won't see p k b very much. 115 00:06:23,530 --> 00:06:25,930 It's not used very much, most things are 116 00:06:25,930 --> 00:06:27,880 converted to a p k a. 117 00:06:27,880 --> 00:06:32,900 So you'll see p k a's quite a bit, and you will see p k a's 118 00:06:32,900 --> 00:06:36,630 if you take organic chemistry, if you take biochemistry, if 119 00:06:36,630 --> 00:06:41,010 you take biology, you'll be hearing a lot about p k a's as 120 00:06:41,010 --> 00:06:41,830 we go along. 121 00:06:41,830 --> 00:06:45,060 Not so much about p k a's, but p k b's. 122 00:06:45,060 --> 00:06:49,110 And so what I want you to do when you hear about p k a's, 123 00:06:49,110 --> 00:06:51,510 is remember that you've learned about it. 124 00:06:51,510 --> 00:06:54,470 Because I have been confronted by some of my colleagues who 125 00:06:54,470 --> 00:06:58,050 teach in advanced levels, and they said our students tell us 126 00:06:58,050 --> 00:07:03,050 that you never talked about p k a's in freshman chemistry. 127 00:07:03,050 --> 00:07:04,910 And I assure them that I did. 128 00:07:04,910 --> 00:07:06,500 So I'll be emphasizing this. 129 00:07:06,500 --> 00:07:09,510 And so I want to make sure that by the end of this unit 130 00:07:09,510 --> 00:07:12,020 you're really familiar with p k a's because you'll need them 131 00:07:12,020 --> 00:07:15,940 later on, and I want you to really impress my colleagues 132 00:07:15,940 --> 00:07:19,350 in later classes, and they'll say oh, that's a 511-1 133 00:07:19,350 --> 00:07:22,090 student, of course, they know what a p k a is. 134 00:07:22,090 --> 00:07:25,460 Even my six month old daughter who's over there, she's like 135 00:07:25,460 --> 00:07:27,730 what, people didn't know what a p k a is? 136 00:07:27,730 --> 00:07:34,080 You don't want to get her upset. 137 00:07:34,080 --> 00:07:37,650 OK. 138 00:07:37,650 --> 00:07:40,110 So all of these things are related with the acids and the 139 00:07:40,110 --> 00:07:44,470 bases, because for every acid it has a conjugate base, every 140 00:07:44,470 --> 00:07:46,520 base is a conjugate acid. 141 00:07:46,520 --> 00:07:50,350 And so, if you have a stronger acid, the stronger the acid, 142 00:07:50,350 --> 00:07:51,620 the weaker its conjugate base. 143 00:07:51,620 --> 00:07:56,620 And the stronger the base, the weaker its conjugate acid. 144 00:07:56,620 --> 00:07:59,580 And this becomes very important in doing these 145 00:07:59,580 --> 00:08:03,910 problems. So here's a little table that 146 00:08:03,910 --> 00:08:05,900 emphasizes that fact. 147 00:08:05,900 --> 00:08:08,080 So we talk about a strong acid. 148 00:08:08,080 --> 00:08:11,840 Most people are familiar with h c l, hydrochloric acid. 149 00:08:11,840 --> 00:08:14,610 So it's a very strong acid. 150 00:08:14,610 --> 00:08:18,540 And its conjugate base, c l minus, is not really a base, 151 00:08:18,540 --> 00:08:20,960 it's completely ineffectual as being a base. 152 00:08:20,960 --> 00:08:22,630 It doesn't really do anything at all. 153 00:08:22,630 --> 00:08:25,720 A strong acid really drives you all the way to hydronium 154 00:08:25,720 --> 00:08:28,030 ion concentrations. 155 00:08:28,030 --> 00:08:30,020 It doesn't go back the other way, it's not really 156 00:08:30,020 --> 00:08:32,660 equilibrium, it's just going to completion there. 157 00:08:32,660 --> 00:08:36,660 So the conjugate is really, really weak, basically not a 158 00:08:36,660 --> 00:08:37,790 base at all. 159 00:08:37,790 --> 00:08:40,860 Then we get into this middle range and here things that are 160 00:08:40,860 --> 00:08:44,820 moderately weak or very weak acids also have their 161 00:08:44,820 --> 00:08:47,100 conjugates in the weak range. 162 00:08:47,100 --> 00:08:50,810 But if you get to something that is a very strong base 163 00:08:50,810 --> 00:08:54,560 down here, its conjugate is going to be also ineffective 164 00:08:54,560 --> 00:08:55,700 as an acid. 165 00:08:55,700 --> 00:08:58,980 So if something is very strong, its conjugate is 166 00:08:58,980 --> 00:09:02,170 pretty much non-existent in those properties, but when you 167 00:09:02,170 --> 00:09:07,390 have weak-weak, then you can start talking about buffers, 168 00:09:07,390 --> 00:09:11,470 which we're going to get into later in today's class. 169 00:09:11,470 --> 00:09:12,220 OK. 170 00:09:12,220 --> 00:09:18,230 So let's prove that, in fact, it has to be true that there's 171 00:09:18,230 --> 00:09:20,560 a relationship between the conjugate acid and its 172 00:09:20,560 --> 00:09:23,270 conjugate base or conjugate base and its conjugate acid, 173 00:09:23,270 --> 00:09:26,730 that they both can't be strong. 174 00:09:26,730 --> 00:09:31,080 One has to be -- you have to be weak-weak, strong or 175 00:09:31,080 --> 00:09:32,320 ineffectual. 176 00:09:32,320 --> 00:09:37,380 All right, so let's look at the first one up here. 177 00:09:37,380 --> 00:09:41,400 So first let's look at what is this acting as? 178 00:09:41,400 --> 00:09:43,730 What is n h 3 acting as, an acid or a 179 00:09:43,730 --> 00:09:45,950 base in this equation? 180 00:09:45,950 --> 00:09:48,750 So it's acting as a base. 181 00:09:48,750 --> 00:09:52,150 And that means water is acting as an acid. 182 00:09:52,150 --> 00:09:57,190 The water gives up a proton or hydrogen ion to the n h 3 183 00:09:57,190 --> 00:10:02,630 forming the conjugate acid of that base. 184 00:10:02,630 --> 00:10:06,890 And then the conjugate base is the hydroxide ion. 185 00:10:06,890 --> 00:10:14,390 All right, so now let's write term for k, and so we're 186 00:10:14,390 --> 00:10:20,120 talking about a base in water, so we're talking about k b. 187 00:10:20,120 --> 00:10:29,420 So k b is going to equal what -- what do I put up here? 188 00:10:29,420 --> 00:10:32,020 Tell me one thing to put up there. 189 00:10:32,020 --> 00:10:33,490 Yup. 190 00:10:33,490 --> 00:10:46,350 OK, n h 4 plus, and hydroxide ion over n h 3. 191 00:10:46,350 --> 00:10:49,720 OK, so we don't have water in there. 192 00:10:49,720 --> 00:11:02,540 All right, so let's look at the next reaction. 193 00:11:02,540 --> 00:11:05,850 So what is n h 4 plus acting as? 194 00:11:05,850 --> 00:11:11,060 It's acting as an acid here, so it's giving up its proton 195 00:11:11,060 --> 00:11:14,640 or hydrogen ion to the water, which is going to act as a 196 00:11:14,640 --> 00:11:17,170 base and accept that hydrogen atom. 197 00:11:17,170 --> 00:11:21,790 And when this gives up its hydrogen ion or proton, it 198 00:11:21,790 --> 00:11:27,380 forms its conjugate base, and the water is conjugate here, 199 00:11:27,380 --> 00:11:28,960 is an acid, hydronium ion. 200 00:11:28,960 --> 00:11:34,320 So we have our conjugate acid base pairs here. 201 00:11:34,320 --> 00:11:38,710 So now am I talking about a k a or a k b? 202 00:11:38,710 --> 00:11:42,550 I'm talking about a k a, so I'm talking about an acid in 203 00:11:42,550 --> 00:11:45,640 water, and we know this is an acid in water if we look at 204 00:11:45,640 --> 00:11:48,100 what's happening over here. 205 00:11:48,100 --> 00:11:51,900 So we have an acid in water, and so we'll put our 206 00:11:51,900 --> 00:11:54,280 concentration of hydronium ions and n h 207 00:11:54,280 --> 00:12:04,470 3 over n h 4 plus. 208 00:12:04,470 --> 00:12:08,290 So now we have k a's and k b's written for the conjugates, 209 00:12:08,290 --> 00:12:15,480 the conjugate acid of n h 3 and n h 3 itself. 210 00:12:15,480 --> 00:12:21,860 So now, we can think about what happens if we take these 211 00:12:21,860 --> 00:12:27,600 k's and we multiply them out together. 212 00:12:27,600 --> 00:12:39,300 All right, so we have a k a and we have a k b. 213 00:12:39,300 --> 00:12:43,200 All right, so we have a k a and a k b, so if we take our k 214 00:12:43,200 --> 00:12:48,280 a and times our k b, we're going to just multiply these 215 00:12:48,280 --> 00:12:50,500 out together. 216 00:12:50,500 --> 00:13:04,330 So I'll do this one first, k a -- just copy from above -- 217 00:13:04,330 --> 00:13:18,310 times k b, hydroxide ion, over n h 3. 218 00:13:18,310 --> 00:13:28,180 And things are going to cancel out, and so I'm left with 219 00:13:28,180 --> 00:13:34,400 hydronium ion and hydroxide ion. 220 00:13:34,400 --> 00:13:36,340 What is this, when you have hydronium ion 221 00:13:36,340 --> 00:13:37,790 times hydroxide ion? 222 00:13:37,790 --> 00:13:39,700 What is that called? 223 00:13:39,700 --> 00:13:41,850 It's another k. 224 00:13:41,850 --> 00:13:44,280 K w. 225 00:13:44,280 --> 00:13:52,780 So we just showed that k a times k b equals k w. 226 00:13:52,780 --> 00:13:58,220 So we can take the logs of all our k terms here, and if we 227 00:13:58,220 --> 00:14:10,670 take the log of k a plus the log of k b equal the log of k 228 00:14:10,670 --> 00:14:33,240 w, or p k a plus p k b equals p k w equals 14. 229 00:14:33,240 --> 00:14:36,220 So there's this relationship with a conjugate acid and its 230 00:14:36,220 --> 00:14:39,530 base between its k a and its k b. 231 00:14:39,530 --> 00:14:43,450 So if one is really big, the other has to be small or they 232 00:14:43,450 --> 00:14:45,690 can both be sort of in the middle. 233 00:14:45,690 --> 00:14:48,790 But they're always going to add up in terms of the p k a 234 00:14:48,790 --> 00:14:50,850 and the p k b to 14. 235 00:14:50,850 --> 00:14:54,040 And the thing about these problems is if you're given a 236 00:14:54,040 --> 00:14:59,340 k a for an acid, you can calculate the k b for its 237 00:14:59,340 --> 00:15:02,530 conjugate base, and you'll be doing that a lot in titration 238 00:15:02,530 --> 00:15:05,510 problems that are coming up. 239 00:15:05,510 --> 00:15:08,270 All right, so there's this relationship between the 240 00:15:08,270 --> 00:15:10,890 strength of an acid and the strength of its conjugate 241 00:15:10,890 --> 00:15:14,490 base, and let's just think for a minute again about this 242 00:15:14,490 --> 00:15:17,260 concept of strong and weak, because this is really 243 00:15:17,260 --> 00:15:19,320 important for the next unit. 244 00:15:19,320 --> 00:15:34,430 So if we have a strong acid, h a, in water, it's going to go 245 00:15:34,430 --> 00:15:39,180 pretty much completely over to hydronium ion and the 246 00:15:39,180 --> 00:15:44,470 conjugate, and this conjugate is going to be really 247 00:15:44,470 --> 00:15:49,000 ineffective as a conjugate base, as a base at all. 248 00:15:49,000 --> 00:15:53,620 So it's going to really go all to that hydronium ion 249 00:15:53,620 --> 00:15:54,740 concentration. 250 00:15:54,740 --> 00:15:56,760 And so, in talking about a strong base, you don't really 251 00:15:56,760 --> 00:15:59,390 have to worry about an equilibrium situation. 252 00:15:59,390 --> 00:16:02,540 Just remember it goes pretty much to completion, and so you 253 00:16:02,540 --> 00:16:06,110 can do complete subtractions when you're doing this. 254 00:16:06,110 --> 00:16:09,250 And the same is true for a strong base. 255 00:16:09,250 --> 00:16:14,075 So, for a strong base, any b, in water, it's going all the 256 00:16:14,075 --> 00:16:21,470 way down, it's driving the reaction all the way over 257 00:16:21,470 --> 00:16:24,070 here, and you're forming, you can consider it that however 258 00:16:24,070 --> 00:16:32,740 much strong base you added is how much hydroxide 259 00:16:32,740 --> 00:16:35,500 ion you have here. 260 00:16:35,500 --> 00:16:39,110 How much strong acid you add is equal to how much hydronium 261 00:16:39,110 --> 00:16:41,920 ion concentration you have here. 262 00:16:41,920 --> 00:16:45,300 So however much of a strong acid or a strong base, you 263 00:16:45,300 --> 00:16:48,510 think they go all the way to completion, but for a weak 264 00:16:48,510 --> 00:16:50,920 acid we're going to have equilibrium, and so you'll 265 00:16:50,920 --> 00:16:53,860 have to set up equilibrium tables to figure out if you 266 00:16:53,860 --> 00:16:58,840 added this much weak base, how much did it ionize. 267 00:16:58,840 --> 00:17:02,650 So remember that -- people get worried about the strong acid, 268 00:17:02,650 --> 00:17:05,970 and you just assume it goes right to completion. 269 00:17:05,970 --> 00:17:10,320 And you can tell again by the k a's and the k b's what's 270 00:17:10,320 --> 00:17:11,870 going to be strong or not. 271 00:17:11,870 --> 00:17:16,160 And so our definition for strong acids is that you have 272 00:17:16,160 --> 00:17:20,430 a k a greater than one, strong base pretty much, the only 273 00:17:20,430 --> 00:17:24,010 problems you use, people are adding sodium hydroxide or 274 00:17:24,010 --> 00:17:27,510 potassium hydroxide, there are not a lot of options for 275 00:17:27,510 --> 00:17:28,380 strong bases. 276 00:17:28,380 --> 00:17:31,230 But for strong acids, people are always worrying about 277 00:17:31,230 --> 00:17:36,060 whether they've identified those correctly or not. 278 00:17:36,060 --> 00:17:36,530 OK. 279 00:17:36,530 --> 00:17:42,760 So let's look at this relative strength of acid problem and 280 00:17:42,760 --> 00:17:46,890 do an example here. 281 00:17:46,890 --> 00:17:52,700 So in this equation we have acid in a base going on one 282 00:17:52,700 --> 00:17:55,690 side, another different acid on the other side. 283 00:17:55,690 --> 00:18:00,100 So we can look at whether the reaction is favored toward 284 00:18:00,100 --> 00:18:02,890 this direction, toward the right or the left, depending 285 00:18:02,890 --> 00:18:05,230 on which acid is stronger. 286 00:18:05,230 --> 00:18:07,500 Will the reaction ride to the right or left. 287 00:18:07,500 --> 00:18:11,700 So if this acid is stronger, then it should drive the 288 00:18:11,700 --> 00:18:12,940 reaction this way. 289 00:18:12,940 --> 00:18:16,310 If this is the acid on the other side, if it's stronger, 290 00:18:16,310 --> 00:18:19,640 then you would expect to drive the reaction the other way. 291 00:18:19,640 --> 00:18:23,390 So we can take a look at that. 292 00:18:23,390 --> 00:18:27,250 So we can consider the k for the overall reaction, again, 293 00:18:27,250 --> 00:18:30,390 just products over reactants here. 294 00:18:30,390 --> 00:18:34,960 And we can also consider the reaction from just each acid 295 00:18:34,960 --> 00:18:38,120 alone in water. 296 00:18:38,120 --> 00:18:41,680 So we can consider it separately as well. 297 00:18:41,680 --> 00:18:45,230 So first we can take a look at one acid. 298 00:18:45,230 --> 00:18:48,530 So if we take a look at this acid alone in water, it'll 299 00:18:48,530 --> 00:18:53,500 form h 3 o plus and n h 3 minus. 300 00:18:53,500 --> 00:18:57,490 So when it gives up its hydrogen ion to the water and 301 00:18:57,490 --> 00:19:00,650 then it forms its conjugate over here. 302 00:19:00,650 --> 00:19:07,510 So now are we going to look at a k a or a k b? 303 00:19:07,510 --> 00:19:07,800 K a? 304 00:19:07,800 --> 00:19:11,200 So we're going to have our products over our reactants, 305 00:19:11,200 --> 00:19:16,450 and the number is quite large, 20, for that k a. 306 00:19:16,450 --> 00:19:21,020 And now we can look at the other reaction as well. 307 00:19:21,020 --> 00:19:25,420 So we can look at this acid in water here and form your 308 00:19:25,420 --> 00:19:29,710 hydronium ion concentrations and your conjugates -- again, 309 00:19:29,710 --> 00:19:33,860 we're looking at that acid in water, so it's a k a, and we 310 00:19:33,860 --> 00:19:35,870 have our products over reactants. 311 00:19:35,870 --> 00:19:37,640 Now we have a number of 5 . 312 00:19:37,640 --> 00:19:42,730 6 times 10 to the minus 10. 313 00:19:42,730 --> 00:19:48,860 So, we can consider those two equations back together, and 314 00:19:48,860 --> 00:19:51,490 this time we're going to be subtracting the equations from 315 00:19:51,490 --> 00:19:54,230 each other to get our sum equation. 316 00:19:54,230 --> 00:19:56,720 And because we are subtracting, we're going to 317 00:19:56,720 --> 00:19:59,430 end up to dividing the equilibrium constants. 318 00:19:59,430 --> 00:20:02,550 So when we add the equilibrium constants together, we 319 00:20:02,550 --> 00:20:05,900 multiply things, and if we're subtracting we divide. 320 00:20:05,900 --> 00:20:10,190 So the k in this case is going to be equal to the k a of the 321 00:20:10,190 --> 00:20:13,780 first acid over the second acid, and you can prove this 322 00:20:13,780 --> 00:20:17,980 to yourself, you write up the k a here, and the k a here, 323 00:20:17,980 --> 00:20:22,160 and then some of your terms, the hydronium ions cancel, and 324 00:20:22,160 --> 00:20:24,940 you get the k overall equilibrium constant that we 325 00:20:24,940 --> 00:20:28,040 wrote in the beginning, again products over reactants. 326 00:20:28,040 --> 00:20:31,890 We know the value for k a for one acid, we know the value of 327 00:20:31,890 --> 00:20:35,780 k a for the other acid, and we can divide those to get the k 328 00:20:35,780 --> 00:20:37,940 for the overall reaction. 329 00:20:37,940 --> 00:20:42,000 And then you can tell me what that k means in terms of which 330 00:20:42,000 --> 00:20:47,960 is the stronger acid of the two, and which side of the 331 00:20:47,960 --> 00:20:51,940 equation does the reaction lie to, the right or the left? 332 00:20:51,940 --> 00:21:18,440 All right, let's give 10 more seconds. 333 00:21:18,440 --> 00:21:35,270 Yup, people did pretty well on this. 334 00:21:35,270 --> 00:21:37,860 So you could have thought about it in terms of the 335 00:21:37,860 --> 00:21:41,720 overall k or of the individual k a's. 336 00:21:41,720 --> 00:21:45,320 So, the stronger acid is the one with the larger number. 337 00:21:45,320 --> 00:21:49,265 And h n o 3 had a number of 20, so that was pretty big, so 338 00:21:49,265 --> 00:21:52,030 that's a really strong acid. 339 00:21:52,030 --> 00:21:55,530 And because it's a strong acid, it'll lie to the right, 340 00:21:55,530 --> 00:21:59,100 so it's going to push toward products -- it's a strong 341 00:21:59,100 --> 00:22:02,560 acid, so it wants to disassociate a lot, so would 342 00:22:02,560 --> 00:22:04,750 push it that direction. 343 00:22:04,750 --> 00:22:07,830 And you can see that also in terms of the equilibrium. 344 00:22:07,830 --> 00:22:11,690 Overall equilibrium constant, if we go back to the slides 345 00:22:11,690 --> 00:22:12,480 for a minute. 346 00:22:12,480 --> 00:22:17,760 So this number overall, k, is also quite large, very large, 347 00:22:17,760 --> 00:22:21,340 so that means a lot more products than reactants at 348 00:22:21,340 --> 00:22:23,960 equilibrium. 349 00:22:23,960 --> 00:22:27,740 So again, those are what you can determine if you're given 350 00:22:27,740 --> 00:22:32,190 a table of k a values, which on the test you will get a 351 00:22:32,190 --> 00:22:35,780 table of k a values, you can tell me a lot about different 352 00:22:35,780 --> 00:22:39,200 reactions knowing that information about k a's. 353 00:22:39,200 --> 00:22:43,670 All right. 354 00:22:43,670 --> 00:22:50,050 So in this unit there are different types of acid base 355 00:22:50,050 --> 00:22:54,090 problems, and sometimes it feels for people like there's 356 00:22:54,090 --> 00:22:56,950 an infinite number of different types of acid base 357 00:22:56,950 --> 00:23:00,900 problems. But, in fact, there are really only five. 358 00:23:00,900 --> 00:23:03,690 And so one of the things I strongly recommend in this 359 00:23:03,690 --> 00:23:07,990 unit and working problems is figuring out which type of 360 00:23:07,990 --> 00:23:12,060 problem it is, and that will help you a lot in solving it. 361 00:23:12,060 --> 00:23:15,280 So you can either have a weak acid in water, a weak base in 362 00:23:15,280 --> 00:23:18,530 water, and sometimes you can be fooled and say oh, it's a 363 00:23:18,530 --> 00:23:21,830 salt and water problem, but a salt and water problem 364 00:23:21,830 --> 00:23:25,130 actually breaks down to a weak acid and water problem and a 365 00:23:25,130 --> 00:23:26,450 weak base and water problem. 366 00:23:26,450 --> 00:23:29,270 So it's really not a different kind of problem, and we'll see 367 00:23:29,270 --> 00:23:30,390 that in a few minutes. 368 00:23:30,390 --> 00:23:33,420 And you can have a strong acid in water and a strong base in 369 00:23:33,420 --> 00:23:37,560 water, and then you can have my good friend the buffer type 370 00:23:37,560 --> 00:23:38,510 of problem. 371 00:23:38,510 --> 00:23:40,460 So those are the type of problems and being able to 372 00:23:40,460 --> 00:23:44,200 recognize them is key to doing well in this unit. 373 00:23:44,200 --> 00:23:48,760 So let's work a problem in the first type of a weak acid in 374 00:23:48,760 --> 00:23:50,670 water problem. 375 00:23:50,670 --> 00:23:52,220 All right, so what's a weak acid? 376 00:23:52,220 --> 00:23:55,800 Well, vitamin C is a weak acid. 377 00:23:55,800 --> 00:23:58,760 And so sometimes when you're taking your vitamins you get a 378 00:23:58,760 --> 00:24:00,850 bad taste in your mouth. 379 00:24:00,850 --> 00:24:07,220 And if you did take a vitamin C tablet here, which is 500 380 00:24:07,220 --> 00:24:10,690 milligrams in this vitamin C tablet, and dissolved it in 381 00:24:10,690 --> 00:24:13,420 water -- this is not scientifically measured -- but 382 00:24:13,420 --> 00:24:16,410 dissolved it in water, so you would say taking your vitamin 383 00:24:16,410 --> 00:24:19,900 with a lot of water, and it was starting to dissolve and 384 00:24:19,900 --> 00:24:22,980 being pretty unpleasant, then we could calculate at 385 00:24:22,980 --> 00:24:28,400 equilibrium what kind of p h we would have in that mixture. 386 00:24:28,400 --> 00:24:32,900 Now, these vitamins, this Nature's Bounty, they do a 387 00:24:32,900 --> 00:24:36,760 really good job of isolating the vitamin, so it is pretty 388 00:24:36,760 --> 00:24:39,190 much impossible for it to dissolve. 389 00:24:39,190 --> 00:24:41,780 So they have a nice coating around it that's highly 390 00:24:41,780 --> 00:24:47,930 protective, at least at normal water p h's, so it doesn't 391 00:24:47,930 --> 00:24:48,500 really dissolve. 392 00:24:48,500 --> 00:24:50,800 And one year I thought I would do the actual experiment, we 393 00:24:50,800 --> 00:24:53,920 could talk about significant figures, but I could not get 394 00:24:53,920 --> 00:24:55,090 the tablet to dissolve -- 395 00:24:55,090 --> 00:24:59,200 I heated it, I stirred it, I did everything at neutral p h, 396 00:24:59,200 --> 00:25:00,950 at room temperature. 397 00:25:00,950 --> 00:25:02,960 I want to do it at room temperature, it just didn't 398 00:25:02,960 --> 00:25:04,570 work, even the high temperature didn't work. 399 00:25:04,570 --> 00:25:08,840 So if you buy Nature's Bounty, it will not dissolve in the 400 00:25:08,840 --> 00:25:12,330 water that you're taking, so just for that little bit of 401 00:25:12,330 --> 00:25:13,210 information. 402 00:25:13,210 --> 00:25:16,840 But if you had, say, an inferior brand of vitamin C 403 00:25:16,840 --> 00:25:21,380 that readily, that didn't have a nice coating around it, then 404 00:25:21,380 --> 00:25:23,990 you could do this experiment. 405 00:25:23,990 --> 00:25:26,010 So let's take a look at that. 406 00:25:26,010 --> 00:25:28,720 So the first thing that we have to do is calculate the 407 00:25:28,720 --> 00:25:34,130 molarity of the acid that we've added. 408 00:25:34,130 --> 00:25:37,040 So here, just have to make sure that your units are going 409 00:25:37,040 --> 00:25:38,170 to be correct. 410 00:25:38,170 --> 00:25:41,670 So we have grams, we're converting it with a molecular 411 00:25:41,670 --> 00:25:45,570 weight to moles, and then we have the number of moles in 412 00:25:45,570 --> 00:25:49,840 the amount of water, and we can calculate the molarity of 413 00:25:49,840 --> 00:25:51,250 that solution. 414 00:25:51,250 --> 00:25:53,960 And one of the mistakes that people often make in doing 415 00:25:53,960 --> 00:25:56,540 these problems, they forget to do all of the conversions that 416 00:25:56,540 --> 00:26:00,060 are necessary, sometimes they stop at moles, and you're 417 00:26:00,060 --> 00:26:03,100 talking about concentrations here, so don't forget about 418 00:26:03,100 --> 00:26:05,380 your friend the volume. 419 00:26:05,380 --> 00:26:08,170 All right, so then you can write and equation, and I 420 00:26:08,170 --> 00:26:11,570 highly recommend that people do this on the test because it 421 00:26:11,570 --> 00:26:15,360 helps them figure out what type of problem it is and it 422 00:26:15,360 --> 00:26:18,050 voids people making silly mistakes. 423 00:26:18,050 --> 00:26:21,170 So if we're talking about an acid in water, you should make 424 00:26:21,170 --> 00:26:24,670 sure that your equation reflects an acid in water. 425 00:26:24,670 --> 00:26:27,510 If you have hydroxide ion on the other side, something is 426 00:26:27,510 --> 00:26:30,730 very wrong that's going on, and acid and water is going to 427 00:26:30,730 --> 00:26:33,550 be giving hydronium ion concentrations 428 00:26:33,550 --> 00:26:37,710 and a conjugate base. 429 00:26:37,710 --> 00:26:41,160 So then we can set up an equilibrium table here. 430 00:26:41,160 --> 00:26:43,260 We calculated the initial molarity. 431 00:26:43,260 --> 00:26:46,170 And in the beginning there's nothing over on this side, so 432 00:26:46,170 --> 00:26:49,540 we've just added our weak acid to water. 433 00:26:49,540 --> 00:26:52,300 And so then the change, there's going to be some 434 00:26:52,300 --> 00:26:55,880 amount of this that ionizes minus x, some amount of this 435 00:26:55,880 --> 00:26:57,290 that's formed, and some amount out of the 436 00:26:57,290 --> 00:26:59,000 conjugate that's formed. 437 00:26:59,000 --> 00:27:05,080 So we have 0.0284 minus x plus x plus x. 438 00:27:05,080 --> 00:27:09,040 Now we're talking about a weak acid in water, so what term am 439 00:27:09,040 --> 00:27:12,780 I going to want to use next? 440 00:27:12,780 --> 00:27:14,990 K a. 441 00:27:14,990 --> 00:27:18,370 So I'm going to want to use k a next -- k a 442 00:27:18,370 --> 00:27:19,480 value is here, 8 . 443 00:27:19,480 --> 00:27:24,200 0 times 10 to minus 5, we have products over our reactant 444 00:27:24,200 --> 00:27:33,030 here, and we have x squared over 0.0284 minus x. 445 00:27:33,030 --> 00:27:35,500 Now you can make an assumption when you're working on these 446 00:27:35,500 --> 00:27:37,270 problems and check it later. 447 00:27:37,270 --> 00:27:40,640 So you can make the assumption that x is really kind of small 448 00:27:40,640 --> 00:27:45,670 compared to this o.084, and you can just drop this x out 449 00:27:45,670 --> 00:27:47,140 of the term here. 450 00:27:47,140 --> 00:27:51,950 And then check later and see if that worked or not. 451 00:27:51,950 --> 00:27:56,030 So that makes the math easier, and so now we can just solve 452 00:27:56,030 --> 00:28:00,560 for x, and x comes out to be 0.00151. 453 00:28:00,560 --> 00:28:06,500 The really two significant figures, but we're going to 454 00:28:06,500 --> 00:28:10,120 carry an extra one for the moment, so they're just two 455 00:28:10,120 --> 00:28:14,030 figures right here, two significant figures here. 456 00:28:14,030 --> 00:28:17,040 Now we can check and see if x was really small, if that 457 00:28:17,040 --> 00:28:18,600 assumption worked right. 458 00:28:18,600 --> 00:28:27,570 So, is 0.0284 minus 0.00151 really the same as 0.0248, and 459 00:28:27,570 --> 00:28:30,780 we let you make the assumption that it is, we say it's OK if 460 00:28:30,780 --> 00:28:34,010 it's less than 5% of the value. 461 00:28:34,010 --> 00:28:37,820 So, in this case, it's actually not, it's 5 . 462 00:28:37,820 --> 00:28:41,970 3, so that violates our policy. 463 00:28:41,970 --> 00:28:45,240 So it's more than 5%, so then you have to use the quadratic 464 00:28:45,240 --> 00:28:48,700 equation to solve the problem. 465 00:28:48,700 --> 00:28:52,130 I just want to note that this term, this percentage, can be 466 00:28:52,130 --> 00:28:54,890 called sometimes percent ionized or percent 467 00:28:54,890 --> 00:28:58,220 deprotonated, so that you're not thinking that's some kind 468 00:28:58,220 --> 00:29:01,220 of bizarre term if you see that. 469 00:29:01,220 --> 00:29:04,140 And if you use the quadratic equation, you get an answer of 470 00:29:04,140 --> 00:29:08,950 0.00147, again, that's really two significant figures. 471 00:29:08,950 --> 00:29:11,200 So it's not a whole lot different, actually, than the 472 00:29:11,200 --> 00:29:16,980 number you got making the approximation. 473 00:29:16,980 --> 00:29:21,000 So, once you know what x is, x is the hydronium ion 474 00:29:21,000 --> 00:29:24,780 concentration in this problem, and so we can plug that in, p 475 00:29:24,780 --> 00:29:28,780 h is minus log of this, so that's 2 . 476 00:29:28,780 --> 00:29:30,540 83. 477 00:29:30,540 --> 00:29:35,610 And so we had really two significant figures here, and 478 00:29:35,610 --> 00:29:38,450 so we are going to have two significant figures after the 479 00:29:38,450 --> 00:29:40,190 decimal point here. 480 00:29:40,190 --> 00:29:42,690 And if you haven't reviewed your sig fig rules and need 481 00:29:42,690 --> 00:29:46,020 more help if that seems wrong, then you should definitely 482 00:29:46,020 --> 00:29:47,500 review it before the next test. 483 00:29:47,500 --> 00:29:55,240 All right, so now we'll continue with today's lecture 484 00:29:55,240 --> 00:29:58,470 notes, and we're just going to continue right on and we're 485 00:29:58,470 --> 00:30:03,510 going to talk about weak bases, and well, we're start 486 00:30:03,510 --> 00:30:05,460 working on our way through, we're also going to try to get 487 00:30:05,460 --> 00:30:06,850 to buffers today. 488 00:30:06,850 --> 00:30:10,140 So, we've done a problem for a weak acid in water. 489 00:30:10,140 --> 00:30:19,620 So now let's talk about a weak base in water, and you can 490 00:30:19,620 --> 00:30:20,870 start us off. 491 00:30:20,870 --> 00:30:25,340 So in this problem we're given a molarity, so you didn't have 492 00:30:25,340 --> 00:30:29,930 to calculate that, and now you can help me fill out the table 493 00:30:29,930 --> 00:31:15,270 so we know what to do here. 494 00:31:15,270 --> 00:31:30,120 OK, let's just do 10 more seconds. 495 00:31:30,120 --> 00:31:33,070 Very good. 496 00:31:33,070 --> 00:31:38,600 So you're going to be losing some of the amount -- some of 497 00:31:38,600 --> 00:31:40,540 the amount of the weak base you have in, it's going to 498 00:31:40,540 --> 00:31:44,100 ionize, and so then you'll be forming the conjugate acid 499 00:31:44,100 --> 00:31:48,340 plus x, and you're going to be forming hydroxide ions plus x. 500 00:31:48,340 --> 00:31:51,980 And so the one with the minus sign is important, and 501 00:31:51,980 --> 00:31:54,590 sometimes there will be 2's involved, and that depends on 502 00:31:54,590 --> 00:31:57,940 the stoichiometry of the reaction. 503 00:31:57,940 --> 00:32:03,810 All right, so we can use that information now and go on and 504 00:32:03,810 --> 00:32:06,190 look at -- actually you could leave that clicker question up 505 00:32:06,190 --> 00:32:11,790 for a minute, and we're going to talk about the k b. 506 00:32:11,790 --> 00:32:17,420 So the k b is going to be equal to our products, n h 4 507 00:32:17,420 --> 00:32:25,670 plus and our hydroxide ion concentration over n h 3. 508 00:32:25,670 --> 00:32:30,190 And so now I can fill in the values that you told me. 509 00:32:30,190 --> 00:32:34,650 So we have, on the top we're going to have x squared, on 510 00:32:34,650 --> 00:32:38,880 the bottom we're going to have 0.15 minus x. 511 00:32:38,880 --> 00:32:44,030 And now we can make an approximation here that x is 512 00:32:44,030 --> 00:32:48,520 going to be small compared to 0.15, and so we can say that's 513 00:32:48,520 --> 00:32:54,390 just going to be equal to x squared over 0.15, and the k b 514 00:32:54,390 --> 00:32:56,290 value that was given was 1 . 515 00:32:56,290 --> 00:33:02,840 8 times 10 to the minus 5. 516 00:33:02,840 --> 00:33:08,180 So now we can solve for x using this approximation, and 517 00:33:08,180 --> 00:33:11,950 using this approximation x comes out to be 0 . 518 00:33:11,950 --> 00:33:18,470 00164, and we can look at whether that 519 00:33:18,470 --> 00:33:20,490 approximation was OK. 520 00:33:20,490 --> 00:33:31,710 So is this number less than 5% of 0.15, so we can say 0.00164 521 00:33:31,710 --> 00:33:40,310 over 0.15 times 100, and that comes out to be 1.1%, so 522 00:33:40,310 --> 00:33:43,760 that's OK, that's less than 5%. 523 00:33:43,760 --> 00:33:46,690 So that's good, we don't have to use the 524 00:33:46,690 --> 00:33:50,910 quadratic equation here. 525 00:33:50,910 --> 00:33:56,810 So now, we want to calculate the p h. 526 00:33:56,810 --> 00:34:03,650 So can I just plug that number for x into my p h equation? 527 00:34:03,650 --> 00:34:05,620 What is x? 528 00:34:05,620 --> 00:34:13,060 What is x equal to here? 529 00:34:13,060 --> 00:34:14,280 It's equal to two different things. 530 00:34:14,280 --> 00:34:17,900 What's one of them? 531 00:34:17,900 --> 00:34:20,490 Hydroxide ion concentration. 532 00:34:20,490 --> 00:34:23,950 So what we can do is calculate a p o h. 533 00:34:23,950 --> 00:34:29,890 So, p o h is minus log of the hydroxide ion concentration, 534 00:34:29,890 --> 00:34:34,090 or minus log of 0 . 535 00:34:34,090 --> 00:34:41,490 001647, and we really only have two significant figures 536 00:34:41,490 --> 00:34:45,910 here, and that is going to come out to 2 . 537 00:34:45,910 --> 00:34:48,050 79. 538 00:34:48,050 --> 00:34:50,740 And so, we would have two significant figures after the 539 00:34:50,740 --> 00:34:53,350 decimal point, because this number had two significant 540 00:34:53,350 --> 00:34:54,530 figures in it. 541 00:34:54,530 --> 00:34:55,430 But I'm not done. 542 00:34:55,430 --> 00:34:59,810 I've calculated p o h and the problem wanted p h. 543 00:34:59,810 --> 00:35:05,320 So how do I go from p o h to p h? 544 00:35:05,320 --> 00:35:08,920 14 minus, yup. 545 00:35:08,920 --> 00:35:10,230 So 14 . 546 00:35:10,230 --> 00:35:14,500 00 at room temperature minus 2 . 547 00:35:14,500 --> 00:35:19,160 79 is going to be equal to 11 . 548 00:35:19,160 --> 00:35:23,460 21. 549 00:35:23,460 --> 00:35:24,910 And so that makes sense. 550 00:35:24,910 --> 00:35:28,140 Now in doing these problems always consider, sometimes 551 00:35:28,140 --> 00:35:31,720 you're rushing and you get done you say OK, my p h is 2. 552 00:35:31,720 --> 00:35:34,080 But go back and think about the type of 553 00:35:34,080 --> 00:35:35,180 problem you're doing. 554 00:35:35,180 --> 00:35:37,470 It's a base in water problem. 555 00:35:37,470 --> 00:35:40,980 Would it make sense that the p h was 2 if it was a base in 556 00:35:40,980 --> 00:35:41,530 water problem? 557 00:35:41,530 --> 00:35:42,220 No. 558 00:35:42,220 --> 00:35:46,440 And so then you realize oh, I have to do another step. 559 00:35:46,440 --> 00:35:50,050 So that kind of thinking can save you a lot of points on 560 00:35:50,050 --> 00:35:53,350 the exam, to remember what it is you're trying to calculate 561 00:35:53,350 --> 00:35:57,020 and go back and make sure that your answer makes sense. 562 00:35:57,020 --> 00:35:59,970 And sometimes people run into weird math problems and some 563 00:35:59,970 --> 00:36:05,590 they'll write and say this p h should be above 7, it's 2, I 564 00:36:05,590 --> 00:36:08,640 don't know what I did wrong, clearly I did something wrong. 565 00:36:08,640 --> 00:36:11,070 I know that's wrong but I don't have time to figure out 566 00:36:11,070 --> 00:36:12,100 what I did wrong. 567 00:36:12,100 --> 00:36:13,620 That will get you points. 568 00:36:13,620 --> 00:36:17,560 So just recognizing that if something makes sense or not 569 00:36:17,560 --> 00:36:20,490 tells us you know what's going on, and sometimes math issues 570 00:36:20,490 --> 00:36:23,610 can get you into a place that you can't get out of quickly. 571 00:36:23,610 --> 00:36:26,180 So just thinking about whether the problem makes 572 00:36:26,180 --> 00:36:27,710 sense is a big step. 573 00:36:27,710 --> 00:36:30,280 All right. 574 00:36:30,280 --> 00:36:34,550 So now, we're going to talk about salt problems and I'm 575 00:36:34,550 --> 00:36:39,070 going to try to convince you that salts are actually the 576 00:36:39,070 --> 00:36:44,520 same as the weak acid and weak bases that we just did. 577 00:36:44,520 --> 00:36:46,810 So a salt is formed when you mix an 578 00:36:46,810 --> 00:36:49,070 acid and a base together. 579 00:36:49,070 --> 00:36:53,490 So, for example, if you have h c l and sodium hydroxide, 580 00:36:53,490 --> 00:36:58,270 you're going to get table salt and a c l and water. 581 00:36:58,270 --> 00:37:04,460 So the p h of a salt in water is not always neutral. 582 00:37:04,460 --> 00:37:07,770 Sometimes it's neutral, sometimes it's not neutral. 583 00:37:07,770 --> 00:37:09,960 Well, when would it not be neutral? 584 00:37:09,960 --> 00:37:13,960 Well, if a salt contained a conjugate acid of a weak base, 585 00:37:13,960 --> 00:37:15,610 then that conjugate acid is going to 586 00:37:15,610 --> 00:37:18,300 make it weakly acidic. 587 00:37:18,300 --> 00:37:21,930 Salts that contain things like iron 3 588 00:37:21,930 --> 00:37:24,860 plus also may be acidic. 589 00:37:24,860 --> 00:37:28,250 So when you're drinking water and you measure the p h of 590 00:37:28,250 --> 00:37:31,840 that water and it's not neutral, this could be part of 591 00:37:31,840 --> 00:37:34,550 the reason that there's some salt in the water, some 592 00:37:34,550 --> 00:37:36,420 different ions in the water. 593 00:37:36,420 --> 00:37:40,320 So a general rule from the periodic table group one and 594 00:37:40,320 --> 00:37:45,520 group two metals, so lithium, calcium, sodium, those are all 595 00:37:45,520 --> 00:37:47,720 going to be neutral in solution, so you can just 596 00:37:47,720 --> 00:37:48,820 remember that. 597 00:37:48,820 --> 00:37:53,070 And if a salt contains a conjugate base of a weak acid, 598 00:37:53,070 --> 00:37:55,340 then it'll form a basic solution. 599 00:37:55,340 --> 00:37:59,460 So it's all about whether the salt derived from a weak acid 600 00:37:59,460 --> 00:38:02,500 or a weak base that's going to give you a clue as whether 601 00:38:02,500 --> 00:38:04,930 it's an acid or a basic solution. 602 00:38:04,930 --> 00:38:07,880 If it derived, say, from a strong acid mixed with a 603 00:38:07,880 --> 00:38:09,680 strong base, then it's going to give you 604 00:38:09,680 --> 00:38:11,280 a salt that's neutral. 605 00:38:11,280 --> 00:38:13,900 So let's look at some examples. 606 00:38:13,900 --> 00:38:19,250 So here we have n h 4 c l. 607 00:38:19,250 --> 00:38:22,750 So we can break this down and think about where a salt like 608 00:38:22,750 --> 00:38:25,240 this would have come from. 609 00:38:25,240 --> 00:38:29,150 So it would have come from n h 4 plus, and it would have come 610 00:38:29,150 --> 00:38:32,470 from c l minus. 611 00:38:32,470 --> 00:38:36,360 So n h 4 plus, let's think about where this came from. 612 00:38:36,360 --> 00:38:38,090 What is it? 613 00:38:38,090 --> 00:38:42,980 So, we want to ask the question is n h 4 plus a 614 00:38:42,980 --> 00:38:45,750 conjugate acid of a weak base? 615 00:38:45,750 --> 00:38:47,020 And what is its conjugate base? 616 00:38:47,020 --> 00:38:50,430 Well, it's conjugate base is n h 3. 617 00:38:50,430 --> 00:38:53,740 If you lose a hydrogen ion or proton from n h 4 618 00:38:53,740 --> 00:38:55,290 plus, you get n h 3. 619 00:38:55,290 --> 00:38:58,020 And so you're really asking, if that's a weak base, then 620 00:38:58,020 --> 00:39:00,210 its conjugate is also going to be weak. 621 00:39:00,210 --> 00:39:03,280 So you need to know about these guys to see what would 622 00:39:03,280 --> 00:39:06,660 happen if you have n h 4 plus in solution. 623 00:39:06,660 --> 00:39:08,600 So how do you know about this. 624 00:39:08,600 --> 00:39:12,080 Well, you know about things being weak or strong based on 625 00:39:12,080 --> 00:39:14,200 their k a's and their k b's. 626 00:39:14,200 --> 00:39:17,290 So, is ammonia a weak base? 627 00:39:17,290 --> 00:39:19,480 It has a k b of 1 . 628 00:39:19,480 --> 00:39:24,010 8 times 10 to the minus 5, so yeah, that's a small number, 629 00:39:24,010 --> 00:39:26,700 so that's a weak base. 630 00:39:26,700 --> 00:39:29,820 And so it's actually in this table, so if you have 631 00:39:29,820 --> 00:39:32,620 something that's weak over here, then its conjugate is 632 00:39:32,620 --> 00:39:35,430 also going to be weak over there. 633 00:39:35,430 --> 00:39:37,160 These are totally lined up right. 634 00:39:37,160 --> 00:39:42,560 And so the conjugate over here is also going to be weak. 635 00:39:42,560 --> 00:39:45,340 And if you weren't really sure you could always look it up, 636 00:39:45,340 --> 00:39:49,070 so here you have ammonium ion and it has a k a of 5 . 637 00:39:49,070 --> 00:39:51,150 6 times 10 to the minus 10. 638 00:39:51,150 --> 00:39:56,920 Yup, that's a very small number, that is a weak acid. 639 00:39:56,920 --> 00:40:02,250 So yes, the conjugate is weak, the base is weak, and the 640 00:40:02,250 --> 00:40:05,520 conjugate acid of that weak base is also weak. 641 00:40:05,520 --> 00:40:09,500 So, n h 4 plus does have acidic properties. 642 00:40:09,500 --> 00:40:12,280 It's not a strong acid, it's a weak acid, but it will make 643 00:40:12,280 --> 00:40:15,370 things acidic. 644 00:40:15,370 --> 00:40:17,900 So this should be acidic. 645 00:40:17,900 --> 00:40:19,950 What about c l minus? 646 00:40:19,950 --> 00:40:23,920 Do you think it's going to do anything useful for you? 647 00:40:23,920 --> 00:40:27,900 Where do you think c l minus came from? 648 00:40:27,900 --> 00:40:29,170 From h c l. 649 00:40:29,170 --> 00:40:32,170 So we could ask again, is a c l minus a conjugate 650 00:40:32,170 --> 00:40:34,780 base of a weak acid. 651 00:40:34,780 --> 00:40:38,770 The acid is h c l, is that a weak acid? 652 00:40:38,770 --> 00:40:39,990 No. 653 00:40:39,990 --> 00:40:44,300 So we can look it up if you didn't remember it, 10 to the 654 00:40:44,300 --> 00:40:49,470 7, definitely not weak -- very, very, very strong acid, 655 00:40:49,470 --> 00:40:53,270 and if something is strong acid, its conjugate is 656 00:40:53,270 --> 00:40:55,330 ineffective as a base. 657 00:40:55,330 --> 00:41:00,840 So c l minus is ineffective as a base so it's going to be 658 00:41:00,840 --> 00:41:02,510 neutral here. 659 00:41:02,510 --> 00:41:05,690 So overall, you have something that's going to be acidic with 660 00:41:05,690 --> 00:41:07,750 something that's going to be neutral. 661 00:41:07,750 --> 00:41:09,730 So overall it'll be acidic. 662 00:41:09,730 --> 00:41:13,420 So this particular salt and water is going to be acidic 663 00:41:13,420 --> 00:41:16,930 because the things that were mixed together to get it, one 664 00:41:16,930 --> 00:41:19,725 of them included a weak base, and so that's going to form a 665 00:41:19,725 --> 00:41:25,590 weak conjugate acid, so it'll be acidic in solution. 666 00:41:25,590 --> 00:41:27,590 All right, so let's look at another one. 667 00:41:27,590 --> 00:41:33,240 I'm giving you the k a value in this problem, and knowing 668 00:41:33,240 --> 00:41:37,150 that particular k a value, tell me what you think is 669 00:41:37,150 --> 00:41:41,170 going to be true about this particular salt in solution, 670 00:41:41,170 --> 00:42:27,350 whether it'll be acidic, neutral, or basic. 671 00:42:27,350 --> 00:42:46,160 OK, 10 more seconds. 672 00:42:46,160 --> 00:42:49,960 So, some people were a little fooled by the information I 673 00:42:49,960 --> 00:42:53,700 gave you, so let's take a look at this. 674 00:42:53,700 --> 00:43:00,960 So if we go to my presentation here. 675 00:43:00,960 --> 00:43:07,700 So we can break this up into n a plus and c h 3 c o minus. 676 00:43:07,700 --> 00:43:13,440 N a plus, is that a conjugate acid of a weak base? 677 00:43:13,440 --> 00:43:14,960 Is that going to be acidic? 678 00:43:14,960 --> 00:43:18,920 What do we know about things in group one, a column of the 679 00:43:18,920 --> 00:43:20,900 periodic table. 680 00:43:20,900 --> 00:43:22,250 They're going to be neutral. 681 00:43:22,250 --> 00:43:26,100 Where do you think that this came from, n a plus, where 682 00:43:26,100 --> 00:43:30,060 might it have come from? 683 00:43:30,060 --> 00:43:33,010 It might have come from n a o h, that might have been a base 684 00:43:33,010 --> 00:43:34,060 that was added. 685 00:43:34,060 --> 00:43:36,220 So it's not going to do anything for you -- things in 686 00:43:36,220 --> 00:43:39,320 group one and group two are going to be neutral. 687 00:43:39,320 --> 00:43:41,870 All right, so we can ask the question about 688 00:43:41,870 --> 00:43:45,350 a c h 3 c o o minus. 689 00:43:45,350 --> 00:43:49,030 Is it a conjugate base of a weak acid? 690 00:43:49,030 --> 00:43:53,310 So then we can say is the acid that it came from a weak acid? 691 00:43:53,310 --> 00:43:55,720 Is its conjugate a weak acid? 692 00:43:55,720 --> 00:43:57,060 Well, how do we know about that? 693 00:43:57,060 --> 00:44:01,540 Well, we know about that from the k a value that I gave you. 694 00:44:01,540 --> 00:44:04,330 So is this a weak acid? 695 00:44:04,330 --> 00:44:05,120 Yes. 696 00:44:05,120 --> 00:44:09,440 So is its conjugate going to be a weak base? 697 00:44:09,440 --> 00:44:11,130 Yes. 698 00:44:11,130 --> 00:44:14,400 So, given that information we can say yes. 699 00:44:14,400 --> 00:44:17,370 So its conjugate acid is weak, so then it 700 00:44:17,370 --> 00:44:19,930 would be a weak base. 701 00:44:19,930 --> 00:44:25,680 So if it's a weak base then it'll be basic in solution, 702 00:44:25,680 --> 00:44:29,550 and we have something that's neutral plus basic, so overall 703 00:44:29,550 --> 00:44:30,970 you get basic. 704 00:44:30,970 --> 00:44:34,700 All right. 705 00:44:34,700 --> 00:44:45,780 So let's look at a general example now of this as well. 706 00:44:45,780 --> 00:44:48,980 So here we can talk about a general rule. 707 00:44:48,980 --> 00:44:53,150 So if you have compound x y, we can talk about 708 00:44:53,150 --> 00:44:55,180 x plus and y minus. 709 00:44:55,180 --> 00:44:58,730 And for x plus you're going to be asking about is it a 710 00:44:58,730 --> 00:45:01,010 conjugate acid of a weak base. 711 00:45:01,010 --> 00:45:04,560 For y minus you're going to be asking is it a conjugate base 712 00:45:04,560 --> 00:45:05,870 of a weak acid. 713 00:45:05,870 --> 00:45:08,180 So for the first part you're asking about if it's a 714 00:45:08,180 --> 00:45:11,510 conjugate acid, in the second half you're asking about if 715 00:45:11,510 --> 00:45:13,120 it's a conjugate base. 716 00:45:13,120 --> 00:45:16,860 So if something is a conjugate acid of a weak base, and 717 00:45:16,860 --> 00:45:19,370 that's yes -- if you know that's a weak base or you know 718 00:45:19,370 --> 00:45:22,970 that it's a weak acid, then it's going to be acidic, if 719 00:45:22,970 --> 00:45:24,940 it's not, it'll be neutral. 720 00:45:24,940 --> 00:45:27,180 Same thing is true over here. 721 00:45:27,180 --> 00:45:29,550 You might know about that something is a weak base, you 722 00:45:29,550 --> 00:45:32,520 might know that its conjugate is a weak acid, and if you 723 00:45:32,520 --> 00:45:35,120 have a conjugate base of something that's a weak acid, 724 00:45:35,120 --> 00:45:36,930 then it is a base. 725 00:45:36,930 --> 00:45:39,540 If it's a strong acid it would be ineffectual, but if it's a 726 00:45:39,540 --> 00:45:42,380 conjugate of a weak acid, it'll be basic. 727 00:45:42,380 --> 00:45:44,230 No, it's neutral. 728 00:45:44,230 --> 00:45:47,760 So overall, you can have three possibilities. 729 00:45:47,760 --> 00:45:52,230 Acidic plus neutral is acidic, basic plus neutral is basic, 730 00:45:52,230 --> 00:45:54,730 and neutral plus neutral is neutral. 731 00:45:54,730 --> 00:45:57,420 Now some people might come up with another option here. 732 00:45:57,420 --> 00:46:03,090 What other thing am I leaving off of this overall? 733 00:46:03,090 --> 00:46:04,200 Acidic plus basic. 734 00:46:04,200 --> 00:46:06,540 That's because I'm never going to ask you that when 735 00:46:06,540 --> 00:46:07,720 it comes to a salt. 736 00:46:07,720 --> 00:46:09,850 Because pretty much, salts are formed when you're doing a 737 00:46:09,850 --> 00:46:12,980 titration, and you're going to be either titrating a strong 738 00:46:12,980 --> 00:46:15,870 acid, a strong base, you're going to be titrating a weak 739 00:46:15,870 --> 00:46:19,270 acid with a strong base, or you're going to be titrating a 740 00:46:19,270 --> 00:46:21,400 strong acid with a weak base. 741 00:46:21,400 --> 00:46:25,120 You are never going to titrate a weak acid with a weak base. 742 00:46:25,120 --> 00:46:26,770 That would yield no interesting 743 00:46:26,770 --> 00:46:28,210 results of any kind. 744 00:46:28,210 --> 00:46:30,350 So you're not going to be forming salts that are 745 00:46:30,350 --> 00:46:32,490 conjugates of both those things. 746 00:46:32,490 --> 00:46:35,700 So, if you want to think about it that way that's fine, or 747 00:46:35,700 --> 00:46:37,750 you could just remember that that is not what I'm 748 00:46:37,750 --> 00:46:38,700 going to ask you. 749 00:46:38,700 --> 00:46:42,190 These are great little short answer questions on an exam, 750 00:46:42,190 --> 00:46:44,860 so if you're good at thinking about this, it'll definitely 751 00:46:44,860 --> 00:46:51,410 give you a couple of points on one of the exams. 752 00:46:51,410 --> 00:46:54,570 All right, so now and last, just a couple minutes, I just 753 00:46:54,570 --> 00:46:58,420 want to introduce very briefly buffers. 754 00:46:58,420 --> 00:47:02,990 So a buffer is something that maintains the p h of a 755 00:47:02,990 --> 00:47:06,890 solution, so it's going to buffer that solution. 756 00:47:06,890 --> 00:47:10,050 So if you add a little bit of strong acid or a little bit of 757 00:47:10,050 --> 00:47:12,300 strong base, it doesn't matter the p h is 758 00:47:12,300 --> 00:47:13,930 going to stay the same. 759 00:47:13,930 --> 00:47:16,580 So there are two kinds of buffers. 760 00:47:16,580 --> 00:47:20,510 You have an acid buffer, which is going to buffer/maintain 761 00:47:20,510 --> 00:47:23,130 the p h on the acidic side of neutral. 762 00:47:23,130 --> 00:47:27,600 And a basic buffer, which will maintain the p h on the basic 763 00:47:27,600 --> 00:47:31,080 end of the p h scale. 764 00:47:31,080 --> 00:47:35,220 So let me just give you a brief example of a buffer and 765 00:47:35,220 --> 00:47:37,360 just get you thinking about buffers. 766 00:47:37,360 --> 00:47:40,940 So here, in about a buffer problem you're going to mix an 767 00:47:40,940 --> 00:47:43,560 acid with its conjugate base. 768 00:47:43,560 --> 00:47:49,360 So, you have acetate, and then probably the acetate salt of 769 00:47:49,360 --> 00:47:50,010 the acetic acid. 770 00:47:50,010 --> 00:47:53,500 So over here, you have the acetic acid, on this side you 771 00:47:53,500 --> 00:47:56,000 have its conjugate base usually added in 772 00:47:56,000 --> 00:47:57,640 the form of a salt. 773 00:47:57,640 --> 00:48:00,220 And then you have an equilibrium. 774 00:48:00,220 --> 00:48:03,470 So, what's going to happen if you add a strong 775 00:48:03,470 --> 00:48:06,530 acid to this solution? 776 00:48:06,530 --> 00:48:13,190 If you add strong acid, if you add more h 3 o plus, what 777 00:48:13,190 --> 00:48:14,280 happens if you add more? 778 00:48:14,280 --> 00:48:19,650 What direction will the reaction shift? 779 00:48:19,650 --> 00:48:22,240 So you'll get the back reaction, it'll try to 780 00:48:22,240 --> 00:48:25,550 minimize that stress and move the other way, and it'll use 781 00:48:25,550 --> 00:48:29,310 up some of that acid and maintain the p h. 782 00:48:29,310 --> 00:48:33,400 Then you can think, so these amount of acid added is 783 00:48:33,400 --> 00:48:37,800 effectively removed and the p h stays the same. 784 00:48:37,800 --> 00:48:41,170 What about if you add a strong base? 785 00:48:41,170 --> 00:48:47,210 Well, that strong base will react with the acid, it will 786 00:48:47,210 --> 00:48:53,030 remove protons from this acid, or the hydrogen ion here, 787 00:48:53,030 --> 00:48:55,320 forming this water and its conjugate. 788 00:48:55,320 --> 00:48:57,680 So you'll make more of these and the p h will 789 00:48:57,680 --> 00:48:58,880 also stay the same. 790 00:48:58,880 --> 00:49:01,860 So the base is going to be removed by reacting. 791 00:49:01,860 --> 00:49:06,350 So they're effectively removed and the p h stays the same. 792 00:49:06,350 --> 00:49:10,850 So in this you have a weak acid, h a, it'll transfer 793 00:49:10,850 --> 00:49:17,570 protons to o h minus supplied by the strong base. 794 00:49:17,570 --> 00:49:21,220 The conjugate of that weak acid, which would be a weak 795 00:49:21,220 --> 00:49:25,720 base, is going to accept protons from any acid that is 796 00:49:25,720 --> 00:49:27,340 going to be supplied. 797 00:49:27,340 --> 00:49:32,340 So, in this way, you maintain the p h. 798 00:49:32,340 --> 00:49:37,550 And so I just to emphasize that in a buffer solution you 799 00:49:37,550 --> 00:49:42,130 have h a, acidic buffer solution, you have h a, and 800 00:49:42,130 --> 00:49:43,990 you have its conjugate. 801 00:49:43,990 --> 00:49:46,470 And if you only have one or the other, it's not going to 802 00:49:46,470 --> 00:49:47,830 make a good buffer. 803 00:49:47,830 --> 00:49:52,470 So, I want you to remember that in buffers you have both 804 00:49:52,470 --> 00:49:55,500 conjugates -- one alone is not going to work. 805 00:49:55,500 --> 00:49:59,330 And people forget this in the class, and so you can remember 806 00:49:59,330 --> 00:50:02,740 that for Halloween, your chemistry professor dressed up 807 00:50:02,740 --> 00:50:07,620 as a buffer to help you remember that in a buffer 808 00:50:07,620 --> 00:50:09,150 you're going to have both. 809 00:50:09,150 --> 00:50:12,200 You've got to have the conjugate acid base set, 810 00:50:12,200 --> 00:50:15,640 otherwise it will not buffer.