1 00:00:00,030 --> 00:00:02,529 NARRATOR: The following content is provided under a Creative 2 00:00:02,529 --> 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 --> 00:00:10,090 continue to offer high quality educational resources for free. 5 00:00:10,090 --> 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 --> 00:00:17,257 at ocw.mit.edu. 8 00:00:26,290 --> 00:00:28,670 CATHERINE DRENNAN: There are five types of problems. 9 00:00:28,670 --> 00:00:30,880 We just did two. 10 00:00:30,880 --> 00:00:33,430 And now I'm going to convince you that salt and water 11 00:00:33,430 --> 00:00:39,600 problems are in fact the same as weak acid weak base problems. 12 00:00:39,600 --> 00:00:41,985 So we're going to move now to today's handout. 13 00:00:46,950 --> 00:00:53,340 So the pH of a salt solution is determined 14 00:00:53,340 --> 00:00:56,850 by what made that salt. So a salt 15 00:00:56,850 --> 00:01:01,290 is formed when you neutralize an acid with a base or a base 16 00:01:01,290 --> 00:01:03,660 with an acid. 17 00:01:03,660 --> 00:01:06,310 And so depending on what acid and which 18 00:01:06,310 --> 00:01:09,550 base were used and mixed together, 19 00:01:09,550 --> 00:01:13,050 the salt might have a different pH. 20 00:01:13,050 --> 00:01:18,930 So an example here if we had HCl and NaOH, 21 00:01:18,930 --> 00:01:22,635 that's going to give our friend table salt NaCl and water. 22 00:01:25,300 --> 00:01:29,300 The pH of the salt and water is not always neutral. 23 00:01:29,300 --> 00:01:32,980 In this case it would be, but it's not always neutral. 24 00:01:32,980 --> 00:01:35,450 It depends on the nature of the acid 25 00:01:35,450 --> 00:01:38,440 and the nature of the base that were mixed together 26 00:01:38,440 --> 00:01:42,960 to form that salt. So there are some rules 27 00:01:42,960 --> 00:01:46,130 to help you figure out whether something's 28 00:01:46,130 --> 00:01:49,700 going to be acidic or basic. 29 00:01:49,700 --> 00:01:54,790 And if you have a salt that contains 30 00:01:54,790 --> 00:01:57,330 a conjugate acid of a weak base, so it 31 00:01:57,330 --> 00:02:00,160 has a conjugate acid in it, it will 32 00:02:00,160 --> 00:02:03,630 produce an acidic solution. 33 00:02:03,630 --> 00:02:07,940 And if you have a salt that has small highly charged metal 34 00:02:07,940 --> 00:02:14,470 cations like iron plus 3, that will also be acidic. 35 00:02:14,470 --> 00:02:18,090 So we saw last time I had the prescription medicine 36 00:02:18,090 --> 00:02:21,615 for my daughter, which was iron sulfate, was highly acidic. 37 00:02:24,120 --> 00:02:27,620 So note that periodic table group 1 and group 38 00:02:27,620 --> 00:02:34,750 2 ions, lithium plus 1, calcium plus 2, sodium plus 1-- 39 00:02:34,750 --> 00:02:39,190 these are all going to be neutral 40 00:02:39,190 --> 00:02:43,710 and in fact, anything plus 1 neutral. 41 00:02:43,710 --> 00:02:48,150 And then salts that can take conjugate bases of weak acids 42 00:02:48,150 --> 00:02:50,840 will produce basic solution. 43 00:02:50,840 --> 00:02:52,710 So you're going to be asking yourself, 44 00:02:52,710 --> 00:02:56,560 does this solution have a conjugate acid of a weak base? 45 00:02:56,560 --> 00:02:59,280 Does it have a conjugate base of weak acid? 46 00:02:59,280 --> 00:03:03,122 Does it have group 1, group 2-- what's in there? 47 00:03:03,122 --> 00:03:05,080 And then you can think about whether it's going 48 00:03:05,080 --> 00:03:08,100 to be acidic, neutral or basic. 49 00:03:08,100 --> 00:03:11,180 So let's look at some examples. 50 00:03:11,180 --> 00:03:14,850 So we have a salt, NH4Cl. 51 00:03:14,850 --> 00:03:16,300 And we want to figure out is this 52 00:03:16,300 --> 00:03:19,360 going to produce an acidic solution, a neutral solution 53 00:03:19,360 --> 00:03:21,130 or a basic solution. 54 00:03:21,130 --> 00:03:25,110 So we need to think about what went in to making this salt. 55 00:03:25,110 --> 00:03:27,610 And we're going to break apart the salt and we're going 56 00:03:27,610 --> 00:03:33,340 to think about NH 4 plus and we're going to think about Cl-. 57 00:03:33,340 --> 00:03:37,380 So let's think about NH 4 plus first. 58 00:03:37,380 --> 00:03:41,230 And we're going to ask the question is NH 4 59 00:03:41,230 --> 00:03:43,680 plus a conjugate acid of a weak base 60 00:03:43,680 --> 00:03:46,970 and therefore, a weak acid itself? 61 00:03:46,970 --> 00:03:48,920 One way to answer this question is 62 00:03:48,920 --> 00:03:53,220 to look up the Ka for NH 4 plus, which happens 63 00:03:53,220 --> 00:03:56,670 to be 5.6 x 10 to the -10. 64 00:03:56,670 --> 00:04:01,890 You could also ask about the weak base that it came from. 65 00:04:01,890 --> 00:04:04,790 The weak base in this case, its conjugate base, 66 00:04:04,790 --> 00:04:08,380 is NH3 and ask if that's a weak base. 67 00:04:08,380 --> 00:04:11,936 So if you have something that is an acid here, 68 00:04:11,936 --> 00:04:13,560 you can think about what the base would 69 00:04:13,560 --> 00:04:15,480 be by removing H plus. 70 00:04:15,480 --> 00:04:20,029 If you remove H plus from NH 4 plus you get NH3. 71 00:04:20,029 --> 00:04:23,970 And you can say, is NH3 a weak base and look up a Kb. 72 00:04:23,970 --> 00:04:28,410 In this case, it's 1.8 x 10 to the minus 5. 73 00:04:28,410 --> 00:04:30,150 So what do you think? 74 00:04:30,150 --> 00:04:35,330 Is this a weak base and this a weak acid with those numbers? 75 00:04:39,760 --> 00:04:40,550 What do you think? 76 00:04:40,550 --> 00:04:42,080 Is that a weak acid? 77 00:04:42,080 --> 00:04:45,550 Is that a strong acid? 78 00:04:45,550 --> 00:04:47,410 So it's a weak acid. 79 00:04:47,410 --> 00:04:49,450 Is this a weak base? 80 00:04:49,450 --> 00:04:50,950 Yeah. 81 00:04:50,950 --> 00:04:55,400 So we can look also in this chart down here 82 00:04:55,400 --> 00:04:59,030 and so NH3 is a moderately weak base and NH 4 plus 83 00:04:59,030 --> 00:05:03,440 is a very weak base, but it is very weak acid, 84 00:05:03,440 --> 00:05:04,870 but it is a weak acid. 85 00:05:04,870 --> 00:05:07,980 So we do have a conjugate acid of a weak base. 86 00:05:07,980 --> 00:05:12,330 So it is acidic, not super acidic, but it is acidic. 87 00:05:12,330 --> 00:05:16,050 NH3 is a weak base and so we probably 88 00:05:16,050 --> 00:05:18,890 had NH3 as our base that was being added 89 00:05:18,890 --> 00:05:22,180 and it formed the conjugate acid in this. 90 00:05:22,180 --> 00:05:27,340 So then this would produce an acidic solution, 91 00:05:27,340 --> 00:05:29,900 because we have this conjugate acid that's formed. 92 00:05:29,900 --> 00:05:33,000 So from this part, just from the NH 4 plus, 93 00:05:33,000 --> 00:05:39,230 it should be acidic and now we can consider Cl-. 94 00:05:39,230 --> 00:05:43,070 So here we're asking the question is Cl-, 95 00:05:43,070 --> 00:05:46,570 a conjugate base of a weak acid and therefore, 96 00:05:46,570 --> 00:05:48,900 itself a weak base. 97 00:05:48,900 --> 00:05:52,110 And if you tried to look up the Kb of Cl-, 98 00:05:52,110 --> 00:05:54,490 you would not find it in any table. 99 00:05:54,490 --> 00:05:59,240 But you could find in a table its conjugate acid. 100 00:05:59,240 --> 00:06:03,660 So if you add H to Cl-, you get HCL, 101 00:06:03,660 --> 00:06:07,570 and so you could look up a Ka for that in the table and its 102 00:06:07,570 --> 00:06:09,510 times 10 to the 7. 103 00:06:09,510 --> 00:06:11,660 So is this a weak acid? 104 00:06:11,660 --> 00:06:12,350 AUDIENCE: No. 105 00:06:12,350 --> 00:06:14,450 CATHERINE DRENNAN: It's a strong acid. 106 00:06:14,450 --> 00:06:17,840 So it's not a weak acid, it's a strong acid. 107 00:06:17,840 --> 00:06:20,660 So is Cl- going to be basic? 108 00:06:23,530 --> 00:06:24,570 No. 109 00:06:24,570 --> 00:06:30,160 So it is not, it is not going to be a basic solution. 110 00:06:30,160 --> 00:06:34,700 If we look over here, we see HCl is a strong acid 111 00:06:34,700 --> 00:06:38,270 and so its conjugate base is ineffective in it's base. 112 00:06:38,270 --> 00:06:41,180 It just doesn't work as a base at all. 113 00:06:41,180 --> 00:06:42,900 So with a number of 10 to the seventh, 114 00:06:42,900 --> 00:06:44,240 it really goes to completion. 115 00:06:44,240 --> 00:06:46,800 It's a strong acid by our definition. 116 00:06:46,800 --> 00:06:50,740 And so Cl- is not going to do anything to the solution. 117 00:06:50,740 --> 00:06:54,210 It's not going to be useful as a weak base. 118 00:06:54,210 --> 00:06:56,610 So this solution is going to be neutral, 119 00:06:56,610 --> 00:06:59,740 or at least the part of the solution due to Cl- 120 00:06:59,740 --> 00:07:01,130 will be neutral. 121 00:07:01,130 --> 00:07:04,310 So here we have something that is acidic and something that's 122 00:07:04,310 --> 00:07:05,400 neutral. 123 00:07:05,400 --> 00:07:08,745 And so overall, the solution is acidic. 124 00:07:11,260 --> 00:07:13,560 So you need to break down the two parts. 125 00:07:13,560 --> 00:07:16,080 You had an acid and a base being mixed. 126 00:07:16,080 --> 00:07:25,810 Here we had HCl mixed with NH3 and formed this NH4 salt. 127 00:07:25,810 --> 00:07:30,160 And that's going to be acidic, because HCL was a weak acid 128 00:07:30,160 --> 00:07:32,120 and its conjugate is ineffective. 129 00:07:32,120 --> 00:07:34,220 Whereas, NH3 was a weak base. 130 00:07:34,220 --> 00:07:35,860 So it has a conjugate acid that's 131 00:07:35,860 --> 00:07:39,990 a weak acid, a very weak acid, but still a weak acid. 132 00:07:39,990 --> 00:07:40,810 All right. 133 00:07:40,810 --> 00:07:43,120 So that's how you think about these salt problems. 134 00:07:43,120 --> 00:07:45,860 So they really break down to weak acid and weak base 135 00:07:45,860 --> 00:07:46,810 problems. 136 00:07:46,810 --> 00:07:48,650 So why don't you give this one a try? 137 00:07:48,650 --> 00:07:52,578 [AUDIENCE MUMBLING] 138 00:07:58,930 --> 00:08:01,180 CATHERINE DRENNAN: All right, let's do 10 more seconds 139 00:08:01,180 --> 00:08:04,680 [AUDIENCE MUMBLING] 140 00:08:17,222 --> 00:08:18,680 CATHERINE DRENNAN: That is correct. 141 00:08:18,680 --> 00:08:20,690 All right, let's look at why that's correct. 142 00:08:23,340 --> 00:08:27,170 So it will produce a basic solution. 143 00:08:27,170 --> 00:08:31,210 So we're going to break it down to NA . 144 00:08:31,210 --> 00:08:33,110 That's one of the things in there. 145 00:08:33,110 --> 00:08:37,250 And we ask is NA a conjugate acid of a weak base? 146 00:08:37,250 --> 00:08:40,250 Is it going to be neutral, acidic or basic? 147 00:08:40,250 --> 00:08:44,070 And what's it going to be? 148 00:08:44,070 --> 00:08:45,380 It's going to be neutral. 149 00:08:45,380 --> 00:08:47,760 It is not a conjugate acid of a weak base. 150 00:08:47,760 --> 00:08:49,170 It's group 1. 151 00:08:49,170 --> 00:08:49,930 It'll be neutral. 152 00:08:52,860 --> 00:08:56,590 So this is not contributing to the pH. 153 00:08:56,590 --> 00:09:00,320 So let's look at the second part and look at CH3COO-. 154 00:09:02,990 --> 00:09:08,970 So we have-- this can be divided into Na+ and HC3OO-. 155 00:09:08,970 --> 00:09:13,080 And then we ask is this a conjugate base of a weak acid? 156 00:09:13,080 --> 00:09:15,370 Or we could ask the other question 157 00:09:15,370 --> 00:09:18,420 is this over here a weak acid? 158 00:09:18,420 --> 00:09:20,700 And the Ka for that was given. 159 00:09:20,700 --> 00:09:24,165 So is that a weak acid or a strong acid? 160 00:09:24,165 --> 00:09:28,710 Is it weak or strong? 161 00:09:28,710 --> 00:09:29,890 Weak, yes. 162 00:09:29,890 --> 00:09:30,970 So the answer is yes. 163 00:09:30,970 --> 00:09:37,130 It's weak and so it's conjugate base would also be a weak base. 164 00:09:37,130 --> 00:09:40,830 And so therefore, this will be acidic. 165 00:09:40,830 --> 00:09:44,650 So sometimes you'll be given Ka, sometimes you'll be given Kb's. 166 00:09:44,650 --> 00:09:46,220 If you're given either one of them 167 00:09:46,220 --> 00:09:47,750 you can answer the question. 168 00:09:47,750 --> 00:09:50,930 If you want, you can convert your Ka to your Kb. 169 00:09:50,930 --> 00:09:54,890 What would you use to convert Ka to Kb? 170 00:09:54,890 --> 00:09:56,450 Kw, right. 171 00:09:56,450 --> 00:09:58,950 But just with one of these pieces of information, 172 00:09:58,950 --> 00:10:02,200 you should be able to answer this question. 173 00:10:02,200 --> 00:10:05,380 So let's just look at the general rule now. 174 00:10:05,380 --> 00:10:08,570 You have a compound XY. 175 00:10:08,570 --> 00:10:12,480 That can be broken up into X and you'll 176 00:10:12,480 --> 00:10:17,110 ask yourself is X a conjugate acid of a weak base? 177 00:10:17,110 --> 00:10:19,490 If yes, then the solution will be acidic. 178 00:10:19,490 --> 00:10:22,270 If no, it will be neutral. 179 00:10:22,270 --> 00:10:24,790 For Y, you have Y-. 180 00:10:24,790 --> 00:10:27,750 Is Y- a conjugate base of a weak acid? 181 00:10:27,750 --> 00:10:32,260 If yes, then it's going to be add some basic nature to this. 182 00:10:32,260 --> 00:10:34,240 If no, neutral. 183 00:10:34,240 --> 00:10:36,610 And then over all, you say, if it's 184 00:10:36,610 --> 00:10:40,300 acidic plus neutral, that's acidic, basic plus 185 00:10:40,300 --> 00:10:43,520 neutral, basic, neutral plus neutral, neutral. 186 00:10:43,520 --> 00:10:45,840 And if I give you a question of something 187 00:10:45,840 --> 00:10:49,300 that is acidic and basic, you can answer the question 188 00:10:49,300 --> 00:10:50,940 without doing math, because it'll 189 00:10:50,940 --> 00:10:55,730 depend on the Ka of one thing and the Kb of the other thing. 190 00:10:55,730 --> 00:10:58,760 And you'll have to see which one's a stronger acid 191 00:10:58,760 --> 00:11:01,420 and which one's a stronger base. 192 00:11:01,420 --> 00:11:04,290 And so you can't just simply answer the question. 193 00:11:04,290 --> 00:11:06,950 Now you may note, that I did mention 194 00:11:06,950 --> 00:11:08,520 that problem set 7 is long. 195 00:11:08,520 --> 00:11:11,110 It takes a long time to do these problems. 196 00:11:11,110 --> 00:11:13,930 And so on the exam, writing a fair exam when the problems 197 00:11:13,930 --> 00:11:15,570 take a long time to do is hard. 198 00:11:15,570 --> 00:11:17,190 But these are short little things. 199 00:11:17,190 --> 00:11:20,690 I could ask you to predict the pH of salt by just thinking 200 00:11:20,690 --> 00:11:24,290 about things and looking up some Kb's and some Ka's. 201 00:11:24,290 --> 00:11:26,870 So these are good short questions on the exam. 202 00:11:26,870 --> 00:11:28,950 So I'm probably not going to give you one 203 00:11:28,950 --> 00:11:31,060 if it's supposed to be a short question that 204 00:11:31,060 --> 00:11:34,280 is an acid and a base mixed together, because then you'd 205 00:11:34,280 --> 00:11:35,430 have to do some math. 206 00:11:35,430 --> 00:11:37,930 So keep that in mind. 207 00:11:37,930 --> 00:11:41,070 All right, so salts, salt and water problems, 208 00:11:41,070 --> 00:11:43,770 are really just weak acid in water or weak base 209 00:11:43,770 --> 00:11:47,410 in water problems, which you already know how to do. 210 00:11:47,410 --> 00:11:49,575 So we're going to move on now to buffers. 211 00:11:52,590 --> 00:11:55,210 Buffers. 212 00:11:55,210 --> 00:12:00,960 So a buffer is a solution that maintains approximately 213 00:12:00,960 --> 00:12:06,000 a constant pH, there'll be a little range of pH can change, 214 00:12:06,000 --> 00:12:09,440 but the buffer tries to keep the pH constant. 215 00:12:09,440 --> 00:12:13,570 You use a buffer to keep pH constant. 216 00:12:13,570 --> 00:12:14,810 Why is this important? 217 00:12:14,810 --> 00:12:18,820 Well, buffers are very important in biology. 218 00:12:18,820 --> 00:12:21,390 We all work at a constant pH. 219 00:12:21,390 --> 00:12:24,560 Living things need to be at a constant pH, 220 00:12:24,560 --> 00:12:26,800 but a lot of times chemical processes 221 00:12:26,800 --> 00:12:29,560 also need to be at a constant pH. 222 00:12:29,560 --> 00:12:34,800 And here's just one example of why buffers are important. 223 00:12:34,800 --> 00:12:36,850 We're looking for alternative energy. 224 00:12:36,850 --> 00:12:39,160 We're trying to develop alternative energy. 225 00:12:39,160 --> 00:12:41,770 There's this idea of creating fuel cells that 226 00:12:41,770 --> 00:12:43,070 employ microbes. 227 00:12:43,070 --> 00:12:46,350 These little red circles are microbes here. 228 00:12:46,350 --> 00:12:49,350 There are microbes like shewanella 229 00:12:49,350 --> 00:12:53,390 that will live, adhere, to electrodes. 230 00:12:53,390 --> 00:12:57,460 Will live on surfaces of electrodes, metal electrodes. 231 00:12:57,460 --> 00:13:00,640 And they will eat things like sugar and organics. 232 00:13:00,640 --> 00:13:03,640 And they will respirate electrons 233 00:13:03,640 --> 00:13:05,640 into the metal electrodes. 234 00:13:05,640 --> 00:13:09,530 So the electrons go in and you create an electric current 235 00:13:09,530 --> 00:13:12,120 and you can do-- have a fuel cell. 236 00:13:12,120 --> 00:13:14,870 So it's a way of generating an electric current. 237 00:13:14,870 --> 00:13:17,980 And then people have other ideas that some like some microbes 238 00:13:17,980 --> 00:13:20,690 live in electrodes, other microbes like moorella 239 00:13:20,690 --> 00:13:24,480 thermoaceticum, they live on carbon dioxide. 240 00:13:24,480 --> 00:13:27,530 They convert carbon dioxide to things like acetyl CoA, which 241 00:13:27,530 --> 00:13:28,490 is a fuel. 242 00:13:28,490 --> 00:13:31,910 So you throw those in there and then you like pump in CO2 243 00:13:31,910 --> 00:13:33,890 and you make electric current. 244 00:13:33,890 --> 00:13:36,980 So there's a lot of great ideas of using microbes. 245 00:13:36,980 --> 00:13:39,560 So some of my research relates to understanding 246 00:13:39,560 --> 00:13:41,252 the fundamentals of these pathways. 247 00:13:41,252 --> 00:13:42,710 And every once in a while I venture 248 00:13:42,710 --> 00:13:45,920 to hear one of these big energy talks 249 00:13:45,920 --> 00:13:49,580 about how is the development coming of these ideas. 250 00:13:49,580 --> 00:13:53,480 And so I heard one talk and I was listening I wanted to know 251 00:13:53,480 --> 00:13:54,727 is this really working? 252 00:13:54,727 --> 00:13:56,060 Are people getting this to work? 253 00:13:56,060 --> 00:13:57,300 What's the status? 254 00:13:57,300 --> 00:13:59,760 And they talked about a lot of technical difficulties 255 00:13:59,760 --> 00:14:01,870 of getting it to work, but they had 256 00:14:01,870 --> 00:14:04,520 discovered something amazing. 257 00:14:04,520 --> 00:14:08,970 Something that might really change the ability to do this. 258 00:14:08,970 --> 00:14:11,340 And it turned out that when you're 259 00:14:11,340 --> 00:14:13,280 generating all this negative charge 260 00:14:13,280 --> 00:14:16,490 you need to have positive charge around, like H plus, 261 00:14:16,490 --> 00:14:17,670 for example. 262 00:14:17,670 --> 00:14:20,150 And so you need to have protons, but it 263 00:14:20,150 --> 00:14:23,020 mattered, the concentration of those protons. 264 00:14:23,020 --> 00:14:26,060 And they kind of had to be kept constant 265 00:14:26,060 --> 00:14:29,230 and they discovered buffers. 266 00:14:32,170 --> 00:14:33,510 Fifty minutes of my life. 267 00:14:33,510 --> 00:14:35,330 This was a talk at MIT. 268 00:14:35,330 --> 00:14:40,320 The person discovered buffers. 269 00:14:40,320 --> 00:14:43,200 I just felt like writing a letter to Washington saying, 270 00:14:43,200 --> 00:14:45,690 can I have their research dollars because I already 271 00:14:45,690 --> 00:14:47,440 know about buffers. 272 00:14:47,440 --> 00:14:49,280 I don't need to do a big research 273 00:14:49,280 --> 00:14:53,760 project to discover that buffers keep things at constant pH. 274 00:14:53,760 --> 00:14:56,130 And that when you're using living organisms 275 00:14:56,130 --> 00:15:00,000 and doing experiments and doing reactions, pH matters. 276 00:15:00,000 --> 00:15:01,150 I already knew that. 277 00:15:01,150 --> 00:15:04,480 Everyone who takes 5.111 already knows that. 278 00:15:04,480 --> 00:15:07,070 And so this is part of the reason I'm teaching you this. 279 00:15:07,070 --> 00:15:10,884 Because some of you will not go on and study chemistry. 280 00:15:10,884 --> 00:15:12,050 You'll be doing engineering. 281 00:15:12,050 --> 00:15:13,300 You'll be doing alternative energy. 282 00:15:13,300 --> 00:15:14,674 Doing all sorts of things and you 283 00:15:14,674 --> 00:15:17,380 can be the person in the room that raises your hand to go, 284 00:15:17,380 --> 00:15:19,480 it's called a buffer. 285 00:15:19,480 --> 00:15:24,310 And that is going to change the speed of that research. 286 00:15:24,310 --> 00:15:25,920 They're just like right away they're 287 00:15:25,920 --> 00:15:27,540 going to be using buffers. 288 00:15:27,540 --> 00:15:29,350 And when you do Problem Set 7, you're 289 00:15:29,350 --> 00:15:30,860 going to know how to make a buffer. 290 00:15:30,860 --> 00:15:32,980 So you cannot only raise your hand and say, 291 00:15:32,980 --> 00:15:36,270 it's called the buffer, you can tell people how to make 292 00:15:36,270 --> 00:15:39,160 a buffer to use in their experiments. 293 00:15:39,160 --> 00:15:42,500 So this is why the material in teaching was really important. 294 00:15:42,500 --> 00:15:44,410 I go out there and I hear these talks 295 00:15:44,410 --> 00:15:47,120 of people who are trying to do these innovative things, 296 00:15:47,120 --> 00:15:49,260 but they're lacking the fundamental chemistry 297 00:15:49,260 --> 00:15:51,780 knowledge that's hindering their ability to do it. 298 00:15:51,780 --> 00:15:54,040 So I want all of you to have this knowledge 299 00:15:54,040 --> 00:15:55,640 whether you study chemistry anymore 300 00:15:55,640 --> 00:15:58,270 or not, so that you can be the person who 301 00:15:58,270 --> 00:16:00,770 brings the case of chemical principles 302 00:16:00,770 --> 00:16:02,710 into the design process. 303 00:16:02,710 --> 00:16:04,842 And I care a lot about alternative energy. 304 00:16:04,842 --> 00:16:06,800 It's going to be super important in the future. 305 00:16:06,800 --> 00:16:09,580 I care that we don't destroy our planet going 306 00:16:09,580 --> 00:16:11,350 after some forms of energy. 307 00:16:11,350 --> 00:16:14,390 And this idea of using microbes is a great one, 308 00:16:14,390 --> 00:16:16,230 but we just need people who know how 309 00:16:16,230 --> 00:16:19,140 to work with them to get it to go right. 310 00:16:19,140 --> 00:16:21,380 All right, so buffers. 311 00:16:21,380 --> 00:16:22,850 What are buffers? 312 00:16:22,850 --> 00:16:26,620 You've got to know about buffers. 313 00:16:26,620 --> 00:16:28,940 A buffer consists, an acid buffer, 314 00:16:28,940 --> 00:16:32,030 consists of a weak acid in its conjugate base. 315 00:16:32,030 --> 00:16:34,420 They're often supplied as a salt, 316 00:16:34,420 --> 00:16:37,450 which means you have a counter ion that comes with it, 317 00:16:37,450 --> 00:16:40,620 like sodium plus or Cl-. 318 00:16:40,620 --> 00:16:44,400 And it buffers on the acidic side of neutral. 319 00:16:44,400 --> 00:16:48,050 A base buffer or a basic buffer has a weak base 320 00:16:48,050 --> 00:16:51,880 in its conjugate acid, often supplied as a salt, 321 00:16:51,880 --> 00:16:55,340 and it buffers on the basic side of neutral. 322 00:16:55,340 --> 00:17:01,030 So the key to a buffer is that it has a conjugate acid. 323 00:17:01,030 --> 00:17:02,770 It has NHA. 324 00:17:02,770 --> 00:17:05,410 But it also has the conjugate base. 325 00:17:05,410 --> 00:17:07,400 It has A-. 326 00:17:07,400 --> 00:17:11,770 If it just has HA, it's a weak acid. 327 00:17:11,770 --> 00:17:13,530 That's not a buffer, that's a weak acid. 328 00:17:13,530 --> 00:17:18,930 If you just have A-, you just have your base. 329 00:17:18,930 --> 00:17:20,410 That's not a buffer. 330 00:17:20,410 --> 00:17:27,869 You need to be both an acid and a base, a base and an acid, 331 00:17:27,869 --> 00:17:31,990 an acid and a base. 332 00:17:31,990 --> 00:17:34,380 On the exam people do buffer problems 333 00:17:34,380 --> 00:17:36,730 and they just have one, they don't have the other. 334 00:17:36,730 --> 00:17:38,290 That's not a buffer. 335 00:17:38,290 --> 00:17:40,120 And I'm going to write on your little exam, 336 00:17:40,120 --> 00:17:42,410 remember me twirling around the room. 337 00:17:42,410 --> 00:17:44,300 It was an acid and a base. 338 00:17:44,300 --> 00:17:45,930 I don't think you can ever forget this. 339 00:17:45,930 --> 00:17:49,050 This will be in your brain forever. 340 00:17:49,050 --> 00:17:51,660 Buffer has an acid and a conjugate base, 341 00:17:51,660 --> 00:17:54,480 a conjugate base and a conjugate acid. 342 00:17:54,480 --> 00:17:55,985 OK, let's look at some examples. 343 00:17:59,010 --> 00:18:04,230 So if you mix acetic acid and acetate salt, 344 00:18:04,230 --> 00:18:08,470 it's conjugate base supplied in the form of a salt. 345 00:18:08,470 --> 00:18:10,690 So here we have the acetic acid. 346 00:18:10,690 --> 00:18:13,980 Here we have the acetate, the conjugate base 347 00:18:13,980 --> 00:18:15,230 often will be supplied. 348 00:18:15,230 --> 00:18:18,550 There are probably going to be some NA around. 349 00:18:18,550 --> 00:18:20,660 And you get this dynamic equilibrium. 350 00:18:20,660 --> 00:18:25,350 It goes forward, it goes back, it goes forward, it goes back. 351 00:18:25,350 --> 00:18:28,340 So when you have this, if you added 352 00:18:28,340 --> 00:18:34,420 a strong acid to this solution, you put in more acid into this. 353 00:18:34,420 --> 00:18:38,620 And you had equal amounts of this and this, 354 00:18:38,620 --> 00:18:40,390 then you're conjugate base is going 355 00:18:40,390 --> 00:18:43,680 to react with the strong acid. 356 00:18:43,680 --> 00:18:48,680 And you're going to go in the back direction. 357 00:18:48,680 --> 00:18:52,640 And you're going to neutralize that acid that was added 358 00:18:52,640 --> 00:18:54,930 and the pH should stay more or less the same 359 00:18:54,930 --> 00:18:58,840 if you did a good job making your buffer. 360 00:18:58,840 --> 00:19:04,000 So the buffer can respond to the changes in pH 361 00:19:04,000 --> 00:19:06,450 by shifts in different directions 362 00:19:06,450 --> 00:19:10,930 to use up the extra added acid or the other way, 363 00:19:10,930 --> 00:19:12,740 the extra added base. 364 00:19:12,740 --> 00:19:17,870 So in this case, the acid that's added is effectively removed 365 00:19:17,870 --> 00:19:21,570 and the pH stays the same. 366 00:19:21,570 --> 00:19:26,830 So a buffer can respond to add acid. 367 00:19:26,830 --> 00:19:30,160 A buffer can also respond to added base. 368 00:19:30,160 --> 00:19:33,200 If you add a base, a strong base or any kind 369 00:19:33,200 --> 00:19:37,250 of base, and in this case hydroxide ion, 370 00:19:37,250 --> 00:19:40,640 would then remove a proton from the weak acid 371 00:19:40,640 --> 00:19:44,300 from the acetic acid forming water and some more 372 00:19:44,300 --> 00:19:46,890 of the conjugate base. 373 00:19:46,890 --> 00:19:50,510 And it's effectively removed as well 374 00:19:50,510 --> 00:19:54,360 and those OH ions are effectively removed. 375 00:19:54,360 --> 00:19:56,930 And again, if you did a good job designing this buffer, 376 00:19:56,930 --> 00:19:59,490 the pH should stay constant. 377 00:19:59,490 --> 00:20:01,830 So that's why you need the conjugate acid 378 00:20:01,830 --> 00:20:03,370 and the conjugate base. 379 00:20:03,370 --> 00:20:06,980 You need the acid to react with the base that's added 380 00:20:06,980 --> 00:20:08,560 and keep the pH constant. 381 00:20:08,560 --> 00:20:11,660 You need the base to react with the acid that's added 382 00:20:11,660 --> 00:20:13,490 and keep the pH constant. 383 00:20:13,490 --> 00:20:18,000 If you just have one, it's not going to be a buffer. 384 00:20:18,000 --> 00:20:19,020 So buffer action. 385 00:20:19,020 --> 00:20:24,350 Again, weak acid transfers, its protons, it's H plus, to OH 386 00:20:24,350 --> 00:20:26,440 supplied by the base. 387 00:20:26,440 --> 00:20:30,910 So here we have added base and it's 388 00:20:30,910 --> 00:20:36,510 going to be reacted with the acid to be neutralized. 389 00:20:36,510 --> 00:20:40,621 The conjugate base, A-, can accept protons from a supplied 390 00:20:40,621 --> 00:20:41,120 acid. 391 00:20:41,120 --> 00:20:43,730 If you add an acid, it will take those 392 00:20:43,730 --> 00:20:46,390 and will neutralize the acid that's added. 393 00:20:49,000 --> 00:20:53,880 So let's think about what type of acids and conjugate bases 394 00:20:53,880 --> 00:20:56,470 are going to work to make a good buffer. 395 00:20:56,470 --> 00:20:59,060 A strong acid in its conjugate base 396 00:20:59,060 --> 00:21:01,060 does not make a good buffer. 397 00:21:01,060 --> 00:21:02,270 Why don't you tell me why. 398 00:21:28,790 --> 00:21:32,000 Yeah, so 67%. 399 00:21:32,000 --> 00:21:37,970 So it's really about the fact that the conjugate base 400 00:21:37,970 --> 00:21:40,640 of a strong acid is really not a base at all. 401 00:21:40,640 --> 00:21:42,510 It's ineffective as a base. 402 00:21:42,510 --> 00:21:45,060 And that's why it's not going to work. 403 00:21:45,060 --> 00:21:48,360 Buffer solution has to be in a dynamic equilibrium. 404 00:21:48,360 --> 00:21:50,950 You have to be able to push it both ways, otherwise, 405 00:21:50,950 --> 00:21:52,340 it's not going to work. 406 00:21:52,340 --> 00:21:57,580 So a conjugate base of a strong acid is ineffective as a base. 407 00:21:57,580 --> 00:22:00,710 And so if you added acid, it won't be neutralized 408 00:22:00,710 --> 00:22:02,250 and the pH will change. 409 00:22:02,250 --> 00:22:06,370 So you need to have something that works as a conjugate base. 410 00:22:06,370 --> 00:22:08,650 Something that's able to interact 411 00:22:08,650 --> 00:22:12,010 with the hydro ion or the acid that's added and take 412 00:22:12,010 --> 00:22:14,100 its proton to neutralize it. 413 00:22:14,100 --> 00:22:17,130 If this doesn't happen, the pH will be affected. 414 00:22:17,130 --> 00:22:20,880 So you need to have weak acids and weak bases in your buffers. 415 00:22:20,880 --> 00:22:24,410 They could be moderately weak or very weak or whatever, 416 00:22:24,410 --> 00:22:27,520 but you have to have a conjugate set that's 417 00:22:27,520 --> 00:22:31,100 going to be able to push both directions where the acid can 418 00:22:31,100 --> 00:22:34,000 interact with the added base and the base can interact 419 00:22:34,000 --> 00:22:35,810 with the added acid. 420 00:22:35,810 --> 00:22:37,780 All right, so all of this is really 421 00:22:37,780 --> 00:22:41,490 the same for a basic buffer. 422 00:22:41,490 --> 00:22:45,420 So we can look at an example here of ammonia 423 00:22:45,420 --> 00:22:49,720 plus water going to ammonium ion and hydroxide ion. 424 00:22:49,720 --> 00:22:52,180 And we can think about the same things. 425 00:22:52,180 --> 00:22:57,050 When you add a strong acid, the base will accept the protons 426 00:22:57,050 --> 00:23:01,640 and make more of your conjugate acid. 427 00:23:01,640 --> 00:23:04,660 When a strong base is added, the conjugate acid 428 00:23:04,660 --> 00:23:06,390 will donate its proton. 429 00:23:06,390 --> 00:23:11,050 And it will form NH3 again, that conjugate base, and water 430 00:23:11,050 --> 00:23:13,740 and the pH will stay the same. 431 00:23:13,740 --> 00:23:16,900 So it's the same idea, the only real difference 432 00:23:16,900 --> 00:23:19,460 between an acidic buffer and a basic buffer 433 00:23:19,460 --> 00:23:24,120 is whether it pH's in the acidic range or in the basic range. 434 00:23:24,120 --> 00:23:26,580 But the buffers work the same way. 435 00:23:26,580 --> 00:23:31,470 So I can redraw this picture to have our different symbols 436 00:23:31,470 --> 00:23:33,950 on it, but the idea is the same. 437 00:23:33,950 --> 00:23:37,910 The weak base will take the proton supplied 438 00:23:37,910 --> 00:23:41,640 by the acid and the conjugate acid 439 00:23:41,640 --> 00:23:50,410 BH is going to probate the OHH- and again keep the pH constant. 440 00:23:50,410 --> 00:23:53,530 So the idea again of the buffer then 441 00:23:53,530 --> 00:23:57,590 is that a buffer is a mixture of weak conjugate acids and bases. 442 00:23:57,590 --> 00:23:59,780 Again, they're weak because its partner 443 00:23:59,780 --> 00:24:02,335 needs to be effective as an asset or a base. 444 00:24:02,335 --> 00:24:04,350 It can't be ineffective. 445 00:24:04,350 --> 00:24:07,380 Weak acid base mixtures that stabilize pH 446 00:24:07,380 --> 00:24:12,170 by providing a source or a sink for protons. 447 00:24:12,170 --> 00:24:15,260 It either adds protons or takes protons away. 448 00:24:15,260 --> 00:24:17,370 It can respond to add acid. 449 00:24:17,370 --> 00:24:20,580 It can respond to add base. 450 00:24:20,580 --> 00:24:26,730 So again, pH is important and buffering is important. 451 00:24:26,730 --> 00:24:30,150 Our body has its own buffering system. 452 00:24:30,150 --> 00:24:34,170 So we have carbonic acid and bicarbonate buffering 453 00:24:34,170 --> 00:24:35,990 agents in our blood. 454 00:24:35,990 --> 00:24:39,300 And our blood has to be kept in a sort of neutral range 455 00:24:39,300 --> 00:24:40,340 over here. 456 00:24:40,340 --> 00:24:44,710 If it gets too acidic, that is very unhealthy for us, 457 00:24:44,710 --> 00:24:47,250 leading to some pretty severe symptoms 458 00:24:47,250 --> 00:24:49,450 and too much leads to death. 459 00:24:49,450 --> 00:24:51,980 If it's too basic, that's really bad. 460 00:24:51,980 --> 00:24:54,170 Also, we have death and notice these are not 461 00:24:54,170 --> 00:24:56,210 all that far away. 462 00:24:56,210 --> 00:24:58,750 There are a number of different medical conditions 463 00:24:58,750 --> 00:25:01,640 that can affect the pH, diabetes, 464 00:25:01,640 --> 00:25:03,540 there are metabolic diseases. 465 00:25:03,540 --> 00:25:08,890 Something else that is under your control, hydration, 466 00:25:08,890 --> 00:25:11,600 you need to drink enough water. 467 00:25:11,600 --> 00:25:15,580 And as the weather gets colder in Boston, 468 00:25:15,580 --> 00:25:18,195 people so often stop drinking as much water 469 00:25:18,195 --> 00:25:20,210 or start drinking like hot beverages, 470 00:25:20,210 --> 00:25:21,930 which don't hydrate as well. 471 00:25:21,930 --> 00:25:23,440 So keep hydrated. 472 00:25:23,440 --> 00:25:27,000 If you're not hydrated, if it's gets really bad, 473 00:25:27,000 --> 00:25:30,090 it can start affecting the pH of your blood, which is really not 474 00:25:30,090 --> 00:25:30,880 good. 475 00:25:30,880 --> 00:25:34,660 So buffering, very important. 476 00:25:34,660 --> 00:25:38,930 All right, so let's do a sample buffer problem. 477 00:25:38,930 --> 00:25:41,500 So sample buffer problem-- and this is important. 478 00:25:41,500 --> 00:25:43,960 I actually know of at least one professor 479 00:25:43,960 --> 00:25:45,900 that if you want to do a UROP with them 480 00:25:45,900 --> 00:25:47,510 or come to their office and they're 481 00:25:47,510 --> 00:25:52,870 going to say, write down how you would design a buffer for me 482 00:25:52,870 --> 00:25:55,190 and do those calculations. 483 00:25:55,190 --> 00:25:58,220 So I can tell you later who that is, maybe I won't, I 484 00:25:58,220 --> 00:25:59,150 don't know. 485 00:25:59,150 --> 00:26:01,280 But this is one of the tasks that someone 486 00:26:01,280 --> 00:26:04,550 uses to see if they want you as a UROP. 487 00:26:04,550 --> 00:26:09,480 OK, so here we have one mol of formic acid 488 00:26:09,480 --> 00:26:13,430 and 0.5 mols of the conjugate base supplied 489 00:26:13,430 --> 00:26:16,570 in the form of a salt with sodium ions. 490 00:26:16,570 --> 00:26:18,820 And those are added to water and diluted 491 00:26:18,820 --> 00:26:22,760 to a total final concentration of one liter. 492 00:26:22,760 --> 00:26:27,920 And you're given the Ka and told to calculate the pH. 493 00:26:27,920 --> 00:26:32,660 So the first step is to write out the equation. 494 00:26:32,660 --> 00:26:36,370 So we have an acid and water going 495 00:26:36,370 --> 00:26:41,540 to hydronium ions and a conjugate base over here. 496 00:26:41,540 --> 00:26:44,580 And we want to think about what's there now 497 00:26:44,580 --> 00:26:48,290 and what's at equilibrium. 498 00:26:48,290 --> 00:26:52,880 So initial molarity, change in molarity and your equilibrium 499 00:26:52,880 --> 00:26:54,070 molarity. 500 00:26:54,070 --> 00:26:57,770 So it's important this word molarity. 501 00:26:57,770 --> 00:27:02,030 Don't put mols in here, put molarity in here. 502 00:27:02,030 --> 00:27:03,880 But I made it really easy for you, 503 00:27:03,880 --> 00:27:08,360 because we have one mol and one liter so the molarity is 1. 504 00:27:08,360 --> 00:27:12,260 0.5 mols and one liter, the molarity is 0.5. 505 00:27:12,260 --> 00:27:14,120 And this is also really important. 506 00:27:14,120 --> 00:27:17,080 You're so used to only putting things here. 507 00:27:17,080 --> 00:27:18,990 When it's a buffer problem, you've 508 00:27:18,990 --> 00:27:21,310 got to put something here, too. 509 00:27:21,310 --> 00:27:25,730 So you're adding the acid with the conjugate base 510 00:27:25,730 --> 00:27:27,210 at the same time. 511 00:27:27,210 --> 00:27:28,750 This is not zero. 512 00:27:28,750 --> 00:27:30,940 This is 0.5. 513 00:27:30,940 --> 00:27:34,800 So now as this dynamic equilibrium goes 514 00:27:34,800 --> 00:27:39,410 you're losing some of this and gaining some of these. 515 00:27:39,410 --> 00:27:44,020 So at equilibrium we have one molar minus x. 516 00:27:44,020 --> 00:27:46,680 X is our hydronium ion concentration, 517 00:27:46,680 --> 00:27:49,740 which is what we want to know to calculate pH. 518 00:27:49,740 --> 00:27:57,250 And our conjugate base our formic ion is 0.5 plus x. 519 00:27:57,250 --> 00:28:01,990 We're given a Ka and we've written this expression as acid 520 00:28:01,990 --> 00:28:04,410 in water so we can use Ka. 521 00:28:04,410 --> 00:28:08,800 The equilibrium constant is for the expression as written 522 00:28:08,800 --> 00:28:16,310 so we can put that in and we can fill the rest out. 523 00:28:16,310 --> 00:28:17,420 We have our products. 524 00:28:17,420 --> 00:28:20,120 We have the concentration of hydronium ion 525 00:28:20,120 --> 00:28:24,410 times the concentration of formic ion, the conjugate base, 526 00:28:24,410 --> 00:28:27,910 over the concentration of our formic acid, 527 00:28:27,910 --> 00:28:30,430 and again water does not appear in the expression 528 00:28:30,430 --> 00:28:32,420 because it's the solvent. 529 00:28:32,420 --> 00:28:35,740 Hydronium ion concentration is x. 530 00:28:35,740 --> 00:28:37,840 The conjugate base concentration is 531 00:28:37,840 --> 00:28:42,640 0.5 plus x and the conjugate acid concentration 532 00:28:42,640 --> 00:28:46,480 is 1 minus x. 533 00:28:46,480 --> 00:28:51,820 So we can now try that assumption that x is small 534 00:28:51,820 --> 00:28:54,850 or we can use the quadratic equation, 535 00:28:54,850 --> 00:28:58,032 but why don't you just try that assumption. 536 00:28:58,032 --> 00:28:59,490 I'm going to take this expression-- 537 00:28:59,490 --> 00:29:02,820 if you haven't written it all down yet, put it right up here. 538 00:29:02,820 --> 00:29:04,450 And now with the clicker why don't you 539 00:29:04,450 --> 00:29:08,030 tell me, if we use the approximation 540 00:29:08,030 --> 00:29:11,400 that x is small compared to 1 and 0.5, what 541 00:29:11,400 --> 00:29:12,980 does this simplify to? 542 00:29:26,270 --> 00:29:27,420 All right, 10 more seconds. 543 00:29:43,470 --> 00:29:46,720 So 70 something percent. 544 00:29:46,720 --> 00:29:51,020 Yep, so let's take a look at that. 545 00:29:51,020 --> 00:29:52,660 So here we're making the assumption 546 00:29:52,660 --> 00:29:56,560 that x is small compared to 0.5, compared to 1. 547 00:29:56,560 --> 00:30:01,370 So that means that we drop the plus x from the 0.5 term 548 00:30:01,370 --> 00:30:04,620 and drop the minus x from the one term. 549 00:30:04,620 --> 00:30:07,230 So we're saying that these are going 550 00:30:07,230 --> 00:30:09,190 to be small enough that it's still going 551 00:30:09,190 --> 00:30:11,670 to be pretty much 0.5 and 1. 552 00:30:11,670 --> 00:30:14,640 And then we have to test that in a minute. 553 00:30:14,640 --> 00:30:17,800 So if we use this, we can calculate x 554 00:30:17,800 --> 00:30:22,280 and it comes out to 3.54 times 10 to the minus 4, 555 00:30:22,280 --> 00:30:24,660 and then we can check the assumption. 556 00:30:24,660 --> 00:30:28,300 And before you told me how to check assumptions. 557 00:30:28,300 --> 00:30:31,140 And that was I take x and I'll divide it 558 00:30:31,140 --> 00:30:37,150 by 0.5, which is the smaller of these two, and times by 100% 559 00:30:37,150 --> 00:30:41,790 and we get 0.69%, which is less than 5% 560 00:30:41,790 --> 00:30:44,590 so the assumption is OK. 561 00:30:44,590 --> 00:30:47,020 And we don't have to check it against 1, 562 00:30:47,020 --> 00:30:50,060 because if the assumption x is small compared to the smaller 563 00:30:50,060 --> 00:30:53,380 number is valid, it will also be valid for the bigger number. 564 00:30:53,380 --> 00:30:56,280 So you only check it for the smaller number 565 00:30:56,280 --> 00:30:59,750 and then we can solve. 566 00:30:59,750 --> 00:31:02,260 So we have to think about-- we solved for x. 567 00:31:02,260 --> 00:31:03,160 We're happy with x. 568 00:31:03,160 --> 00:31:04,470 Our assumption was OK. 569 00:31:04,470 --> 00:31:05,845 But then we have to remember what 570 00:31:05,845 --> 00:31:11,030 x is and x is the hydronium ion concentration 571 00:31:11,030 --> 00:31:13,640 and you always want to make sure to think about this, 572 00:31:13,640 --> 00:31:16,980 because if it's hydroxide ion that's different. 573 00:31:16,980 --> 00:31:18,620 So we can calculate pH. 574 00:31:18,620 --> 00:31:24,110 pH is minus log of 3.54 times 10 to the minus 4, 575 00:31:24,110 --> 00:31:29,120 which is equal to pH of 3.45. 576 00:31:29,120 --> 00:31:31,710 And here we have two significant figures 577 00:31:31,710 --> 00:31:34,010 after the decimal point, because the volume 578 00:31:34,010 --> 00:31:36,320 we had before only had two significant figures. 579 00:31:36,320 --> 00:31:39,420 Everything else had three, but the volume was just 1.0. 580 00:31:39,420 --> 00:31:42,340 So that is our answer there. 581 00:31:42,340 --> 00:31:48,380 All right, so I'm going to just briefly start 582 00:31:48,380 --> 00:31:52,120 what happens when you-- actually we'll do-- this 583 00:31:52,120 --> 00:31:54,450 is how you would design the buffer for this. 584 00:31:54,450 --> 00:31:56,850 Next time we'll see what happens if we stress 585 00:31:56,850 --> 00:32:00,760 the buffer we design by adding acid to it 586 00:32:00,760 --> 00:32:04,390 and see what the new result is. 587 00:32:04,390 --> 00:32:07,450 All right, so today we're going to finish lecture 22. 588 00:32:07,450 --> 00:32:10,691 So take out those lecture notes on acids and bases. 589 00:32:10,691 --> 00:32:12,190 And when we're done with that, we'll 590 00:32:12,190 --> 00:32:14,240 just continue with acids and bases. 591 00:32:14,240 --> 00:32:16,360 And we're going to continue with some acids 592 00:32:16,360 --> 00:32:18,190 and bases on Wednesday. 593 00:32:18,190 --> 00:32:20,610 We should finish on Wednesday and move on 594 00:32:20,610 --> 00:32:26,710 to oxidation reduction and that will end exam three material. 595 00:32:26,710 --> 00:32:30,080 So exam three is sneaking up on us 596 00:32:30,080 --> 00:32:32,780 and it'll have thermodynamics, chemical equilibrium, 597 00:32:32,780 --> 00:32:34,518 solubility, and acid base. 598 00:32:37,680 --> 00:32:40,230 So you should have already filled 599 00:32:40,230 --> 00:32:42,950 in your handout with all the information. 600 00:32:42,950 --> 00:32:47,590 The parts that you were filling in are in bold here, 601 00:32:47,590 --> 00:32:51,080 but I just want to remind you of what we were talking about. 602 00:32:51,080 --> 00:32:53,580 So we had a sample buffer problem. 603 00:32:53,580 --> 00:32:56,720 We had one mol of the conjugate acid and a half 604 00:32:56,720 --> 00:33:01,220 a mol of its conjugate base supplied in the form of a salt, 605 00:33:01,220 --> 00:33:05,240 were put in water and diluted into one liter. 606 00:33:05,240 --> 00:33:10,510 So we calculated what the hydronium ion concentration 607 00:33:10,510 --> 00:33:14,230 would be and therefore, the pH, using information 608 00:33:14,230 --> 00:33:17,230 that was given to us about the Ka, 609 00:33:17,230 --> 00:33:20,140 the acid ionization constant. 610 00:33:20,140 --> 00:33:24,470 So we got a pH of 3.45. 611 00:33:24,470 --> 00:33:27,820 Someone asked me after class, but isn't this conjugate base 612 00:33:27,820 --> 00:33:33,080 also reacting with water and forming some of this weak acid. 613 00:33:33,080 --> 00:33:35,550 And the answer is yes. 614 00:33:35,550 --> 00:33:38,620 That we are writing this as a problem of acid 615 00:33:38,620 --> 00:33:41,310 in water, because we can use the Ka. 616 00:33:41,310 --> 00:33:44,854 But we could've also written it the other way. 617 00:33:44,854 --> 00:33:46,770 And written it, and this is not in your notes, 618 00:33:46,770 --> 00:33:50,530 but I just want to let you know, that the weak base and water 619 00:33:50,530 --> 00:33:53,910 could be written that way forming a conjugate acid 620 00:33:53,910 --> 00:33:56,180 and hydroxide ions. 621 00:33:56,180 --> 00:34:01,340 And so then it would be 0.5 minus x and 1 plus x and x. 622 00:34:01,340 --> 00:34:05,240 Use Kb then, because it's a base in water to solve for x 623 00:34:05,240 --> 00:34:10,840 and calculate pH from POH and if you try it, you get 3.45. 624 00:34:10,840 --> 00:34:13,690 So you can do these problems either way. 625 00:34:13,690 --> 00:34:15,909 You can think about them as a weak base 626 00:34:15,909 --> 00:34:17,889 problem or a weak acid problem. 627 00:34:17,889 --> 00:34:21,370 You just have to remember that this is not zero 628 00:34:21,370 --> 00:34:22,580 when you start. 629 00:34:22,580 --> 00:34:24,960 The conjugate has some amount. 630 00:34:24,960 --> 00:34:26,650 That's what makes it a good buffer. 631 00:34:26,650 --> 00:34:29,360 So this can work either way, but you also 632 00:34:29,360 --> 00:34:32,239 want to remember if you write it as an acid in water, use Ka. 633 00:34:32,239 --> 00:34:34,650 If you write it as a base in water, use Kb. 634 00:34:34,650 --> 00:34:36,330 Because those equilibrium constants 635 00:34:36,330 --> 00:34:40,100 are telling you about those ratios of the conjugate acid 636 00:34:40,100 --> 00:34:42,400 and the conjugate base at equilibrium. 637 00:34:42,400 --> 00:34:46,239 So within the Ka and the Kb is information 638 00:34:46,239 --> 00:34:50,389 that you need about this dynamic chemical equilibrium. 639 00:34:50,389 --> 00:34:54,520 So both of these ways work and by the end of, sort of halfway 640 00:34:54,520 --> 00:34:57,240 through today's lecture, you'll see yet another way 641 00:34:57,240 --> 00:34:59,130 to solve buffer problems. 642 00:34:59,130 --> 00:35:01,960 And probably will be for most of you, the preferred way 643 00:35:01,960 --> 00:35:03,060 to do it. 644 00:35:03,060 --> 00:35:04,880 So there's a number of different ways 645 00:35:04,880 --> 00:35:07,500 which makes grading the exam lots of fun. 646 00:35:07,500 --> 00:35:10,470 And we have option 1, option 2, option 3-- 647 00:35:10,470 --> 00:35:14,400 to make sure that all the options are possibly 648 00:35:14,400 --> 00:35:16,100 accounted for. 649 00:35:16,100 --> 00:35:18,780 All right, so the purpose of a buffer 650 00:35:18,780 --> 00:35:21,500 is to keep the pH pretty much constant. 651 00:35:21,500 --> 00:35:24,140 So you can add a strong acid to it 652 00:35:24,140 --> 00:35:26,640 and the acid will be neutralized, 653 00:35:26,640 --> 00:35:27,840 keeping the pH constant. 654 00:35:27,840 --> 00:35:29,720 You can add a strong base to it and the base 655 00:35:29,720 --> 00:35:32,500 will be neutralized, keeping the pH pretty constant. 656 00:35:32,500 --> 00:35:36,590 So what you want in a buffer is to have a weak acid conjugate 657 00:35:36,590 --> 00:35:40,570 base pairing that allows it to be a source 658 00:35:40,570 --> 00:35:43,790 or sink for a strong acid or a strong base 659 00:35:43,790 --> 00:35:45,710 to keep the pH constant. 660 00:35:45,710 --> 00:35:47,370 So now in this problem we're asked 661 00:35:47,370 --> 00:35:49,940 to calculate what would happen to the pH 662 00:35:49,940 --> 00:35:53,360 if 0.1 mols of the strong acid had been included 663 00:35:53,360 --> 00:35:57,380 in our 1 liter solution. 664 00:35:57,380 --> 00:36:01,350 And because 0.1 mols of our strong acid 665 00:36:01,350 --> 00:36:04,480 would react with equal number of mols, 666 00:36:04,480 --> 00:36:06,140 all of that conjugate base, that's 667 00:36:06,140 --> 00:36:08,310 what the conjugate base does in a buffer, 668 00:36:08,310 --> 00:36:11,970 it reacts with the strong acid that is supplied. 669 00:36:11,970 --> 00:36:17,150 And it will form equal mols then of the conjugate, 670 00:36:17,150 --> 00:36:19,050 its conjugate acid. 671 00:36:19,050 --> 00:36:22,070 So we have to do some subtraction. 672 00:36:22,070 --> 00:36:25,930 So for the conjugate acid, we had 0.5 mols. 673 00:36:29,080 --> 00:36:33,080 For the conjugate base, we had 0.5 mols. 674 00:36:33,080 --> 00:36:36,430 We used up one of that, 0.1 of that, 675 00:36:36,430 --> 00:36:40,800 reacting with the strong acid and we have 0.4 left. 676 00:36:40,800 --> 00:36:46,150 So we have a new concentration for our conjugate base, 0.4 677 00:36:46,150 --> 00:36:51,300 divided by 1, keep the math simple, 0.4 molar. 678 00:36:51,300 --> 00:36:55,320 So for our conjugate acid, we had one mol to begin with, 679 00:36:55,320 --> 00:37:00,260 but we formed 0.1 more when that strong acid was added. 680 00:37:00,260 --> 00:37:06,400 So now we have 1.1 mols and a new molarity. 681 00:37:06,400 --> 00:37:11,680 So now we have to go back and we can use this expression again 682 00:37:11,680 --> 00:37:16,290 and determine what x is now, what the hydronium 683 00:37:16,290 --> 00:37:18,490 concentration is now using Ka. 684 00:37:18,490 --> 00:37:22,290 So why don't you tell me how I would solve this. 685 00:37:22,290 --> 00:37:25,610 What is the correct expression for Ka? 686 00:37:25,610 --> 00:37:27,800 You can fill in your little table as you do it. 687 00:37:39,518 --> 00:37:41,226 All right, let's just do 10 more seconds. 688 00:37:55,570 --> 00:38:00,240 All right, so let's just fill in that table 689 00:38:00,240 --> 00:38:01,970 if you haven't already. 690 00:38:01,970 --> 00:38:06,690 So here now, we can write this as an acid and water problem. 691 00:38:06,690 --> 00:38:09,090 Again, again, you could have done the base in water 692 00:38:09,090 --> 00:38:13,010 and used Kb, but we'll just do it the same way we did before. 693 00:38:13,010 --> 00:38:16,630 So we have our acid 1.1 molar. 694 00:38:16,630 --> 00:38:18,437 Now remember, this is molarity. 695 00:38:18,437 --> 00:38:20,020 So you don't want to put mols in here, 696 00:38:20,020 --> 00:38:22,770 you want to put a concentration and some of that 697 00:38:22,770 --> 00:38:24,220 will be minus x. 698 00:38:24,220 --> 00:38:27,680 Then we have our 0.4, 0.4 plus x. 699 00:38:27,680 --> 00:38:29,370 We can fill out the Ka. 700 00:38:29,370 --> 00:38:32,220 The Ka again, weak acid and water problem. 701 00:38:32,220 --> 00:38:37,910 We have products-- hydronium ion times our conjugate base over 702 00:38:37,910 --> 00:38:44,620 our conjugate acid equals 0.400 plus x times x, these 2, 703 00:38:44,620 --> 00:38:49,490 and 1.1 minus x on the bottom. 704 00:38:49,490 --> 00:38:53,020 So you can always solve then for x, which will tell you 705 00:38:53,020 --> 00:38:57,040 about the pH by using the Ka. 706 00:38:57,040 --> 00:38:59,670 And you just have to remember molarity and remember 707 00:38:59,670 --> 00:39:04,060 these are the new molarity after the reaction has occurred. 708 00:39:04,060 --> 00:39:07,660 All right, so we can use the approximation 709 00:39:07,660 --> 00:39:13,430 that x is small compared to 1.1 and 0.4 to simplify this. 710 00:39:13,430 --> 00:39:16,310 Or we could use the quadratic equation. 711 00:39:16,310 --> 00:39:21,060 If we simplify it, we calculate x as 4.87 times 10 712 00:39:21,060 --> 00:39:22,440 to the minus 4. 713 00:39:22,440 --> 00:39:25,060 And remember again, that the approximation 714 00:39:25,060 --> 00:39:30,510 is you get rid of the plus x and the minus x here. 715 00:39:30,510 --> 00:39:32,909 And then, you have to check it with the 5% rule. 716 00:39:32,909 --> 00:39:34,700 If you did that, if you used the quadratic, 717 00:39:34,700 --> 00:39:36,030 then you don't have to check. 718 00:39:36,030 --> 00:39:38,230 But if you use the 5% rule, then you're 719 00:39:38,230 --> 00:39:43,690 asking is this number 4.87 times 10 to the minus 4 720 00:39:43,690 --> 00:39:48,210 within 5% of the smaller value, which is 0.4 and it is here. 721 00:39:48,210 --> 00:39:49,980 It's 0.12%. 722 00:39:49,980 --> 00:39:51,430 So the assumption is OK. 723 00:39:51,430 --> 00:39:52,920 That's less than 5%. 724 00:39:52,920 --> 00:39:54,790 X is small. 725 00:39:54,790 --> 00:39:58,540 And then we can solve for the pH. 726 00:39:58,540 --> 00:40:02,420 So pH is minus log of the hydronium ion concentration, 727 00:40:02,420 --> 00:40:04,720 which here is 3.31. 728 00:40:04,720 --> 00:40:07,710 This only has two significant figures after the decimal, 729 00:40:07,710 --> 00:40:09,780 because if you remember, back on the other page, 730 00:40:09,780 --> 00:40:11,360 the volume only had two. 731 00:40:11,360 --> 00:40:13,870 So actually all these numbers here really just 732 00:40:13,870 --> 00:40:15,970 have two significant figures. 733 00:40:15,970 --> 00:40:20,300 We carried an extra, but it really just had two. 734 00:40:20,300 --> 00:40:23,050 All right, so our buffer was pretty good 735 00:40:23,050 --> 00:40:27,390 so we added 0.1 mols of a very strong acid. 736 00:40:27,390 --> 00:40:29,260 And we only changed the pH. 737 00:40:29,260 --> 00:40:34,680 We only lowered it from 3.45 to 3.31. 738 00:40:34,680 --> 00:40:40,970 So that was a pretty good buffer that we had designed there. 739 00:40:40,970 --> 00:40:44,831 All right, so that is how you do a buffer problem. 740 00:40:44,831 --> 00:40:46,830 And there are other kinds of things that you can 741 00:40:46,830 --> 00:40:49,170 be asked about with buffers. 742 00:40:49,170 --> 00:40:51,490 In particular, you can also often 743 00:40:51,490 --> 00:40:54,780 be asked to design a buffer. 744 00:40:54,780 --> 00:40:56,620 And when you design a buffer, you 745 00:40:56,620 --> 00:40:59,070 must consider the following things. 746 00:40:59,070 --> 00:41:03,280 The ratio of your conjugate acid base pair 747 00:41:03,280 --> 00:41:04,940 and that they should also be weak 748 00:41:04,940 --> 00:41:10,590 conjugate acid base pairs-- the pKa and the pH. 749 00:41:10,590 --> 00:41:13,400 OK, so here comes a derivation. 750 00:41:16,330 --> 00:41:19,090 So here is our expression that should be familiar to you 751 00:41:19,090 --> 00:41:22,690 at this point, for a weak acid, HA and water, 752 00:41:22,690 --> 00:41:27,540 going to hydronium ions and our conjugate base, A-. 753 00:41:27,540 --> 00:41:30,070 It should also be familiar to you 754 00:41:30,070 --> 00:41:32,710 that you can write the equilibrium 755 00:41:32,710 --> 00:41:35,190 expression for this acid in water 756 00:41:35,190 --> 00:41:38,830 as hydronium ions concentration times the concentration, 757 00:41:38,830 --> 00:41:41,150 the conjugate base, over the concentration 758 00:41:41,150 --> 00:41:42,440 of the conjugate acid. 759 00:41:42,440 --> 00:41:44,090 Water, which is our solvent, doesn't 760 00:41:44,090 --> 00:41:46,380 appear in the expression. 761 00:41:46,380 --> 00:41:50,320 We can rearrange this expression now and solve for hydronium ion 762 00:41:50,320 --> 00:41:52,170 concentrations. 763 00:41:52,170 --> 00:41:56,320 We can take a log of both sides, and get this, 764 00:41:56,320 --> 00:42:00,140 log of hydronium ion concentration, log of Ka, 765 00:42:00,140 --> 00:42:07,900 plus log of HA over A-, multiply by negative to get this. 766 00:42:07,900 --> 00:42:10,530 And so we have minus, minus, minus, 767 00:42:10,530 --> 00:42:14,020 and minus the log of the hydronium ion concentration 768 00:42:14,020 --> 00:42:15,450 is what? 769 00:42:15,450 --> 00:42:22,300 pH minus log of Ka is pKa and this is the same as that. 770 00:42:22,300 --> 00:42:29,010 So that gives us this expression that pH equals pKa minus 771 00:42:29,010 --> 00:42:32,300 the log of HA over A-. 772 00:42:32,300 --> 00:42:35,390 Now it's important to point out that these are equilibrium 773 00:42:35,390 --> 00:42:39,870 concentrations of HA and A-, because we derived this 774 00:42:39,870 --> 00:42:44,630 expression from Ka and that's an equilibrium constant. 775 00:42:44,630 --> 00:42:47,140 So these are equilibrium expressions. 776 00:42:47,140 --> 00:42:50,430 And that makes this particular expression slightly less 777 00:42:50,430 --> 00:42:53,040 useful, because we often don't want 778 00:42:53,040 --> 00:42:55,400 to have to calculate the equilibrium concentration. 779 00:42:55,400 --> 00:42:58,200 We know how much we added and so we often want 780 00:42:58,200 --> 00:43:00,520 to use initial concentrations. 781 00:43:00,520 --> 00:43:04,370 But we can consider whether the initial concentrations 782 00:43:04,370 --> 00:43:07,530 are actually pretty similar to the equilibrium concentrations 783 00:43:07,530 --> 00:43:08,170 i.e. 784 00:43:08,170 --> 00:43:10,920 the x is small, less than 5%. 785 00:43:10,920 --> 00:43:13,830 So we know that a weak acid typically 786 00:43:13,830 --> 00:43:17,010 only loses a fraction of its protons, 787 00:43:17,010 --> 00:43:19,030 hence the definition of weak acid. 788 00:43:19,030 --> 00:43:21,880 Weak acid in water doesn't ionize that much. 789 00:43:21,880 --> 00:43:25,380 And a weak base typically only accepts a fraction 790 00:43:25,380 --> 00:43:27,070 of the protons that it could. 791 00:43:27,070 --> 00:43:28,930 It's a weak base. 792 00:43:28,930 --> 00:43:32,700 So the initial concentration is often 793 00:43:32,700 --> 00:43:36,354 approximately equal to the equilibrium concentration 794 00:43:36,354 --> 00:43:37,770 and that's what we've been finding 795 00:43:37,770 --> 00:43:38,811 in a lot of the problems. 796 00:43:38,811 --> 00:43:41,760 We've found that the 5% rule works pretty well. 797 00:43:41,760 --> 00:43:43,710 It's usually less than 5%. 798 00:43:43,710 --> 00:43:48,670 X is usually less than 5% of the initial concentration we had 799 00:43:48,670 --> 00:43:51,180 of HA or A-. 800 00:43:51,180 --> 00:43:55,530 And so therefore, we can say that the pH is in fact, 801 00:43:55,530 --> 00:44:00,760 approximately equal to the pKa minus the log of the initial 802 00:44:00,760 --> 00:44:04,950 concentration of your HA over your initial concentration 803 00:44:04,950 --> 00:44:06,330 of A-. 804 00:44:06,330 --> 00:44:10,080 And this expression is known as the Henderson-Hasselbalch 805 00:44:10,080 --> 00:44:11,420 Equation. 806 00:44:11,420 --> 00:44:18,420 And so this equation is only valid when your hydronium ion 807 00:44:18,420 --> 00:44:23,700 concentration is small compared to HA and A-, i.e., 808 00:44:23,700 --> 00:44:25,740 less than 5%. 809 00:44:25,740 --> 00:44:30,250 So this 5% rule must apply to use the Henderson-Hasselbalch 810 00:44:30,250 --> 00:44:31,450 Equation. 811 00:44:31,450 --> 00:44:33,590 The Henderson-Hasselbalch Equation 812 00:44:33,590 --> 00:44:37,240 is a great equation to use for buffer problems. 813 00:44:37,240 --> 00:44:39,140 And so, we've showed you a couple 814 00:44:39,140 --> 00:44:41,170 different ways of doing buffer problems. 815 00:44:41,170 --> 00:44:44,170 This is the final way I'm going to show you using 816 00:44:44,170 --> 00:44:46,050 the Henderson-Hasselbalch Equation, 817 00:44:46,050 --> 00:44:49,620 but you can only use it if x is small. 818 00:44:49,620 --> 00:44:51,860 But most of the time, x is going to be small, 819 00:44:51,860 --> 00:44:53,970 because buffers are only buffers when 820 00:44:53,970 --> 00:44:57,600 you have a weak acid with a conjugate weak base. 821 00:44:57,600 --> 00:45:01,090 And when you're talking about weak acids and bases, 822 00:45:01,090 --> 00:45:02,310 x is small. 823 00:45:02,310 --> 00:45:04,690 They don't ionize that much. 824 00:45:04,690 --> 00:45:07,440 And importantly, and I'll emphasize this, 825 00:45:07,440 --> 00:45:11,410 this equation only works for buffers. 826 00:45:11,410 --> 00:45:15,970 Don't apply it for just regular weak acid in water or weak base 827 00:45:15,970 --> 00:45:19,770 in water, a strong acid in water, a strong base in water-- 828 00:45:19,770 --> 00:45:22,805 it only applies for buffers. 829 00:45:25,400 --> 00:45:28,420 Buffers, buffers. 830 00:45:28,420 --> 00:45:31,700 Now I know that MIT students love equations 831 00:45:31,700 --> 00:45:33,990 and they love doing math, and so they 832 00:45:33,990 --> 00:45:36,910 try to apply this equation to every type 833 00:45:36,910 --> 00:45:39,080 of acid-base problem. 834 00:45:39,080 --> 00:45:40,830 Don't do it. 835 00:45:40,830 --> 00:45:42,715 Buffers, buffers. 836 00:45:45,530 --> 00:45:49,270 You'll remember that now, right? 837 00:45:49,270 --> 00:45:51,870 Because I'll make noises again if you don't. 838 00:45:51,870 --> 00:45:53,215 You remember that now, right? 839 00:45:55,780 --> 00:45:59,890 All right, so let's use the Henderson-Hasselbalch 840 00:45:59,890 --> 00:46:01,760 to design a buffer then. 841 00:46:01,760 --> 00:46:06,450 So say we want to design a buffer of pH 4.6. 842 00:46:06,450 --> 00:46:10,560 Say you're interviewing for a UROP position and the UROP 843 00:46:10,560 --> 00:46:14,030 supervisor, the PI says, tell me how you 844 00:46:14,030 --> 00:46:17,630 would design a buffer pH 4.6. 845 00:46:17,630 --> 00:46:20,790 And you might go to the shelf and see what was available 846 00:46:20,790 --> 00:46:25,150 and then look up what the Ka and the pKa are, 847 00:46:25,150 --> 00:46:28,350 because a buffer solution is most effective in the range 848 00:46:28,350 --> 00:46:30,860 of the pKa plus or minus 1. 849 00:46:30,860 --> 00:46:35,500 In fact, the closer the pH you want is to the pKa, 850 00:46:35,500 --> 00:46:37,460 the better the buffer is going to be. 851 00:46:37,460 --> 00:46:39,760 So we can look and see what's in the range. 852 00:46:39,760 --> 00:46:44,340 There are several that are close to 4.6 that we can choose from. 853 00:46:44,340 --> 00:46:46,370 And probably would end up choosing 854 00:46:46,370 --> 00:46:49,110 acetic acid, because that would be on the shelf, 855 00:46:49,110 --> 00:46:51,980 whereas, some of the others of these would not be there. 856 00:46:51,980 --> 00:46:53,830 All right, so acetate is going to work. 857 00:46:53,830 --> 00:46:56,480 It's a pretty common buffer. 858 00:46:56,480 --> 00:47:04,220 So we can use acetic acid with a suitable pKa of 4.75. 859 00:47:04,220 --> 00:47:07,370 Then we can use the Henderson-Hasselbalch Equation 860 00:47:07,370 --> 00:47:09,170 because we're designing a buffer. 861 00:47:09,170 --> 00:47:12,110 So we can use that equation and figure out 862 00:47:12,110 --> 00:47:18,110 what the ratio of acetic acid to its conjugate base should be. 863 00:47:18,110 --> 00:47:20,980 So we can rearrange this expression. 864 00:47:20,980 --> 00:47:26,360 We know the pKa and we know the pH that we want, 865 00:47:26,360 --> 00:47:29,390 and so we can subtract those. 866 00:47:29,390 --> 00:47:35,630 And we get that the log of the ratio should be 0.15. 867 00:47:35,630 --> 00:47:38,280 And then inverse log will tell us 868 00:47:38,280 --> 00:47:42,100 that the ratio then of the acid to the conjugate base 869 00:47:42,100 --> 00:47:44,560 should be 1.4. 870 00:47:44,560 --> 00:47:48,820 So you should use things with that ratio. 871 00:47:48,820 --> 00:47:53,330 So for example, you could use 1.4 molar of the acid 872 00:47:53,330 --> 00:47:56,300 to one molar of the conjugate base 873 00:47:56,300 --> 00:48:03,070 and the total amounts are often less important than the ratio. 874 00:48:03,070 --> 00:48:07,980 The ratio is very important, but if you go too low 875 00:48:07,980 --> 00:48:10,640 in concentrations, then that will 876 00:48:10,640 --> 00:48:13,900 affect what's known as the buffering capacity, which 877 00:48:13,900 --> 00:48:17,040 is the ability of the buffer to resist changes. 878 00:48:17,040 --> 00:48:19,640 So if it's too dilute, and a lot of strong acid 879 00:48:19,640 --> 00:48:21,810 or a lot of strong base is added, 880 00:48:21,810 --> 00:48:24,280 then it won't be a good buffer anymore. 881 00:48:24,280 --> 00:48:26,600 So the ratio is very important. 882 00:48:26,600 --> 00:48:30,750 The amounts are important such that you have some resistance 883 00:48:30,750 --> 00:48:32,350 to change. 884 00:48:32,350 --> 00:48:36,520 So the higher concentrations, the more resistant to change. 885 00:48:40,430 --> 00:48:45,460 And also, if you use too low a concentration, 886 00:48:45,460 --> 00:48:49,680 the Henderson-Hasselbalch Equation is no longer valid. 887 00:48:49,680 --> 00:48:51,280 So you could be asked to calculate 888 00:48:51,280 --> 00:48:53,490 sort of what the minimum concentration is 889 00:48:53,490 --> 00:48:55,570 that you would need to use. 890 00:48:55,570 --> 00:49:02,580 And so for a pH 4.60, you can back calculate what 891 00:49:02,580 --> 00:49:04,910 the hydronium ion concentration is. 892 00:49:04,910 --> 00:49:08,370 And it's 2.5 times 10 to the minus 5. 893 00:49:08,370 --> 00:49:12,740 So for our 5% assumption to hold or to be valid, 894 00:49:12,740 --> 00:49:16,740 that this number, the hydronium ion concentration, 895 00:49:16,740 --> 00:49:20,390 is less than 5% of either one of these. 896 00:49:20,390 --> 00:49:22,700 The concentrations here would have 897 00:49:22,700 --> 00:49:26,510 to be greater than 5 times 10 to the minus 4. 898 00:49:26,510 --> 00:49:28,360 So if we use one molar or something, 899 00:49:28,360 --> 00:49:30,980 that's way above this, but that would be the minimum. 900 00:49:30,980 --> 00:49:34,250 If it's less than that, that 5% doesn't really hold 901 00:49:34,250 --> 00:49:37,240 and you would not have a very good buffer in that case. 902 00:49:37,240 --> 00:49:39,570 It wouldn't be very resistant to change. 903 00:49:39,570 --> 00:49:42,490 X would be big compared to those numbers. 904 00:49:42,490 --> 00:49:44,916 So that's how you go about designing a buffer. 905 00:49:44,916 --> 00:49:46,540 And in that, there were two things that 906 00:49:46,540 --> 00:49:48,190 are really common mistakes. 907 00:49:48,190 --> 00:49:50,630 When I'm reviewing a paper for publication, 908 00:49:50,630 --> 00:49:53,340 two of the things that I see the most often 909 00:49:53,340 --> 00:49:56,720 is that people one, do not use the right buffer 910 00:49:56,720 --> 00:49:57,730 for their experiment. 911 00:49:57,730 --> 00:50:00,410 They say, oh, I'm at pH 8. 912 00:50:00,410 --> 00:50:02,160 And they'll use a buffer that is in fact, 913 00:50:02,160 --> 00:50:04,150 not a good buffer at pH 8. 914 00:50:04,150 --> 00:50:07,120 So who knows what their data is telling us. 915 00:50:07,120 --> 00:50:09,610 And the other thing that people will do 916 00:50:09,610 --> 00:50:11,810 is that they'll say, oh yes, it was buffered. 917 00:50:11,810 --> 00:50:15,140 And the pKa might be right, but the concentration of the buffer 918 00:50:15,140 --> 00:50:18,190 is so low that you don't really imagine 919 00:50:18,190 --> 00:50:19,680 the buffer's doing anything. 920 00:50:19,680 --> 00:50:24,050 So now you know how to avoid both of those pitfalls.