1 00:00:00,000 --> 00:00:00,730 2 00:00:00,730 --> 00:00:06,070 Let's have 1 molar of-- let me pick a strong acid. 3 00:00:06,070 --> 00:00:07,730 Let's say it's hydrochloric acid. 4 00:00:07,730 --> 00:00:10,880 5 00:00:10,880 --> 00:00:11,580 HCl. 6 00:00:11,580 --> 00:00:14,810 And we know, of course, that since it's a strong acid it 7 00:00:14,810 --> 00:00:16,309 disassociates completely. 8 00:00:16,309 --> 00:00:20,370 So HCl in an aqueous solution-- or a solution of 9 00:00:20,370 --> 00:00:25,700 water-- disassociates completely into hydrogen ions 10 00:00:25,699 --> 00:00:28,660 in an aqueous solution, which is really hydronium. 11 00:00:28,660 --> 00:00:30,390 I've said that multiple times. 12 00:00:30,390 --> 00:00:34,670 Plus your chlorine anion in an aqueous solution. 13 00:00:34,670 --> 00:00:37,300 So if you actually have a molar of this, as soon as you 14 00:00:37,299 --> 00:00:40,579 put it in the water, you really have 1 molar of this. 15 00:00:40,579 --> 00:00:44,119 So your hydrogen concentration is really 1 molar. 16 00:00:44,119 --> 00:00:47,959 And of course we know, what's the pH of that? 17 00:00:47,960 --> 00:00:53,170 Well, the pH is just the minus log base 10 of your hydrogen 18 00:00:53,170 --> 00:00:55,270 concentration, which is 1. 19 00:00:55,270 --> 00:00:58,760 Which is the minus log of 0. 20 00:00:58,759 --> 00:01:00,609 So minus 0, which is equal to 0. 21 00:01:00,609 --> 00:01:04,209 So your pH is going to be equal to 0. 22 00:01:04,209 --> 00:01:07,560 And let's say for use in the later part of this, let's say 23 00:01:07,560 --> 00:01:10,939 I have a liter of this. 24 00:01:10,939 --> 00:01:11,109 Right. 25 00:01:11,109 --> 00:01:13,290 This just tells me how many moles per liter, and now I 26 00:01:13,290 --> 00:01:14,680 just told you that I have 1 liter. 27 00:01:14,680 --> 00:01:17,210 So you know that you have 1 mole. 28 00:01:17,209 --> 00:01:19,809 Let me make a little chart here. 29 00:01:19,810 --> 00:01:23,650 I'll make a little box here. 30 00:01:23,650 --> 00:01:29,100 And let's make this access right here. 31 00:01:29,099 --> 00:01:32,049 This is going to be my pH. 32 00:01:32,049 --> 00:01:34,799 And right when I take that first measurement-- let's say 33 00:01:34,799 --> 00:01:41,700 this is 0 here, this is 7, and this is 14 up here. 34 00:01:41,700 --> 00:01:45,659 When I take that first measurement, my solution of 35 00:01:45,659 --> 00:01:52,269 hydrogen chloride, or hydrochloric acid, has a pH 0. 36 00:01:52,269 --> 00:01:56,170 Now, what I'm going to do is I'm going to titrate this 37 00:01:56,170 --> 00:01:57,530 aqueous solution of acid. 38 00:01:57,530 --> 00:02:01,239 And titrate just means drop something else into it in 39 00:02:01,239 --> 00:02:03,649 controlled increments to see what happens. 40 00:02:03,650 --> 00:02:06,121 And what I'm doing here is going to be 41 00:02:06,120 --> 00:02:07,799 an acid-base titration. 42 00:02:07,799 --> 00:02:10,198 I have an acid here, and what I'm actually going to titrate 43 00:02:10,199 --> 00:02:16,110 it with, or the reagent that I'm going to add to it-- 44 00:02:16,110 --> 00:02:19,540 The reagent is essentially a reactant that, when you view 45 00:02:19,539 --> 00:02:21,400 it this way, that you're adding to the solution. 46 00:02:21,400 --> 00:02:22,319 So it's the titrant. 47 00:02:22,319 --> 00:02:24,599 So in this case, my solution already has 48 00:02:24,599 --> 00:02:25,824 hydrochloric acid in it. 49 00:02:25,824 --> 00:02:27,359 The pH is already 0. 50 00:02:27,360 --> 00:02:28,480 But what I'm going to do is I'm going to 51 00:02:28,479 --> 00:02:29,879 add a strong base. 52 00:02:29,879 --> 00:02:32,439 53 00:02:32,439 --> 00:02:34,409 Let's say sodium hydroxide. 54 00:02:34,409 --> 00:02:36,740 That's my favorite strong base. 55 00:02:36,740 --> 00:02:40,500 And let's say I have a solution that I have prepared 56 00:02:40,500 --> 00:02:42,229 ahead of time. 57 00:02:42,229 --> 00:02:45,594 Let's say it's a 1 molar solution. 58 00:02:45,594 --> 00:02:48,669 59 00:02:48,669 --> 00:02:51,329 But I'm not going to add all of it at the same time. 60 00:02:51,330 --> 00:02:52,780 I'm going to add it in increments of 61 00:02:52,780 --> 00:02:54,300 a tenth of a liter. 62 00:02:54,300 --> 00:03:02,020 So this is 1 liter that I've added. 63 00:03:02,020 --> 00:03:06,150 And then this would be 2 liters. 64 00:03:06,150 --> 00:03:09,439 And so this would be 500 milliliters. 65 00:03:09,439 --> 00:03:11,240 500 mL. 66 00:03:11,240 --> 00:03:12,719 This is 1 liter. 67 00:03:12,719 --> 00:03:16,000 This is 1.5 liters. 68 00:03:16,000 --> 00:03:19,129 This 2 liters. 69 00:03:19,129 --> 00:03:22,740 And so on this axis, I'm going to slowly add more and more of 70 00:03:22,740 --> 00:03:24,420 this hydroxide. 71 00:03:24,419 --> 00:03:25,669 So what's going to happen? 72 00:03:25,669 --> 00:03:29,559 73 00:03:29,560 --> 00:03:32,610 As I slowly add hydroxide, it's going to increase the pH. 74 00:03:32,610 --> 00:03:34,750 Why is it going to increase the pH? 75 00:03:34,750 --> 00:03:35,949 Because this OH. 76 00:03:35,949 --> 00:03:40,129 Remember, hydroxide is a strong base. 77 00:03:40,129 --> 00:03:44,750 So the NaOH disassociates completely 78 00:03:44,750 --> 00:03:46,439 in an aqueous solution. 79 00:03:46,439 --> 00:03:51,819 To OH minus plus sodium cation. 80 00:03:51,819 --> 00:03:52,370 Right? 81 00:03:52,370 --> 00:03:54,900 Now if I'm adding this to this stuff here, what do you 82 00:03:54,900 --> 00:03:56,650 think's going to happen? 83 00:03:56,650 --> 00:03:59,840 Well this stuff right here, these OHs are just going to 84 00:03:59,840 --> 00:04:01,960 sop up your hydrogens. 85 00:04:01,960 --> 00:04:04,430 So for the most part-- obviously, everything is 86 00:04:04,430 --> 00:04:04,950 probabalistic. 87 00:04:04,949 --> 00:04:07,379 And whenever we're dealing with chemistry, there's always 88 00:04:07,379 --> 00:04:09,715 some molecules that won't completely react, and there 89 00:04:09,715 --> 00:04:12,229 will always be some concentration of anything. 90 00:04:12,229 --> 00:04:15,000 And that's why you actually can never get to a 0 91 00:04:15,000 --> 00:04:17,990 concentration of hydrogen, no matter how much base you add. 92 00:04:17,990 --> 00:04:20,569 Because there will always be some hydrogen molecules that 93 00:04:20,569 --> 00:04:23,365 are just hiding just perfectly, or they're in 94 00:04:23,365 --> 00:04:24,870 perfect equilibrium so they don't react. 95 00:04:24,870 --> 00:04:26,720 But most of them will react. 96 00:04:26,720 --> 00:04:30,110 But as I add more H, these are going to disappear. 97 00:04:30,110 --> 00:04:33,449 So my hydrogen concentration is going to go down. 98 00:04:33,449 --> 00:04:37,360 The pH is a negative log of that, so your pH will go up. 99 00:04:37,360 --> 00:04:41,949 Your pH will go up slowly-- well, it will look slowly, 100 00:04:41,949 --> 00:04:44,050 because it's a log chart, so maybe it's doing 101 00:04:44,050 --> 00:04:45,610 something like this. 102 00:04:45,610 --> 00:04:48,230 Your pH concentration is going to look something like-- as 103 00:04:48,230 --> 00:04:49,060 you add more and more. 104 00:04:49,060 --> 00:04:50,930 But you might say, oh that's such a slow movement. 105 00:04:50,930 --> 00:04:52,250 Why isn't it occurring faster. 106 00:04:52,250 --> 00:04:54,560 But you have to think about something. 107 00:04:54,560 --> 00:05:02,939 This right here, if this is a pH of 1, this means-- so, at a 108 00:05:02,939 --> 00:05:04,810 pH of 0, this means your hydrogen 109 00:05:04,810 --> 00:05:07,410 concentration is 1 molar. 110 00:05:07,410 --> 00:05:11,130 At a pH of 1, this means your hydrogen concentration is at 111 00:05:11,129 --> 00:05:12,639 10 to the minus 1. 112 00:05:12,639 --> 00:05:15,479 Which is 0.1 molar. 113 00:05:15,480 --> 00:05:20,410 So even though on a log chart it looks like you've made a 114 00:05:20,410 --> 00:05:22,189 very small movement, you've actually gone 115 00:05:22,189 --> 00:05:23,639 by a factor of 10. 116 00:05:23,639 --> 00:05:26,204 In terms of your reduction of your hydrogen concentration. 117 00:05:26,204 --> 00:05:29,870 118 00:05:29,870 --> 00:05:34,360 You started off with 1 molar of your hydrogen. 119 00:05:34,360 --> 00:05:38,910 By the time you've added roughly 900 milliliters of 120 00:05:38,910 --> 00:05:41,530 your solution, you probably would have gotten 121 00:05:41,529 --> 00:05:42,219 rid of most of it. 122 00:05:42,220 --> 00:05:43,580 And obviously the denomenator's changing, 123 00:05:43,579 --> 00:05:46,269 because I'm actually adding volume here. 124 00:05:46,269 --> 00:05:46,599 Right? 125 00:05:46,600 --> 00:05:49,220 Because I'm actually adding some volume here. 126 00:05:49,220 --> 00:05:50,120 So you would have to add them. 127 00:05:50,120 --> 00:05:51,850 But you get the I general idea. 128 00:05:51,850 --> 00:05:55,379 That these actual molecules of hydroxide are going to react 129 00:05:55,379 --> 00:05:56,420 with these things. 130 00:05:56,420 --> 00:05:59,370 They're going to essentially turn into water and disappear 131 00:05:59,370 --> 00:06:02,340 into the solution. 132 00:06:02,339 --> 00:06:04,429 So these are going to disappear with those 133 00:06:04,430 --> 00:06:06,009 disappearing, for the most part. 134 00:06:06,009 --> 00:06:09,120 Which leads in an overall reduction concentration of 135 00:06:09,120 --> 00:06:11,079 your hydrogen ions, which will lead to an 136 00:06:11,079 --> 00:06:12,500 increase in your pH. 137 00:06:12,500 --> 00:06:15,589 Until some point where you have pretty much sopped up as 138 00:06:15,589 --> 00:06:17,539 many of these things as you can. 139 00:06:17,540 --> 00:06:19,520 There will always be a few of them, but you've sopped up as 140 00:06:19,519 --> 00:06:20,779 many of them as you can. 141 00:06:20,779 --> 00:06:27,309 And so any incremental hydroxide you add will 142 00:06:27,310 --> 00:06:30,949 actually go to build the hydroxide concentration. 143 00:06:30,949 --> 00:06:33,139 Where the hydroxide concentration is going to be 144 00:06:33,139 --> 00:06:35,689 greater than your hydrogen concentration. 145 00:06:35,689 --> 00:06:37,480 But even better than that, there's going to be some point 146 00:06:37,480 --> 00:06:40,780 where you've added just enough of this that you have an equal 147 00:06:40,779 --> 00:06:43,029 amount of this and this. 148 00:06:43,029 --> 00:06:46,409 You have an equal amount of hydroxide and hydrogen. 149 00:06:46,410 --> 00:06:48,200 Right? 150 00:06:48,199 --> 00:06:50,370 What happens, first of all, when there's an equal amount 151 00:06:50,370 --> 00:06:52,379 of hydroxide and hydrogen? 152 00:06:52,379 --> 00:06:54,240 Well, then you're neutral. 153 00:06:54,240 --> 00:06:55,319 Right? 154 00:06:55,319 --> 00:06:58,389 If your hydroxide concentration is equal to your 155 00:06:58,389 --> 00:07:02,279 hydrogen concentration, you're neutral. 156 00:07:02,279 --> 00:07:03,319 This is just like water. 157 00:07:03,319 --> 00:07:07,810 You're at a pH of 7. 158 00:07:07,810 --> 00:07:11,009 So there's some point where your hydroxide concentration 159 00:07:11,009 --> 00:07:14,269 is equal to your hydrogen concentration, and your pH is 160 00:07:14,269 --> 00:07:15,729 going to be 7. 161 00:07:15,730 --> 00:07:17,680 And then above that, you're just adding 162 00:07:17,680 --> 00:07:18,990 more and more hydroxide. 163 00:07:18,990 --> 00:07:21,689 The hydroxide concentration is overwhelming your hydrogen 164 00:07:21,689 --> 00:07:22,629 concentration. 165 00:07:22,629 --> 00:07:24,579 And so you're going to get really, really basic. 166 00:07:24,579 --> 00:07:27,039 And since this is a strong base, you're eventually going 167 00:07:27,040 --> 00:07:29,340 to get to a pH of 14. 168 00:07:29,339 --> 00:07:31,489 Because you're essentially going to have-- well, you're 169 00:07:31,490 --> 00:07:32,400 not going to have 1 molar. 170 00:07:32,399 --> 00:07:33,759 Let me erase that point. 171 00:07:33,759 --> 00:07:38,319 You're going to have half a molar of OHs 172 00:07:38,319 --> 00:07:39,240 at the end of this. 173 00:07:39,240 --> 00:07:41,829 So your graph is going to look something like this. 174 00:07:41,829 --> 00:07:48,229 175 00:07:48,230 --> 00:07:49,850 Now, I mean, you're still going to have hydrogen 176 00:07:49,850 --> 00:07:52,070 concentration, but it's going to go really small. 177 00:07:52,069 --> 00:07:55,449 It's going to get really, really small. 178 00:07:55,449 --> 00:07:56,029 Now. 179 00:07:56,029 --> 00:08:00,329 What's interesting about what just happened here? 180 00:08:00,329 --> 00:08:07,569 At this point right here, your OH is equal to your hydrogen. 181 00:08:07,569 --> 00:08:13,459 And the way I think about it is the great majority of your 182 00:08:13,459 --> 00:08:17,209 moles of hydrogen were sopped up at that point. 183 00:08:17,209 --> 00:08:20,979 So in order to stop up 1 mole of hydrogen, you essentially, 184 00:08:20,980 --> 00:08:23,879 for the most part-- and there's always things that 185 00:08:23,879 --> 00:08:25,740 don't get completely sopped up-- but you'll essentially 186 00:08:25,740 --> 00:08:33,918 have to add 1 mole of OH to sop up 187 00:08:33,918 --> 00:08:35,279 your 1 mole of hydrogen. 188 00:08:35,279 --> 00:08:37,288 Remember, this is 1 molar and you had 1 liter of it. 189 00:08:37,288 --> 00:08:41,649 So you had 1 mole of hydrogen to sop up. 190 00:08:41,649 --> 00:08:44,699 So you need 1 mole of OH to sop up, and how do you get 1 191 00:08:44,700 --> 00:08:46,040 mole of OH? 192 00:08:46,039 --> 00:08:49,129 Well, we said the concentration of the solution 193 00:08:49,129 --> 00:08:51,110 was 1 molar. 194 00:08:51,110 --> 00:08:54,169 So once you put a liter in there-- because that's 1 mole 195 00:08:54,169 --> 00:08:56,240 per liter-- once you put a liter in there, you would have 196 00:08:56,240 --> 00:09:00,430 put 1 mole of hydroxide cations in there 197 00:09:00,429 --> 00:09:01,389 to sop up the hydrogen. 198 00:09:01,389 --> 00:09:04,240 So this is the equivalence point. 199 00:09:04,240 --> 00:09:07,180 Where have essentially, you have an equal amount and very 200 00:09:07,179 --> 00:09:10,429 low concentrations of both hydroxide and hydrogen. 201 00:09:10,429 --> 00:09:13,370 You've essentially sopped up all of the hydrogen, but you 202 00:09:13,370 --> 00:09:15,879 also have sopped up all of the hydroxide that 203 00:09:15,879 --> 00:09:17,169 you've added so far. 204 00:09:17,169 --> 00:09:19,659 After this point, any hydroxide you add will 205 00:09:19,659 --> 00:09:23,519 essentially contribute to increasing the pH even more. 206 00:09:23,519 --> 00:09:27,559 Or it'll kind of overwhelm your hydrogen concentration. 207 00:09:27,559 --> 00:09:28,759 Now what's interesting about this? 208 00:09:28,759 --> 00:09:31,509 In this example, I told you, oh, I have hydrochloric acid 209 00:09:31,509 --> 00:09:33,870 and I gave you its concentration ahead of time. 210 00:09:33,870 --> 00:09:35,070 But let's say I didn't tell you that. 211 00:09:35,070 --> 00:09:37,730 Let's say it was some mystery substance, and you just 212 00:09:37,730 --> 00:09:40,000 measured the mystery substance's pH. 213 00:09:40,000 --> 00:09:43,970 At this point, you said OK, it's pH is 0. 214 00:09:43,970 --> 00:09:47,560 And you titrated it with this sodium hydroxide. 215 00:09:47,559 --> 00:09:50,250 And you said, well, gee, when I added 1 mole-- because you 216 00:09:50,250 --> 00:09:56,809 figured out at a liter of the solution-- of hydroxide ions. 217 00:09:56,809 --> 00:10:00,009 At that point, I've essentially eliminated all of 218 00:10:00,009 --> 00:10:03,990 the hydrogen ions that were in my original solution. 219 00:10:03,990 --> 00:10:07,299 So I must have had 1 mole of whatever acid 220 00:10:07,299 --> 00:10:08,889 this was over here. 221 00:10:08,889 --> 00:10:11,620 So this would have told you that my initial concentration 222 00:10:11,620 --> 00:10:18,679 was 1 mole of H plus. 223 00:10:18,679 --> 00:10:21,449 And since we know it's easy to see how much volume you have, 224 00:10:21,450 --> 00:10:22,180 you see I have a liter. 225 00:10:22,179 --> 00:10:23,849 So you say my original concentration of 226 00:10:23,850 --> 00:10:25,730 this acid is 1 molar. 227 00:10:25,730 --> 00:10:28,490 And you could also know that what you're dealing with is a 228 00:10:28,490 --> 00:10:29,740 strong acid. 229 00:10:29,740 --> 00:10:33,060 230 00:10:33,059 --> 00:10:35,459 And I encourage you to watch the next to do I do the same 231 00:10:35,460 --> 00:10:37,389 thing with a weak acid. 232 00:10:37,389 --> 00:10:40,980 But because it's a strong acid and a strong base reacting 233 00:10:40,980 --> 00:10:44,810 with each other, when you have the same number moles of both, 234 00:10:44,809 --> 00:10:47,689 you get exactly to a pH of 7. 235 00:10:47,690 --> 00:10:50,620 They completely neutralize each other, and you have a 236 00:10:50,620 --> 00:10:52,350 completely neutral solution. 237 00:10:52,350 --> 00:10:56,509 If you had a strong base with a weak acid, the strong base-- 238 00:10:56,509 --> 00:10:58,840 once you have the same amount as the weak acid, it would've 239 00:10:58,840 --> 00:10:59,769 neutralized it. 240 00:10:59,769 --> 00:11:01,090 But you would have also had some of the 241 00:11:01,090 --> 00:11:03,370 conjugate weak base left. 242 00:11:03,370 --> 00:11:05,950 So you would have actually ended up with a basic pH. 243 00:11:05,950 --> 00:11:07,060 And I'm going to do that in the next video. 244 00:11:07,059 --> 00:11:12,439 But if you see that the equivalence point occurs at a 245 00:11:12,440 --> 00:11:15,850 pH of 7, you'd know that you were titrating-- what you were 246 00:11:15,850 --> 00:11:19,149 dealing with was a strong acid. 247 00:11:19,149 --> 00:11:21,779 Because it was completely neutralized, not more than 248 00:11:21,779 --> 00:11:25,269 neutralized, by the strong base. 249 00:11:25,269 --> 00:11:27,559 And if you want to eye-ball where is equivalence point, 250 00:11:27,559 --> 00:11:29,149 you look for the steepest point in the graph. 251 00:11:29,149 --> 00:11:31,840 Because that's the point where you're going from having a 252 00:11:31,840 --> 00:11:35,090 majority of hydrogen versus hydroxide, to having a 253 00:11:35,090 --> 00:11:37,120 majority of hydroxide versus hydrogen. 254 00:11:37,120 --> 00:11:39,659 So it creates this inflection in the graph, and you just 255 00:11:39,659 --> 00:11:41,240 would have to look at this chart. 256 00:11:41,240 --> 00:11:43,210 And say, I added a liter of this solution. 257 00:11:43,210 --> 00:11:46,280 The concentration of this solution is 1 mole per liter. 258 00:11:46,279 --> 00:11:47,220 It's 1 molar. 259 00:11:47,220 --> 00:11:48,840 It has a molarity of 1. 260 00:11:48,840 --> 00:11:51,870 So I must have added 1 mole of hydroxide 261 00:11:51,870 --> 00:11:52,820 to get to this point. 262 00:11:52,820 --> 00:11:55,480 So I must have neutralized 1 mole hydrogen. 263 00:11:55,480 --> 00:11:58,139 So there must have been 1 mole of hydrogen to begin with. 264 00:11:58,139 --> 00:12:01,129 265 00:12:01,129 --> 00:12:04,330 And since I had 1 liter of solution, I had 1 mole 266 00:12:04,330 --> 00:12:06,100 hydrogen and 1 liter of solution. 267 00:12:06,100 --> 00:12:10,159 So my original molarity was 1. 268 00:12:10,159 --> 00:12:12,159 Anyway, I don't want to confuse you too much. 269 00:12:12,159 --> 00:12:14,339 In the next video, I'm going to try to do the same thing, 270 00:12:14,340 --> 00:12:16,879 but I'm going to do it with a weak acid.