WEBVTT 00:00:00.000 --> 00:00:00.000 00:00:00.000 --> 00:00:04.087 Sal: We know that if we leave water to its own devices-- so 00:00:04.087 --> 00:00:08.000 you have some H2O-- that it's an equilibrium with the 00:00:08.000 --> 00:00:10.024 autoionized version of itself. 00:00:10.024 --> 00:00:13.084 So a little bit of it will turn into some hydrogen ions, 00:00:13.084 --> 00:00:16.000 and we know that this really takes the form hydronium. 00:00:16.000 --> 00:00:18.039 That these attach themselves to other water molecules. 00:00:18.039 --> 00:00:21.057 And it could be H3O, but we'll just write it 00:00:21.057 --> 00:00:23.064 as a hydrogen ion. 00:00:23.064 --> 00:00:26.023 Which is really just a free-floating proton. 00:00:26.023 --> 00:00:29.087 Plus hydroxide ion. 00:00:29.087 --> 00:00:34.047 And we also know that in kind of an equilibrium state at 25 00:00:34.047 --> 00:00:36.085 degrees Celsius. 00:00:36.085 --> 00:00:39.042 And remember, equilibrium constants and equilibrium 00:00:39.042 --> 00:00:42.003 reactions are only dependent on the temperature. 00:00:42.003 --> 00:00:43.078 Nothing else. 00:00:43.078 --> 00:00:46.047 For a given molecule, of course. 00:00:46.047 --> 00:00:48.090 So 25 degrees Celsius. 00:00:48.090 --> 00:00:51.064 And we also know, we did this two videos ago, that the 00:00:51.064 --> 00:00:54.040 equilibrium constant-- as a review, that's the 00:00:54.040 --> 00:01:01.099 concentration of the products divided by the concentration 00:01:01.099 --> 00:01:03.022 of the reactants. 00:01:03.022 --> 00:01:05.027 But the reactant in this case is just water. 00:01:05.027 --> 00:01:06.059 It's the actual solvent. 00:01:06.059 --> 00:01:09.065 And if the reactant is what you're-- it's everywhere. 00:01:09.065 --> 00:01:11.078 So if you just go back to that intuition example, the 00:01:11.078 --> 00:01:13.087 probability of finding it is 1. 00:01:13.087 --> 00:01:17.029 So it's just always there, so you don't included it. 00:01:17.029 --> 00:01:19.067 So you can just say divided by 1 or whatever, and this is 00:01:19.067 --> 00:01:23.065 equal to the equilibrium constant of water. 00:01:23.065 --> 00:01:27.034 We learned that that's 10 to the minus 14. 00:01:27.034 --> 00:01:32.051 Because water by itself will have a hydrogen concentration 00:01:32.051 --> 00:01:37.062 of 10 to the minus 7 and a hydroxide concentration of 10 00:01:37.062 --> 00:01:38.087 to the minus 7. 00:01:38.087 --> 00:01:41.098 And if you take a log of everything-- so if you take 00:01:41.098 --> 00:01:46.028 the pKw-- 00:01:46.028 --> 00:01:47.021 What was that? 00:01:47.021 --> 00:01:49.018 If you put a p in front of something, that means you're 00:01:49.018 --> 00:01:51.001 taking the negative log of it. 00:01:51.001 --> 00:01:54.046 So the negative log of 10 to the minus 14-- the log base 10 00:01:54.046 --> 00:01:57.004 up to the minus 14 is minus 14. 00:01:57.004 --> 00:01:59.040 So the negative log is just 14. 00:01:59.040 --> 00:02:07.076 So pKw is 14 and that is equal to-- if I take the negative 00:02:07.076 --> 00:02:10.096 log of this side right here-- let me do that. 00:02:10.096 --> 00:02:12.052 This is just a logarithm property. 00:02:12.052 --> 00:02:15.087 This is more math than chemistry. 00:02:15.087 --> 00:02:23.065 So the log of H plus times OH times our hydroxide ion. 00:02:23.065 --> 00:02:25.071 That's the same thing, just the logarithm properties. 00:02:25.071 --> 00:02:32.018 It's the same thing as minus log of H plus minus, or you 00:02:32.018 --> 00:02:40.015 could say plus the minus log of OH minus. 00:02:40.015 --> 00:02:41.072 And what is this? 00:02:41.072 --> 00:02:48.053 well this is just the pH, which is equal 00:02:48.053 --> 00:02:49.090 to the minus log. 00:02:49.090 --> 00:02:52.083 This is 10 to the minus 7, right? 00:02:52.083 --> 00:02:54.025 10 to the minus 7. 00:02:54.025 --> 00:02:55.086 The log of that is minus 7. 00:02:55.086 --> 00:02:56.065 You have the minus in front. 00:02:56.065 --> 00:02:59.056 So its pH is equal to 7. 00:02:59.056 --> 00:03:01.036 And what is this? 00:03:01.036 --> 00:03:02.013 This over here. 00:03:02.013 --> 00:03:05.090 This is our pOH. 00:03:05.090 --> 00:03:08.071 The minus log of the hydroxide concentration. 00:03:08.071 --> 00:03:12.041 And of course, that was also 10 to the minus 7. 00:03:12.041 --> 00:03:16.059 And so our pOH is equal to log of that is minus 7. 00:03:16.059 --> 00:03:17.058 You have a minus in front. 00:03:17.058 --> 00:03:19.000 It's equal to 7. 00:03:19.000 --> 00:03:24.005 So you get right there that little formula that the pKw, 00:03:24.005 --> 00:03:28.052 or the negative log of the equilibrium constant of water, 00:03:28.052 --> 00:03:41.024 pKw is equal to the pH of water plus the pOH of water. 00:03:41.024 --> 00:03:43.078 And this, at 25 degrees Celsius, this is the thing 00:03:43.078 --> 00:03:45.055 that's going to stay constant because we're going to start 00:03:45.055 --> 00:03:47.072 messing with these things by throwing acid and 00:03:47.072 --> 00:03:49.009 base into the water. 00:03:49.009 --> 00:03:55.062 This thing is always going to be 14 at 25 degrees Celsius. 00:03:55.062 --> 00:03:57.081 Remember, as long as you keep temperature constant and 00:03:57.081 --> 00:04:01.022 you're not messing too much with the molecule itself, your 00:04:01.022 --> 00:04:03.019 equilibrium constant stays constant. 00:04:03.019 --> 00:04:04.093 That's why it's called a constant. 00:04:04.093 --> 00:04:08.071 So with all of that out of the way, let's think about what 00:04:08.071 --> 00:04:13.096 happens if I throw some acid into a-- let's say I have some 00:04:13.096 --> 00:04:15.021 hydrochloric acid. 00:04:15.021 --> 00:04:18.099 00:04:18.099 --> 00:04:21.025 I'll use colors more creatively. 00:04:21.025 --> 00:04:23.026 So I have some hydrochloric acid. 00:04:23.026 --> 00:04:26.069 It's in an aqueous solution. 00:04:26.069 --> 00:04:32.089 We know that it disassociates completely, which means that 00:04:32.089 --> 00:04:39.039 we're just left with the hydrogen ion, on which of 00:04:39.039 --> 00:04:42.073 course really attaches itself to another water molecule and 00:04:42.073 --> 00:04:44.075 becomes hydronium. 00:04:44.075 --> 00:04:51.056 Plus the chlorine anion, or negative ion. 00:04:51.056 --> 00:04:53.085 Right there. 00:04:53.085 --> 00:05:09.007 And let's say that I do this with 1 molar-- or, you know, 00:05:09.007 --> 00:05:12.027 this is also sometimes written as 1 capital M-- of 00:05:12.027 --> 00:05:13.076 hydrochloric acid. 00:05:13.076 --> 00:05:15.060 So essentially what am I doing? 00:05:15.060 --> 00:05:18.061 I am taking 1 molar of hydrochloric acid, literally 00:05:18.061 --> 00:05:26.098 means that I am taking 1 mole of HCl per liter 00:05:26.098 --> 00:05:28.004 of our whole solution. 00:05:28.004 --> 00:05:29.016 Which is mainly water. 00:05:29.016 --> 00:05:30.075 It's an aqueous solution. 00:05:30.075 --> 00:05:33.000 Per liter of water, right? 00:05:33.000 --> 00:05:36.075 So what's my concentration going to be of these things 00:05:36.075 --> 00:05:37.037 right here? 00:05:37.037 --> 00:05:39.012 Or in particular, what's the concentration of 00:05:39.012 --> 00:05:41.047 the H going to be? 00:05:41.047 --> 00:05:46.025 Well, if this disassociated completely, right? 00:05:46.025 --> 00:05:49.033 So all of this stuff-- this is not an equilibrium reaction. 00:05:49.033 --> 00:05:49.093 Remember. 00:05:49.093 --> 00:05:52.023 I only drew a one way arrow to the right. 00:05:52.023 --> 00:05:54.018 There's no even small leftwards arrow. 00:05:54.018 --> 00:05:57.025 This is a strong hydrochloric acid. 00:05:57.025 --> 00:06:01.050 So if you really put one molar of this in an aqueous 00:06:01.050 --> 00:06:03.038 solution, you're not going to see any of this. 00:06:03.038 --> 00:06:04.064 You're going to just see this. 00:06:04.064 --> 00:06:11.029 So you're going to have the hydrogen concentration here in 00:06:11.029 --> 00:06:16.008 the aqueous solution is going to be equal to 1 molar. 00:06:16.008 --> 00:06:19.076 And there's also going to be 1 molar of chlorine anions, but 00:06:19.076 --> 00:06:22.054 we don't care about that. 00:06:22.054 --> 00:06:24.091 If I haven't said already, it would be nice to figure out 00:06:24.091 --> 00:06:27.032 what the pH of this solution is. 00:06:27.032 --> 00:06:29.082 Now that I've thrown hydrochloric acid in it. 00:06:29.082 --> 00:06:32.005 Well the pH is just the hydrogen concentration. 00:06:32.005 --> 00:06:36.093 00:06:36.093 --> 00:06:38.063 We already have the hydrogen concentration. 00:06:38.063 --> 00:06:42.019 That's 1 molar, or 1 mole per liter of solution. 00:06:42.019 --> 00:06:53.044 So the pH is going to be equal to the minus log base 10 of 00:06:53.044 --> 00:06:54.076 our hydrogen concentration. 00:06:54.076 --> 00:06:56.089 Of 1. 00:06:56.089 --> 00:06:59.014 10 to the what power is equal to 1? 00:06:59.014 --> 00:07:01.097 Well, anything to the 0 of power is equal to 00:07:01.097 --> 00:07:02.093 1, including 10. 00:07:02.093 --> 00:07:05.098 So this is equal to 0 minus 0 is just 0. 00:07:05.098 --> 00:07:07.038 So your pH is 0. 00:07:07.038 --> 00:07:15.019 So if you have 1 molar of hydrochloric acid, and you 00:07:15.019 --> 00:07:19.005 throw it into an aqueous solution. 00:07:19.005 --> 00:07:21.094 And, well, I guess I'm saying you're putting it into a 00:07:21.094 --> 00:07:23.043 solution when I tell you it's 1 molar. 00:07:23.043 --> 00:07:26.075 So if you have a concentration of 1 mole per liter of 00:07:26.075 --> 00:07:31.019 solution, where the solvent is water, you will end up 00:07:31.019 --> 00:07:33.093 with a pH of 0. 00:07:33.093 --> 00:07:35.018 The pH of 0. 00:07:35.018 --> 00:07:38.004 00:07:38.004 --> 00:07:43.075 So pH of water without any acid in it, that 00:07:43.075 --> 00:07:44.067 was equal to 7. 00:07:44.067 --> 00:07:49.037 And this is considered a neutral pH. 00:07:49.037 --> 00:07:54.000 Now we know that if you were to have an aqueous solution 00:07:54.000 --> 00:07:59.001 with 1 molar of hydrochloric acid, we can say-- I'll do it 00:07:59.001 --> 00:08:07.092 in red because-- pH of HCl in water is equal to 0. 00:08:07.092 --> 00:08:11.043 So obviously a low pH is more acidic. 00:08:11.043 --> 00:08:14.061 And we went over that in previous videos. 00:08:14.061 --> 00:08:18.041 And let's figure out what the pOH of hydrochloric acid is. 00:08:18.041 --> 00:08:24.086 pOH of hydrochloric acid in an aqueous solution. 00:08:24.086 --> 00:08:28.049 Well, this all goes back to Le Chatelier's Principle, right? 00:08:28.049 --> 00:08:29.092 If you go back to what we said before. 00:08:29.092 --> 00:08:32.061 00:08:32.061 --> 00:08:34.037 This is just pure water right here. 00:08:34.037 --> 00:08:37.054 If we may have put 1 molar of hydrochloric acid in here, 00:08:37.054 --> 00:08:46.007 we're essentially just throwing a ton of hydrogen 00:08:46.007 --> 00:08:46.095 protons in there. 00:08:46.095 --> 00:08:50.026 We're substantially increasing the concentration of this. 00:08:50.026 --> 00:08:52.090 And Le Chatelier's Principle says oh, well that means that 00:08:52.090 --> 00:08:55.064 a lot of this is going to be consumed and the reaction will 00:08:55.064 --> 00:08:56.087 go and this direction. 00:08:56.087 --> 00:08:59.015 The equilibrium reaction will go in that direction. 00:08:59.015 --> 00:09:00.012 But remember. 00:09:00.012 --> 00:09:03.040 Water by itself only had a 10 to the minus 7 concentration. 00:09:03.040 --> 00:09:10.086 We're throwing in a million-- I mean it was one ten 00:09:10.086 --> 00:09:13.022 millionth of a mole per liter. 00:09:13.022 --> 00:09:17.011 Now we're throwing in-- what is that? 00:09:17.011 --> 00:09:17.075 10 to the 7th. 00:09:17.075 --> 00:09:22.035 We're throwing in 10 million times as much hydrogen ions 00:09:22.035 --> 00:09:23.048 into that water. 00:09:23.048 --> 00:09:25.029 So all of this stuff just gets consumed. 00:09:25.029 --> 00:09:26.037 Maybe it goes there. 00:09:26.037 --> 00:09:30.048 And so the concentration of this gets thrown down really 00:09:30.048 --> 00:09:32.061 far because we're dumping so much. 00:09:32.061 --> 00:09:34.095 And the concentration of this goes up because it can only 00:09:34.095 --> 00:09:37.001 consume so much of these guys. 00:09:37.001 --> 00:09:38.042 There's not that much of this. 00:09:38.042 --> 00:09:40.088 There's only 10 to the minus 7th molar of this. 00:09:40.088 --> 00:09:43.017 So this ends up being 1 molar. 00:09:43.017 --> 00:09:46.011 And if this ends up being 1 molar-- because 10 to the 00:09:46.011 --> 00:09:48.041 minus 7th molar, essentially, you can kind of view it as it 00:09:48.041 --> 00:09:50.059 all gets consumed with the stuff over here. 00:09:50.059 --> 00:09:53.067 What ends up being the concentration of the OH? 00:09:53.067 --> 00:09:58.084 Well, we already know that the pKw is 14 of water at 25 00:09:58.084 --> 00:10:03.032 degrees, and the pKw of water is equal to the pH of your 00:10:03.032 --> 00:10:05.025 solution plus your pOH. 00:10:05.025 --> 00:10:12.033 So if your pH for hydrochloric acid is 0, right? 00:10:12.033 --> 00:10:14.027 We have 1 molar of hydrochloric acid. 00:10:14.027 --> 00:10:19.059 Then your pOH of 1 molar of hydrochloric acid is 14. 00:10:19.059 --> 00:10:24.007 So right here, our pOH is equal to 14. 00:10:24.007 --> 00:10:26.015 Now let's do the same thing with a base and figure out 00:10:26.015 --> 00:10:26.097 what its pH is. 00:10:26.097 --> 00:10:28.048 A strong base. 00:10:28.048 --> 00:10:30.063 And I think you'll see that it's the opposite. 00:10:30.063 --> 00:10:35.070 So let's say I had potassium hydroxide. 00:10:35.070 --> 00:10:37.050 It's a strong base. 00:10:37.050 --> 00:10:43.062 So it completely disassociates in water to potassium cations. 00:10:43.062 --> 00:10:46.022 Positively charged ions. 00:10:46.022 --> 00:10:50.012 Plus hydroxide anions. 00:10:50.012 --> 00:10:51.041 It completed disassociates. 00:10:51.041 --> 00:10:53.064 So if I put anything in an aqueous solution-- I should 00:10:53.064 --> 00:10:54.089 write that down. 00:10:54.089 --> 00:10:59.019 00:10:59.019 --> 00:11:02.046 Aqueous solution just means we are in water, of course. 00:11:02.046 --> 00:11:05.094 And if we essentially put 1 molar-- remember the 00:11:05.094 --> 00:11:07.008 concentration matters. 00:11:07.008 --> 00:11:07.095 You can't just say, oh, hydrochloric 00:11:07.095 --> 00:11:09.032 acid has a pH of 0. 00:11:09.032 --> 00:11:09.041 No. 00:11:09.041 --> 00:11:11.032 You have to say 1 molar of hydrochloric 00:11:11.032 --> 00:11:13.070 acid has a pH of 0. 00:11:13.070 --> 00:11:15.004 And actually I didn't write that. 00:11:15.004 --> 00:11:15.065 Let me write that. 00:11:15.065 --> 00:11:16.090 1 molar. 00:11:16.090 --> 00:11:19.028 00:11:19.028 --> 00:11:22.037 And I'll leave you to figure out what the pH or the pOH of 00:11:22.037 --> 00:11:24.030 2 molars of hydrochloric acid is. 00:11:24.030 --> 00:11:26.095 Or a 10 molar of hydrochloric acid. 00:11:26.095 --> 00:11:29.070 And figure out what those pH's are. 00:11:29.070 --> 00:11:36.009 But if we have 1 molar, of potassium hydroxide. 00:11:36.009 --> 00:11:39.009 00:11:39.009 --> 00:11:41.016 We have 1 molar of this. 00:11:41.016 --> 00:11:42.094 And it completely disassociates 00:11:42.094 --> 00:11:43.061 when it's in water. 00:11:43.061 --> 00:11:47.061 So you have none of this left over. 00:11:47.061 --> 00:11:50.048 What's your concentration of OH? 00:11:50.048 --> 00:11:55.086 When your OH concentration is going to be 1 molar. 00:11:55.086 --> 00:11:56.007 Right? 00:11:56.007 --> 00:11:58.029 If you had 1 mole per liter of this, you're going to 1 mole 00:11:58.029 --> 00:11:58.084 per liter of this. 00:11:58.084 --> 00:12:01.016 Because all of this just disappears in the water. 00:12:01.016 --> 00:12:02.044 So what is your pOH? 00:12:02.044 --> 00:12:05.035 00:12:05.035 --> 00:12:08.025 POH is just the negative log of this. 00:12:08.025 --> 00:12:10.045 The log of 1 is 0. 00:12:10.045 --> 00:12:12.070 The negative of 0 is 0. 00:12:12.070 --> 00:12:19.012 And then your pH in this circumstance-- well, you could 00:12:19.012 --> 00:12:20.070 say, oh, it was the hydrogen concentration. 00:12:20.070 --> 00:12:22.054 You don't know what the hydrogen concentration is, but 00:12:22.054 --> 00:12:24.032 you know that when you throw a bunch of this stuff, it's 00:12:24.032 --> 00:12:26.016 going to sop up a bunch of hydrogen and the hydrogen is 00:12:26.016 --> 00:12:27.011 going to go down a lot. 00:12:27.011 --> 00:12:28.048 But you're like, well, how do I measure it? 00:12:28.048 --> 00:12:29.064 Well, you remember it. 00:12:29.064 --> 00:12:32.029 25 degrees Celsius. 00:12:32.029 --> 00:12:34.092 The equilibrium constant of water is equal to 00:12:34.092 --> 00:12:37.013 the pH plus the pOH. 00:12:37.013 --> 00:12:38.062 We showed that at the beginning of the video. 00:12:38.062 --> 00:12:43.025 So 14 is equal to your pH plus 0. 00:12:43.025 --> 00:12:45.042 That's our pOH in this case. 00:12:45.042 --> 00:12:49.017 So our pH is 14. 00:12:49.017 --> 00:12:52.072 So if you have 1 molar-- I used potassium hydroxide in 00:12:52.072 --> 00:12:55.082 this case-- but if you have 1 molar of a strong base-- let 00:12:55.082 --> 00:12:57.030 me write that down. 00:12:57.030 --> 00:13:05.044 1 molar of strong base. 00:13:05.044 --> 00:13:08.021 Remember, strong is kind of an official term in chemistry. 00:13:08.021 --> 00:13:11.012 It means complete disassociation. 00:13:11.012 --> 00:13:17.098 You have a pH of 14 and you have a pOH of 0. 00:13:17.098 --> 00:13:22.044 If you have 1 molar of strong acid. 00:13:22.044 --> 00:13:25.067 If someone says that they have something with a pH of 0 that 00:13:25.067 --> 00:13:31.051 they would like to maybe throw at you, you should decline. 00:13:31.051 --> 00:13:34.063 Because it'll probably hurt your 00:13:34.063 --> 00:13:37.025 chances of-- well, anyway. 00:13:37.025 --> 00:13:39.036 So let's say you have 1 molar of strong acid. 00:13:39.036 --> 00:13:47.087 It's a pH of 0 and a pOH of 14. 00:13:47.087 --> 00:13:50.042 Anyway, maybe in the next video I'll actually show you-- 00:13:50.042 --> 00:13:52.058 This might give you the impression that this is an 00:13:52.058 --> 00:13:53.080 absolute scale. 00:13:53.080 --> 00:13:57.049 That 0 is as acidic as you can get, and 14 is as basic as you 00:13:57.049 --> 00:13:59.028 can get when you get the pH, but that's not 00:13:59.028 --> 00:14:00.000 that's not the case. 00:14:00.000 --> 00:14:01.062 You can actually get above this or you 00:14:01.062 --> 00:14:02.045 can get below this. 00:14:02.045 --> 00:14:07.021 This was this when you had one 1 molar of a strong acid. 00:14:07.021 --> 00:14:10.024 If you had 2 molars of a strong acid-- actually if you 00:14:10.024 --> 00:14:11.061 had 10 molars. 00:14:11.061 --> 00:14:11.084 Right? 00:14:11.084 --> 00:14:12.083 Let's say you get your hydrogen 00:14:12.083 --> 00:14:19.090 concentration to 10 molar. 00:14:19.090 --> 00:14:23.014 So if you had 10 molar of a strong acid, you apply that in 00:14:23.014 --> 00:14:24.008 an aqueous solution. 00:14:24.008 --> 00:14:27.009 It is, when I say it's a molar by definition. 00:14:27.009 --> 00:14:28.075 What's your pH going to be? 00:14:28.075 --> 00:14:33.001 Your pH is going to be the minus log base 10 of 10. 00:14:33.001 --> 00:14:34.041 The log, base 10 of 10, is 1. 00:14:34.041 --> 00:14:36.026 10 to the first power is one. 00:14:36.026 --> 00:14:37.083 So this is equal to minus 1. 00:14:37.083 --> 00:14:40.076 So minus 1 pH would-- if you had 10 molar of say 00:14:40.076 --> 00:14:45.013 hydrochloric acid or nitric acid or anything like that. 00:14:45.013 --> 00:14:47.005 Anyway, that's all for this video. 00:14:47.005 --> 00:14:49.007 I'll see you in the next one.