1 00:00:00,000 --> 00:00:00,000 2 00:00:00,000 --> 00:00:04,087 Sal: We know that if we leave water to its own devices-- so 3 00:00:04,087 --> 00:00:08,000 you have some H2O-- that it's an equilibrium with the 4 00:00:08,000 --> 00:00:10,024 autoionized version of itself. 5 00:00:10,024 --> 00:00:13,084 So a little bit of it will turn into some hydrogen ions, 6 00:00:13,084 --> 00:00:16,000 and we know that this really takes the form hydronium. 7 00:00:16,000 --> 00:00:18,039 That these attach themselves to other water molecules. 8 00:00:18,039 --> 00:00:21,057 And it could be H3O, but we'll just write it 9 00:00:21,057 --> 00:00:23,064 as a hydrogen ion. 10 00:00:23,064 --> 00:00:26,023 Which is really just a free-floating proton. 11 00:00:26,023 --> 00:00:29,087 Plus hydroxide ion. 12 00:00:29,087 --> 00:00:34,047 And we also know that in kind of an equilibrium state at 25 13 00:00:34,047 --> 00:00:36,085 degrees Celsius. 14 00:00:36,085 --> 00:00:39,042 And remember, equilibrium constants and equilibrium 15 00:00:39,042 --> 00:00:42,003 reactions are only dependent on the temperature. 16 00:00:42,003 --> 00:00:43,078 Nothing else. 17 00:00:43,078 --> 00:00:46,047 For a given molecule, of course. 18 00:00:46,047 --> 00:00:48,090 So 25 degrees Celsius. 19 00:00:48,090 --> 00:00:51,064 And we also know, we did this two videos ago, that the 20 00:00:51,064 --> 00:00:54,040 equilibrium constant-- as a review, that's the 21 00:00:54,040 --> 00:01:01,099 concentration of the products divided by the concentration 22 00:01:01,099 --> 00:01:03,022 of the reactants. 23 00:01:03,022 --> 00:01:05,027 But the reactant in this case is just water. 24 00:01:05,027 --> 00:01:06,059 It's the actual solvent. 25 00:01:06,059 --> 00:01:09,065 And if the reactant is what you're-- it's everywhere. 26 00:01:09,065 --> 00:01:11,078 So if you just go back to that intuition example, the 27 00:01:11,078 --> 00:01:13,087 probability of finding it is 1. 28 00:01:13,087 --> 00:01:17,029 So it's just always there, so you don't included it. 29 00:01:17,029 --> 00:01:19,067 So you can just say divided by 1 or whatever, and this is 30 00:01:19,067 --> 00:01:23,065 equal to the equilibrium constant of water. 31 00:01:23,065 --> 00:01:27,034 We learned that that's 10 to the minus 14. 32 00:01:27,034 --> 00:01:32,051 Because water by itself will have a hydrogen concentration 33 00:01:32,051 --> 00:01:37,062 of 10 to the minus 7 and a hydroxide concentration of 10 34 00:01:37,062 --> 00:01:38,087 to the minus 7. 35 00:01:38,087 --> 00:01:41,098 And if you take a log of everything-- so if you take 36 00:01:41,098 --> 00:01:46,028 the pKw-- 37 00:01:46,028 --> 00:01:47,021 What was that? 38 00:01:47,021 --> 00:01:49,018 If you put a p in front of something, that means you're 39 00:01:49,018 --> 00:01:51,001 taking the negative log of it. 40 00:01:51,001 --> 00:01:54,046 So the negative log of 10 to the minus 14-- the log base 10 41 00:01:54,046 --> 00:01:57,004 up to the minus 14 is minus 14. 42 00:01:57,004 --> 00:01:59,040 So the negative log is just 14. 43 00:01:59,040 --> 00:02:07,076 So pKw is 14 and that is equal to-- if I take the negative 44 00:02:07,076 --> 00:02:10,096 log of this side right here-- let me do that. 45 00:02:10,096 --> 00:02:12,052 This is just a logarithm property. 46 00:02:12,052 --> 00:02:15,087 This is more math than chemistry. 47 00:02:15,087 --> 00:02:23,065 So the log of H plus times OH times our hydroxide ion. 48 00:02:23,065 --> 00:02:25,071 That's the same thing, just the logarithm properties. 49 00:02:25,071 --> 00:02:32,018 It's the same thing as minus log of H plus minus, or you 50 00:02:32,018 --> 00:02:40,015 could say plus the minus log of OH minus. 51 00:02:40,015 --> 00:02:41,072 And what is this? 52 00:02:41,072 --> 00:02:48,053 well this is just the pH, which is equal 53 00:02:48,053 --> 00:02:49,090 to the minus log. 54 00:02:49,090 --> 00:02:52,083 This is 10 to the minus 7, right? 55 00:02:52,083 --> 00:02:54,025 10 to the minus 7. 56 00:02:54,025 --> 00:02:55,086 The log of that is minus 7. 57 00:02:55,086 --> 00:02:56,065 You have the minus in front. 58 00:02:56,065 --> 00:02:59,056 So its pH is equal to 7. 59 00:02:59,056 --> 00:03:01,036 And what is this? 60 00:03:01,036 --> 00:03:02,013 This over here. 61 00:03:02,013 --> 00:03:05,090 This is our pOH. 62 00:03:05,090 --> 00:03:08,071 The minus log of the hydroxide concentration. 63 00:03:08,071 --> 00:03:12,041 And of course, that was also 10 to the minus 7. 64 00:03:12,041 --> 00:03:16,059 And so our pOH is equal to log of that is minus 7. 65 00:03:16,059 --> 00:03:17,058 You have a minus in front. 66 00:03:17,058 --> 00:03:19,000 It's equal to 7. 67 00:03:19,000 --> 00:03:24,005 So you get right there that little formula that the pKw, 68 00:03:24,005 --> 00:03:28,052 or the negative log of the equilibrium constant of water, 69 00:03:28,052 --> 00:03:41,024 pKw is equal to the pH of water plus the pOH of water. 70 00:03:41,024 --> 00:03:43,078 And this, at 25 degrees Celsius, this is the thing 71 00:03:43,078 --> 00:03:45,055 that's going to stay constant because we're going to start 72 00:03:45,055 --> 00:03:47,072 messing with these things by throwing acid and 73 00:03:47,072 --> 00:03:49,009 base into the water. 74 00:03:49,009 --> 00:03:55,062 This thing is always going to be 14 at 25 degrees Celsius. 75 00:03:55,062 --> 00:03:57,081 Remember, as long as you keep temperature constant and 76 00:03:57,081 --> 00:04:01,022 you're not messing too much with the molecule itself, your 77 00:04:01,022 --> 00:04:03,019 equilibrium constant stays constant. 78 00:04:03,019 --> 00:04:04,093 That's why it's called a constant. 79 00:04:04,093 --> 00:04:08,071 So with all of that out of the way, let's think about what 80 00:04:08,071 --> 00:04:13,096 happens if I throw some acid into a-- let's say I have some 81 00:04:13,096 --> 00:04:15,021 hydrochloric acid. 82 00:04:15,021 --> 00:04:18,099 83 00:04:18,099 --> 00:04:21,025 I'll use colors more creatively. 84 00:04:21,025 --> 00:04:23,026 So I have some hydrochloric acid. 85 00:04:23,026 --> 00:04:26,069 It's in an aqueous solution. 86 00:04:26,069 --> 00:04:32,089 We know that it disassociates completely, which means that 87 00:04:32,089 --> 00:04:39,039 we're just left with the hydrogen ion, on which of 88 00:04:39,039 --> 00:04:42,073 course really attaches itself to another water molecule and 89 00:04:42,073 --> 00:04:44,075 becomes hydronium. 90 00:04:44,075 --> 00:04:51,056 Plus the chlorine anion, or negative ion. 91 00:04:51,056 --> 00:04:53,085 Right there. 92 00:04:53,085 --> 00:05:09,007 And let's say that I do this with 1 molar-- or, you know, 93 00:05:09,007 --> 00:05:12,027 this is also sometimes written as 1 capital M-- of 94 00:05:12,027 --> 00:05:13,076 hydrochloric acid. 95 00:05:13,076 --> 00:05:15,060 So essentially what am I doing? 96 00:05:15,060 --> 00:05:18,061 I am taking 1 molar of hydrochloric acid, literally 97 00:05:18,061 --> 00:05:26,098 means that I am taking 1 mole of HCl per liter 98 00:05:26,098 --> 00:05:28,004 of our whole solution. 99 00:05:28,004 --> 00:05:29,016 Which is mainly water. 100 00:05:29,016 --> 00:05:30,075 It's an aqueous solution. 101 00:05:30,075 --> 00:05:33,000 Per liter of water, right? 102 00:05:33,000 --> 00:05:36,075 So what's my concentration going to be of these things 103 00:05:36,075 --> 00:05:37,037 right here? 104 00:05:37,037 --> 00:05:39,012 Or in particular, what's the concentration of 105 00:05:39,012 --> 00:05:41,047 the H going to be? 106 00:05:41,047 --> 00:05:46,025 Well, if this disassociated completely, right? 107 00:05:46,025 --> 00:05:49,033 So all of this stuff-- this is not an equilibrium reaction. 108 00:05:49,033 --> 00:05:49,093 Remember. 109 00:05:49,093 --> 00:05:52,023 I only drew a one way arrow to the right. 110 00:05:52,023 --> 00:05:54,018 There's no even small leftwards arrow. 111 00:05:54,018 --> 00:05:57,025 This is a strong hydrochloric acid. 112 00:05:57,025 --> 00:06:01,050 So if you really put one molar of this in an aqueous 113 00:06:01,050 --> 00:06:03,038 solution, you're not going to see any of this. 114 00:06:03,038 --> 00:06:04,064 You're going to just see this. 115 00:06:04,064 --> 00:06:11,029 So you're going to have the hydrogen concentration here in 116 00:06:11,029 --> 00:06:16,008 the aqueous solution is going to be equal to 1 molar. 117 00:06:16,008 --> 00:06:19,076 And there's also going to be 1 molar of chlorine anions, but 118 00:06:19,076 --> 00:06:22,054 we don't care about that. 119 00:06:22,054 --> 00:06:24,091 If I haven't said already, it would be nice to figure out 120 00:06:24,091 --> 00:06:27,032 what the pH of this solution is. 121 00:06:27,032 --> 00:06:29,082 Now that I've thrown hydrochloric acid in it. 122 00:06:29,082 --> 00:06:32,005 Well the pH is just the hydrogen concentration. 123 00:06:32,005 --> 00:06:36,093 124 00:06:36,093 --> 00:06:38,063 We already have the hydrogen concentration. 125 00:06:38,063 --> 00:06:42,019 That's 1 molar, or 1 mole per liter of solution. 126 00:06:42,019 --> 00:06:53,044 So the pH is going to be equal to the minus log base 10 of 127 00:06:53,044 --> 00:06:54,076 our hydrogen concentration. 128 00:06:54,076 --> 00:06:56,089 Of 1. 129 00:06:56,089 --> 00:06:59,014 10 to the what power is equal to 1? 130 00:06:59,014 --> 00:07:01,097 Well, anything to the 0 of power is equal to 131 00:07:01,097 --> 00:07:02,093 1, including 10. 132 00:07:02,093 --> 00:07:05,098 So this is equal to 0 minus 0 is just 0. 133 00:07:05,098 --> 00:07:07,038 So your pH is 0. 134 00:07:07,038 --> 00:07:15,019 So if you have 1 molar of hydrochloric acid, and you 135 00:07:15,019 --> 00:07:19,005 throw it into an aqueous solution. 136 00:07:19,005 --> 00:07:21,094 And, well, I guess I'm saying you're putting it into a 137 00:07:21,094 --> 00:07:23,043 solution when I tell you it's 1 molar. 138 00:07:23,043 --> 00:07:26,075 So if you have a concentration of 1 mole per liter of 139 00:07:26,075 --> 00:07:31,019 solution, where the solvent is water, you will end up 140 00:07:31,019 --> 00:07:33,093 with a pH of 0. 141 00:07:33,093 --> 00:07:35,018 The pH of 0. 142 00:07:35,018 --> 00:07:38,004 143 00:07:38,004 --> 00:07:43,075 So pH of water without any acid in it, that 144 00:07:43,075 --> 00:07:44,067 was equal to 7. 145 00:07:44,067 --> 00:07:49,037 And this is considered a neutral pH. 146 00:07:49,037 --> 00:07:54,000 Now we know that if you were to have an aqueous solution 147 00:07:54,000 --> 00:07:59,001 with 1 molar of hydrochloric acid, we can say-- I'll do it 148 00:07:59,001 --> 00:08:07,092 in red because-- pH of HCl in water is equal to 0. 149 00:08:07,092 --> 00:08:11,043 So obviously a low pH is more acidic. 150 00:08:11,043 --> 00:08:14,061 And we went over that in previous videos. 151 00:08:14,061 --> 00:08:18,041 And let's figure out what the pOH of hydrochloric acid is. 152 00:08:18,041 --> 00:08:24,086 pOH of hydrochloric acid in an aqueous solution. 153 00:08:24,086 --> 00:08:28,049 Well, this all goes back to Le Chatelier's Principle, right? 154 00:08:28,049 --> 00:08:29,092 If you go back to what we said before. 155 00:08:29,092 --> 00:08:32,061 156 00:08:32,061 --> 00:08:34,037 This is just pure water right here. 157 00:08:34,037 --> 00:08:37,054 If we may have put 1 molar of hydrochloric acid in here, 158 00:08:37,054 --> 00:08:46,007 we're essentially just throwing a ton of hydrogen 159 00:08:46,007 --> 00:08:46,095 protons in there. 160 00:08:46,095 --> 00:08:50,026 We're substantially increasing the concentration of this. 161 00:08:50,026 --> 00:08:52,090 And Le Chatelier's Principle says oh, well that means that 162 00:08:52,090 --> 00:08:55,064 a lot of this is going to be consumed and the reaction will 163 00:08:55,064 --> 00:08:56,087 go and this direction. 164 00:08:56,087 --> 00:08:59,015 The equilibrium reaction will go in that direction. 165 00:08:59,015 --> 00:09:00,012 But remember. 166 00:09:00,012 --> 00:09:03,040 Water by itself only had a 10 to the minus 7 concentration. 167 00:09:03,040 --> 00:09:10,086 We're throwing in a million-- I mean it was one ten 168 00:09:10,086 --> 00:09:13,022 millionth of a mole per liter. 169 00:09:13,022 --> 00:09:17,011 Now we're throwing in-- what is that? 170 00:09:17,011 --> 00:09:17,075 10 to the 7th. 171 00:09:17,075 --> 00:09:22,035 We're throwing in 10 million times as much hydrogen ions 172 00:09:22,035 --> 00:09:23,048 into that water. 173 00:09:23,048 --> 00:09:25,029 So all of this stuff just gets consumed. 174 00:09:25,029 --> 00:09:26,037 Maybe it goes there. 175 00:09:26,037 --> 00:09:30,048 And so the concentration of this gets thrown down really 176 00:09:30,048 --> 00:09:32,061 far because we're dumping so much. 177 00:09:32,061 --> 00:09:34,095 And the concentration of this goes up because it can only 178 00:09:34,095 --> 00:09:37,001 consume so much of these guys. 179 00:09:37,001 --> 00:09:38,042 There's not that much of this. 180 00:09:38,042 --> 00:09:40,088 There's only 10 to the minus 7th molar of this. 181 00:09:40,088 --> 00:09:43,017 So this ends up being 1 molar. 182 00:09:43,017 --> 00:09:46,011 And if this ends up being 1 molar-- because 10 to the 183 00:09:46,011 --> 00:09:48,041 minus 7th molar, essentially, you can kind of view it as it 184 00:09:48,041 --> 00:09:50,059 all gets consumed with the stuff over here. 185 00:09:50,059 --> 00:09:53,067 What ends up being the concentration of the OH? 186 00:09:53,067 --> 00:09:58,084 Well, we already know that the pKw is 14 of water at 25 187 00:09:58,084 --> 00:10:03,032 degrees, and the pKw of water is equal to the pH of your 188 00:10:03,032 --> 00:10:05,025 solution plus your pOH. 189 00:10:05,025 --> 00:10:12,033 So if your pH for hydrochloric acid is 0, right? 190 00:10:12,033 --> 00:10:14,027 We have 1 molar of hydrochloric acid. 191 00:10:14,027 --> 00:10:19,059 Then your pOH of 1 molar of hydrochloric acid is 14. 192 00:10:19,059 --> 00:10:24,007 So right here, our pOH is equal to 14. 193 00:10:24,007 --> 00:10:26,015 Now let's do the same thing with a base and figure out 194 00:10:26,015 --> 00:10:26,097 what its pH is. 195 00:10:26,097 --> 00:10:28,048 A strong base. 196 00:10:28,048 --> 00:10:30,063 And I think you'll see that it's the opposite. 197 00:10:30,063 --> 00:10:35,070 So let's say I had potassium hydroxide. 198 00:10:35,070 --> 00:10:37,050 It's a strong base. 199 00:10:37,050 --> 00:10:43,062 So it completely disassociates in water to potassium cations. 200 00:10:43,062 --> 00:10:46,022 Positively charged ions. 201 00:10:46,022 --> 00:10:50,012 Plus hydroxide anions. 202 00:10:50,012 --> 00:10:51,041 It completed disassociates. 203 00:10:51,041 --> 00:10:53,064 So if I put anything in an aqueous solution-- I should 204 00:10:53,064 --> 00:10:54,089 write that down. 205 00:10:54,089 --> 00:10:59,019 206 00:10:59,019 --> 00:11:02,046 Aqueous solution just means we are in water, of course. 207 00:11:02,046 --> 00:11:05,094 And if we essentially put 1 molar-- remember the 208 00:11:05,094 --> 00:11:07,008 concentration matters. 209 00:11:07,008 --> 00:11:07,095 You can't just say, oh, hydrochloric 210 00:11:07,095 --> 00:11:09,032 acid has a pH of 0. 211 00:11:09,032 --> 00:11:09,041 No. 212 00:11:09,041 --> 00:11:11,032 You have to say 1 molar of hydrochloric 213 00:11:11,032 --> 00:11:13,070 acid has a pH of 0. 214 00:11:13,070 --> 00:11:15,004 And actually I didn't write that. 215 00:11:15,004 --> 00:11:15,065 Let me write that. 216 00:11:15,065 --> 00:11:16,090 1 molar. 217 00:11:16,090 --> 00:11:19,028 218 00:11:19,028 --> 00:11:22,037 And I'll leave you to figure out what the pH or the pOH of 219 00:11:22,037 --> 00:11:24,030 2 molars of hydrochloric acid is. 220 00:11:24,030 --> 00:11:26,095 Or a 10 molar of hydrochloric acid. 221 00:11:26,095 --> 00:11:29,070 And figure out what those pH's are. 222 00:11:29,070 --> 00:11:36,009 But if we have 1 molar, of potassium hydroxide. 223 00:11:36,009 --> 00:11:39,009 224 00:11:39,009 --> 00:11:41,016 We have 1 molar of this. 225 00:11:41,016 --> 00:11:42,094 And it completely disassociates 226 00:11:42,094 --> 00:11:43,061 when it's in water. 227 00:11:43,061 --> 00:11:47,061 So you have none of this left over. 228 00:11:47,061 --> 00:11:50,048 What's your concentration of OH? 229 00:11:50,048 --> 00:11:55,086 When your OH concentration is going to be 1 molar. 230 00:11:55,086 --> 00:11:56,007 Right? 231 00:11:56,007 --> 00:11:58,029 If you had 1 mole per liter of this, you're going to 1 mole 232 00:11:58,029 --> 00:11:58,084 per liter of this. 233 00:11:58,084 --> 00:12:01,016 Because all of this just disappears in the water. 234 00:12:01,016 --> 00:12:02,044 So what is your pOH? 235 00:12:02,044 --> 00:12:05,035 236 00:12:05,035 --> 00:12:08,025 POH is just the negative log of this. 237 00:12:08,025 --> 00:12:10,045 The log of 1 is 0. 238 00:12:10,045 --> 00:12:12,070 The negative of 0 is 0. 239 00:12:12,070 --> 00:12:19,012 And then your pH in this circumstance-- well, you could 240 00:12:19,012 --> 00:12:20,070 say, oh, it was the hydrogen concentration. 241 00:12:20,070 --> 00:12:22,054 You don't know what the hydrogen concentration is, but 242 00:12:22,054 --> 00:12:24,032 you know that when you throw a bunch of this stuff, it's 243 00:12:24,032 --> 00:12:26,016 going to sop up a bunch of hydrogen and the hydrogen is 244 00:12:26,016 --> 00:12:27,011 going to go down a lot. 245 00:12:27,011 --> 00:12:28,048 But you're like, well, how do I measure it? 246 00:12:28,048 --> 00:12:29,064 Well, you remember it. 247 00:12:29,064 --> 00:12:32,029 25 degrees Celsius. 248 00:12:32,029 --> 00:12:34,092 The equilibrium constant of water is equal to 249 00:12:34,092 --> 00:12:37,013 the pH plus the pOH. 250 00:12:37,013 --> 00:12:38,062 We showed that at the beginning of the video. 251 00:12:38,062 --> 00:12:43,025 So 14 is equal to your pH plus 0. 252 00:12:43,025 --> 00:12:45,042 That's our pOH in this case. 253 00:12:45,042 --> 00:12:49,017 So our pH is 14. 254 00:12:49,017 --> 00:12:52,072 So if you have 1 molar-- I used potassium hydroxide in 255 00:12:52,072 --> 00:12:55,082 this case-- but if you have 1 molar of a strong base-- let 256 00:12:55,082 --> 00:12:57,030 me write that down. 257 00:12:57,030 --> 00:13:05,044 1 molar of strong base. 258 00:13:05,044 --> 00:13:08,021 Remember, strong is kind of an official term in chemistry. 259 00:13:08,021 --> 00:13:11,012 It means complete disassociation. 260 00:13:11,012 --> 00:13:17,098 You have a pH of 14 and you have a pOH of 0. 261 00:13:17,098 --> 00:13:22,044 If you have 1 molar of strong acid. 262 00:13:22,044 --> 00:13:25,067 If someone says that they have something with a pH of 0 that 263 00:13:25,067 --> 00:13:31,051 they would like to maybe throw at you, you should decline. 264 00:13:31,051 --> 00:13:34,063 Because it'll probably hurt your 265 00:13:34,063 --> 00:13:37,025 chances of-- well, anyway. 266 00:13:37,025 --> 00:13:39,036 So let's say you have 1 molar of strong acid. 267 00:13:39,036 --> 00:13:47,087 It's a pH of 0 and a pOH of 14. 268 00:13:47,087 --> 00:13:50,042 Anyway, maybe in the next video I'll actually show you-- 269 00:13:50,042 --> 00:13:52,058 This might give you the impression that this is an 270 00:13:52,058 --> 00:13:53,080 absolute scale. 271 00:13:53,080 --> 00:13:57,049 That 0 is as acidic as you can get, and 14 is as basic as you 272 00:13:57,049 --> 00:13:59,028 can get when you get the pH, but that's not 273 00:13:59,028 --> 00:14:00,000 that's not the case. 274 00:14:00,000 --> 00:14:01,062 You can actually get above this or you 275 00:14:01,062 --> 00:14:02,045 can get below this. 276 00:14:02,045 --> 00:14:07,021 This was this when you had one 1 molar of a strong acid. 277 00:14:07,021 --> 00:14:10,024 If you had 2 molars of a strong acid-- actually if you 278 00:14:10,024 --> 00:14:11,061 had 10 molars. 279 00:14:11,061 --> 00:14:11,084 Right? 280 00:14:11,084 --> 00:14:12,083 Let's say you get your hydrogen 281 00:14:12,083 --> 00:14:19,090 concentration to 10 molar. 282 00:14:19,090 --> 00:14:23,014 So if you had 10 molar of a strong acid, you apply that in 283 00:14:23,014 --> 00:14:24,008 an aqueous solution. 284 00:14:24,008 --> 00:14:27,009 It is, when I say it's a molar by definition. 285 00:14:27,009 --> 00:14:28,075 What's your pH going to be? 286 00:14:28,075 --> 00:14:33,001 Your pH is going to be the minus log base 10 of 10. 287 00:14:33,001 --> 00:14:34,041 The log, base 10 of 10, is 1. 288 00:14:34,041 --> 00:14:36,026 10 to the first power is one. 289 00:14:36,026 --> 00:14:37,083 So this is equal to minus 1. 290 00:14:37,083 --> 00:14:40,076 So minus 1 pH would-- if you had 10 molar of say 291 00:14:40,076 --> 00:14:45,013 hydrochloric acid or nitric acid or anything like that. 292 00:14:45,013 --> 00:14:47,005 Anyway, that's all for this video. 293 00:14:47,005 --> 00:14:49,007 I'll see you in the next one.