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.