-
-
Sal: We know that if we leave
water to its own devices-- so
-
you have some H2O-- that it's
an equilibrium with the
-
autoionized version of itself.
-
So a little bit of it will turn
into some hydrogen ions,
-
and we know that this really
takes the form hydronium.
-
That these attach themselves
to other water molecules.
-
And it could be H3O, but
we'll just write it
-
as a hydrogen ion.
-
Which is really just a
free-floating proton.
-
Plus hydroxide ion.
-
And we also know that in kind of
an equilibrium state at 25
-
degrees Celsius.
-
And remember, equilibrium
constants and equilibrium
-
reactions are only dependent
on the temperature.
-
Nothing else.
-
For a given molecule,
of course.
-
So 25 degrees Celsius.
-
And we also know, we did this
two videos ago, that the
-
equilibrium constant-- as
a review, that's the
-
concentration of the products
divided by the concentration
-
of the reactants.
-
But the reactant in this
case is just water.
-
It's the actual solvent.
-
And if the reactant is what
you're-- it's everywhere.
-
So if you just go back to that
intuition example, the
-
probability of finding
it is 1.
-
So it's just always there,
so you don't included it.
-
So you can just say divided by
1 or whatever, and this is
-
equal to the equilibrium
constant of water.
-
We learned that that's
10 to the minus 14.
-
Because water by itself will
have a hydrogen concentration
-
of 10 to the minus 7 and a
hydroxide concentration of 10
-
to the minus 7.
-
And if you take a log of
everything-- so if you take
-
the pKw--
-
What was that?
-
If you put a p in front of
something, that means you're
-
taking the negative log of it.
-
So the negative log of 10 to the
minus 14-- the log base 10
-
up to the minus 14
is minus 14.
-
So the negative log
is just 14.
-
So pKw is 14 and that is equal
to-- if I take the negative
-
log of this side right here--
let me do that.
-
This is just a logarithm
property.
-
This is more math
than chemistry.
-
So the log of H plus times OH
times our hydroxide ion.
-
That's the same thing, just
the logarithm properties.
-
It's the same thing as minus
log of H plus minus, or you
-
could say plus the minus
log of OH minus.
-
And what is this?
-
well this is just the
pH, which is equal
-
to the minus log.
-
This is 10 to the
minus 7, right?
-
10 to the minus 7.
-
The log of that is minus 7.
-
You have the minus in front.
-
So its pH is equal to 7.
-
And what is this?
-
This over here.
-
This is our pOH.
-
The minus log of the hydroxide
concentration.
-
And of course, that was also
10 to the minus 7.
-
And so our pOH is equal to
log of that is minus 7.
-
You have a minus in front.
-
It's equal to 7.
-
So you get right there that
little formula that the pKw,
-
or the negative log of the
equilibrium constant of water,
-
pKw is equal to the pH of water
plus the pOH of water.
-
And this, at 25 degrees Celsius,
this is the thing
-
that's going to stay constant
because we're going to start
-
messing with these things
by throwing acid and
-
base into the water.
-
This thing is always going to
be 14 at 25 degrees Celsius.
-
Remember, as long as you keep
temperature constant and
-
you're not messing too much with
the molecule itself, your
-
equilibrium constant
stays constant.
-
That's why it's called
a constant.
-
So with all of that out of the
way, let's think about what
-
happens if I throw some acid
into a-- let's say I have some
-
hydrochloric acid.
-
-
I'll use colors more
creatively.
-
So I have some hydrochloric
acid.
-
It's in an aqueous solution.
-
We know that it disassociates
completely, which means that
-
we're just left with the
hydrogen ion, on which of
-
course really attaches itself to
another water molecule and
-
becomes hydronium.
-
Plus the chlorine anion,
or negative ion.
-
Right there.
-
And let's say that I do this
with 1 molar-- or, you know,
-
this is also sometimes written
as 1 capital M-- of
-
hydrochloric acid.
-
So essentially what
am I doing?
-
I am taking 1 molar of
hydrochloric acid, literally
-
means that I am taking 1
mole of HCl per liter
-
of our whole solution.
-
Which is mainly water.
-
It's an aqueous solution.
-
Per liter of water, right?
-
So what's my concentration going
to be of these things
-
right here?
-
Or in particular, what's
the concentration of
-
the H going to be?
-
Well, if this disassociated
completely, right?
-
So all of this stuff-- this is
not an equilibrium reaction.
-
Remember.
-
I only drew a one way
arrow to the right.
-
There's no even small
leftwards arrow.
-
This is a strong hydrochloric
acid.
-
So if you really put one molar
of this in an aqueous
-
solution, you're not going
to see any of this.
-
You're going to just see this.
-
So you're going to have the
hydrogen concentration here in
-
the aqueous solution is going
to be equal to 1 molar.
-
And there's also going to be 1
molar of chlorine anions, but
-
we don't care about that.
-
If I haven't said already, it
would be nice to figure out
-
what the pH of this
solution is.
-
Now that I've thrown
hydrochloric acid in it.
-
Well the pH is just the hydrogen
concentration.
-
-
We already have the hydrogen
concentration.
-
That's 1 molar, or 1 mole
per liter of solution.
-
So the pH is going to be equal
to the minus log base 10 of
-
our hydrogen concentration.
-
Of 1.
-
10 to the what power
is equal to 1?
-
Well, anything to the 0
of power is equal to
-
1, including 10.
-
So this is equal to 0
minus 0 is just 0.
-
So your pH is 0.
-
So if you have 1 molar of
hydrochloric acid, and you
-
throw it into an aqueous
solution.
-
And, well, I guess I'm saying
you're putting it into a
-
solution when I tell
you it's 1 molar.
-
So if you have a concentration
of 1 mole per liter of
-
solution, where the solvent
is water, you will end up
-
with a pH of 0.
-
The pH of 0.
-
-
So pH of water without
any acid in it, that
-
was equal to 7.
-
And this is considered
a neutral pH.
-
Now we know that if you were
to have an aqueous solution
-
with 1 molar of hydrochloric
acid, we can say-- I'll do it
-
in red because-- pH of HCl
in water is equal to 0.
-
So obviously a low pH
is more acidic.
-
And we went over that
in previous videos.
-
And let's figure out what the
pOH of hydrochloric acid is.
-
pOH of hydrochloric acid
in an aqueous solution.
-
Well, this all goes back to Le
Chatelier's Principle, right?
-
If you go back to what
we said before.
-
-
This is just pure water
right here.
-
If we may have put 1 molar of
hydrochloric acid in here,
-
we're essentially just throwing
a ton of hydrogen
-
protons in there.
-
We're substantially increasing
the concentration of this.
-
And Le Chatelier's Principle
says oh, well that means that
-
a lot of this is going to be
consumed and the reaction will
-
go and this direction.
-
The equilibrium reaction will
go in that direction.
-
But remember.
-
Water by itself only had a 10 to
the minus 7 concentration.
-
We're throwing in a million--
I mean it was one ten
-
millionth of a mole per liter.
-
Now we're throwing in--
what is that?
-
10 to the 7th.
-
We're throwing in 10 million
times as much hydrogen ions
-
into that water.
-
So all of this stuff
just gets consumed.
-
Maybe it goes there.
-
And so the concentration of this
gets thrown down really
-
far because we're
dumping so much.
-
And the concentration of this
goes up because it can only
-
consume so much of these guys.
-
There's not that much of this.
-
There's only 10 to the minus
7th molar of this.
-
So this ends up being 1 molar.
-
And if this ends up being 1
molar-- because 10 to the
-
minus 7th molar, essentially,
you can kind of view it as it
-
all gets consumed with
the stuff over here.
-
What ends up being the
concentration of the OH?
-
Well, we already know that the
pKw is 14 of water at 25
-
degrees, and the pKw of water
is equal to the pH of your
-
solution plus your pOH.
-
So if your pH for hydrochloric
acid is 0, right?
-
We have 1 molar of hydrochloric
acid.
-
Then your pOH of 1 molar of
hydrochloric acid is 14.
-
So right here, our pOH
is equal to 14.
-
Now let's do the same thing
with a base and figure out
-
what its pH is.
-
A strong base.
-
And I think you'll see that
it's the opposite.
-
So let's say I had potassium
hydroxide.
-
It's a strong base.
-
So it completely disassociates
in water to potassium cations.
-
Positively charged ions.
-
Plus hydroxide anions.
-
It completed disassociates.
-
So if I put anything in an
aqueous solution-- I should
-
write that down.
-
-
Aqueous solution just means we
are in water, of course.
-
And if we essentially put
1 molar-- remember the
-
concentration matters.
-
You can't just say,
oh, hydrochloric
-
acid has a pH of 0.
-
No.
-
You have to say 1 molar
of hydrochloric
-
acid has a pH of 0.
-
And actually I didn't
write that.
-
Let me write that.
-
1 molar.
-
-
And I'll leave you to figure out
what the pH or the pOH of
-
2 molars of hydrochloric
acid is.
-
Or a 10 molar of hydrochloric
acid.
-
And figure out what
those pH's are.
-
But if we have 1 molar, of
potassium hydroxide.
-
-
We have 1 molar of this.
-
And it completely disassociates
-
when it's in water.
-
So you have none of
this left over.
-
What's your concentration
of OH?
-
When your OH concentration
is going to be 1 molar.
-
Right?
-
If you had 1 mole per liter of
this, you're going to 1 mole
-
per liter of this.
-
Because all of this just
disappears in the water.
-
So what is your pOH?
-
-
POH is just the negative
log of this.
-
The log of 1 is 0.
-
The negative of 0 is 0.
-
And then your pH in this
circumstance-- well, you could
-
say, oh, it was the hydrogen
concentration.
-
You don't know what the hydrogen
concentration is, but
-
you know that when you throw
a bunch of this stuff, it's
-
going to sop up a bunch of
hydrogen and the hydrogen is
-
going to go down a lot.
-
But you're like, well,
how do I measure it?
-
Well, you remember it.
-
25 degrees Celsius.
-
The equilibrium constant
of water is equal to
-
the pH plus the pOH.
-
We showed that at the beginning
of the video.
-
So 14 is equal to
your pH plus 0.
-
That's our pOH in this case.
-
So our pH is 14.
-
So if you have 1 molar-- I used
potassium hydroxide in
-
this case-- but if you have 1
molar of a strong base-- let
-
me write that down.
-
1 molar of strong base.
-
Remember, strong is kind of an
official term in chemistry.
-
It means complete
disassociation.
-
You have a pH of 14 and
you have a pOH of 0.
-
If you have 1 molar
of strong acid.
-
If someone says that they have
something with a pH of 0 that
-
they would like to maybe throw
at you, you should decline.
-
Because it'll probably
hurt your
-
chances of-- well, anyway.
-
So let's say you have 1
molar of strong acid.
-
It's a pH of 0 and
a pOH of 14.
-
Anyway, maybe in the next video
I'll actually show you--
-
This might give you the
impression that this is an
-
absolute scale.
-
That 0 is as acidic as you can
get, and 14 is as basic as you
-
can get when you get the
pH, but that's not
-
that's not the case.
-
You can actually get
above this or you
-
can get below this.
-
This was this when you had one
1 molar of a strong acid.
-
If you had 2 molars of a strong
acid-- actually if you
-
had 10 molars.
-
Right?
-
Let's say you get your hydrogen
-
concentration to 10 molar.
-
So if you had 10 molar of a
strong acid, you apply that in
-
an aqueous solution.
-
It is, when I say it's a
molar by definition.
-
What's your pH going to be?
-
Your pH is going to be the
minus log base 10 of 10.
-
The log, base 10 of 10, is 1.
-
10 to the first power is one.
-
So this is equal to minus 1.
-
So minus 1 pH would-- if
you had 10 molar of say
-
hydrochloric acid or nitric acid
or anything like that.
-
Anyway, that's all
for this video.
-
I'll see you in the next one.
