Main content

## MCAT

### Course: MCAT > Unit 9

Lesson 1: Acid/base equilibria- Acid/base questions
- Acid-base definitions
- Chemistry of buffers and buffers in our blood
- Ka and acid strength
- Autoionization of water
- Definition of pH
- Strong acid solutions
- Strong base solutions
- Weak acid equilibrium
- Weak base equilibrium
- Relationship between Ka and Kb
- Acid–base properties of salts
- pH of salt solutions
- Common ion effect and buffers
- Buffer solutions
- Buffer solution pH calculations

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# Autoionization of water

## Want to join the conversation?

- At5:40, how is it that Kw is used for determining [OH-] for lemon juice. Doesn't Kw apply to water? Shouldn't we use the Ka of lemon juice here?(26 votes)
- If you need to know pH, the Ka of lemon juice is irrelevant. It can be used to find the hydronium ion concentration, but since that's already been given, we don't need to concern ourselves with it. Kw specifically concerns the concentration of hydronium ions and hydroxide ions in an aqueous solution in a given temperature. Lemon juice is an aqueous solution, so we can use Kw(13 votes)

- Does Kahn Academy teach the trig function tricks for the MCAT, since we cannot use a calculator?(7 votes)
- When would Ka be Kw? Thanks in advance for anyone who answers!(9 votes)
- Ka is for an Acid. Water, being amphoteric, can react both as an acid and as a base. Kw is therefore a special case of Ka.(9 votes)

- Will Kw always be 1.0 X 10^-14?(6 votes)
- At temperatures greater than 25°C the degree of self-ionization will increase and at temperatures below it,it will decrease.

Hence Kw will be different at different temperatures.(2 votes)

- In the "Relationship between Ka and Kb" video, Jay says that Ka * Kb = Kw. But in this video he says that Ka = Kw at equilibrium, so does Kb = 1 at equilibrium?(5 votes)
- Do you know of any reactions where there is H3O but no OH? (or the vice versa). And if there is, how do you know when to use Kw? Also, thanks so much for posting these videos, they are great!(4 votes)
- If whatever reactants yields
**H3O**as one of its products, we have to use Ka values because the reaction is*acting like an acid.*

For example: HCl (aq) + H2O (l) → H3O (aq) + Cl (aq) : the hydrochloric acid (HCl) is a strong acid that dissociates into hydroxide (H3O) molecules and chlorine molecules. Since H3O is one of the products due to HCl donating its hydrogen proton, we use the**Ka**value of HCl in our equation.

Ka = (1.3 * 10^6) = [H3O][Cl] / [HCl]

Similarly, if a reaction yielded an**OH-* ion, then we'd use the *Kb**(base-dissociation constant) value in our equation. Keep in mind that Kb = Kw/Ka :)(0 votes)

- I'm confused about the autoionization constant. I thought it only described the relative concentrations of OH and H30 in water.. It can be used for other liquids like lemon juice too?(3 votes)
- We call it as dissociation constant(Ka) and it varies from solution to solution.(1 vote)

- How does Ka = Kw = 1x10^-14 for water at equilibrium, if Ka X Kb = Kw? I would have thought Ka = 1x10^-7 and Kb = 1x10^-7.(2 votes)
- if Ka = [H3O+][OH-], logically then Kb = [OH-][H3O+], so Ka = Kb = 10^-14. In that case, Ka x Kb = 10^28. But we know that Ka x Kb = Kw = 10^-14. I'm confused(2 votes)
- how is 2.2 x10^-3 larger than 4.5 times 10^-12, are basing off of the exponents rather than the coefficient?(2 votes)

## Video transcript

- Water is amphoteric,
which means it can act as an acid or as a base. And so let's say that this water molecule functions as a Bronsted-Lowry acid, so it's gonna be a proton donor, and this water molecule functions as a Bronsted-Lowry
base, so it's going to be a proton accepter. So a lone pair of electrons on the Oxygen take this proton and leave
these electrons behind. So we're gonna form a Hydronium. So we make H3O+, so let me go ahead and draw out H3O+ here. So lone pair of electrons on this Oxygen, plus one formal charge, and let's show these
electrons here in red. So these electrons in red
are going to take this proton to form this bond here. So we make Hydronium, and in the process, these electrons over here in green come off onto this Oxygen. So let's go ahead and draw
out what we would form. So we have that Oxygen, right, and then we had two
lone pairs of electrons on this Oxygen, and we
picked up another lone pair, so the ones in green right here. It's going to give this Oxygen a negative one formal charge. And this, of course,
is the Hydroxide anion. So this is the autoionization of water and we can write an equilibrium expression for this reaction. So we would write Ka,
but for this reaction, it's special, so we write Kw instead of Ka and Kw is called the
autoionization constant. So this is the autoionization constant or you might hear
different terms for this. You might hear ion product constant. So ion product constant. So whatever term you want to use or whatever term your textbook uses. And remember, when we're
writing an equilibrium expression, you're going
to put your products over your reactants, your concentration. So we go over here and
we look at our products and we see H3O+. So we do the concentration
of Hydronium ions times the concentration
of our Hydroxide anions and then we don't worry
about our reactants because we have pure water over here. So we're done writing our
equilibrium expression. Alright, the concentration
of Hydroxide ion... Actually, Hydronium is
what I underlined there. So the concentration of
Hydronium ion and pure water at 25 degrees Celcius has
been determined experimentally to be 1.0 times 10 to
the negative seven more. So this is the concentration
of Hydronium ions and the concentration of Hydroxide, again determined experimentally, is also 1.0 times 10 to
the negative seven more. So the concentration of
Hydroxide at 25 degrees Celcius of pure water turns out to be 1.0 times 10 to the negative seven. So we can calculate the value for the autoionization constant. We can calculate Kw. Kw would just be equal to... This would be 1.0 times
10 to the negative 14. So we can go up here and write it. All this is equal to 1.0
times 10 to the negative 14. And with an equilibrium
constant much less than one, you can think about the
fact that the equilibrium lies far to the left. So you're not gonna have... That's why you have such
low concentration of ions. So this is the autoionization of water. Now let's think about the
concentration of Hydronium compared to the
concentration of Hydroxide. For this example, they're the exact same. So the concentrations are the same. The concentration of Hydronium is equal to the concentration of Hydroxide, and so let me go ahead and write that. And so the concentration of Hydronium ions is equal to the
concentration of Hydroxide. And when that happens,
we say we're dealing with something that's neutral. So water is neutral, pure H2O is neutral. And we use that as a comparison, so if we had a solution where we had a concentration of Hydronium
ions that was greater than the concentration
of Hydroxide anions, that's not a neutral solution. We call this an acidic solution. So this is an acidic solution. And if we think about the opposite, if we have a greater
concentration of Hydroxide anions. So a greater concentration
of Hydroxide anions than Hydronium ions here,
then that's a basic solution. So that would refer to
a basic solution, here. So let's go ahead and do a problem. Let's say that... Let's say we had some lemon juice and the concentration of Hydronium ions of our lemon juice has been
measured experimentally to be 2.2 times 10 to
the negative third more. And let's say we're expected
to find the concentration of Hydroxide anions, and also
to classify our solution. So are we dealing with a neutral solution, an acidic solution, or a basic solution? So we can find the concentration
of Hydroxide anions by using the equation
that we have up here. So let me go back up here and let's look at this
equation that we came up with. So the concentration of Hydronium ions times the concentration
of Hydroxide anions is equal to Kw. So let's go ahead and take our equation and let's plug in our numbers. Let me go ahead and
rewrite that down here. So we have our concentration
of Hydronium ions times concentration of Hydroxide ions is equal to 1.0 times
10 to the negative 14. So we can plug this into here, so now we have 2.2 times
10 to the negative third or we could just make the concentration of Hydroxide anions x is equal to 1.0 times
10 to the negative 14. So we have a simple calculation, so we'll just get out our calculator here and take 1.0 times 10 to the negative 14 and then we just divide that by 2.2 times 10 to the negative third. And that will give us the
concentration of Hydroxide ions. So x is equal to 4.5 times
10 to the negative 12. So x is equal to the
concentration of Hydroxide anions which is equal to 4.5 times
10 to the negative 12 more. And so let's compare our
concentrations, here. I'm gonna go in and write more here. So let's compare the concentration of Hydronium to Hydroxide. The concentration of Hydronium ions was 2.2 times 10 to the negative third and that number is larger
than this number, right? 2.2 times 10 to the
negative third is larger than 4.5 times 10 to the negative 12, so the concentration of Hydronium ions is greater than the
concentration of Hydroxide. So let me go ahead and use red for this. The concentration of Hydronium is greater than the concentration of Hydroxide, and so we're dealing
with an acidic solution. So lemon juice is acidic.