## Kw

Abby Lam 3F
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### Kw

How do we know Kw is 10^-14? Was this just found to be true through experiment or is there another reason?

Kailani_Dial_2K
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### Re: Kw

I'm pretty sure that it was found through experimentation

Alara Aygen 3K
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### Re: Kw

I believe it is found through experimentation

Alara Aygen 3K
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### Re: Kw

Alara Aygen 3K wrote:I believe it is found through experimentation

But can someone explain why [H3O+] and [OH-] concentrations need to be equal?

Inderpal Singh 2L
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### Re: Kw

I am pretty sure that they don't always have to be equal, and sometimes the [H30+] will be larger than [OH-] making it acidic, and the [OH-] can be larger than [H30+] making it basic. However, the Kw will always be equal to 10^-14.

Samuel Flores 1E
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### Re: Kw

Alara Aygen 3K wrote:
Alara Aygen 3K wrote:I believe it is found through experimentation

But can someone explain why [H3O+] and [OH-] concentrations need to be equal?

When [H3O+] and [OH-] are equal, then these two substances will neutralize each other. If we are talking about a neutral solution, then we must make [H3O+] = [OH-] in order for all these substances to neutralize each other. When they aren't equal to each other, then we will either have an acidic or basic solution.

Hope this helps!

JonathanSung_2G
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### Re: Kw

Alara Aygen 3K wrote:
Alara Aygen 3K wrote:I believe it is found through experimentation

But can someone explain why [H3O+] and [OH-] concentrations need to be equal?

For example we have 2H2O -> H3O+ + OH-. If we were to calculate the equilibrium constant, Kw = [H3O+][OH-] and we know that Kw=10^-14. The change from the initial concentrations to the equilibrium concentrations would be x for each ion. So we would have Kw=x^2, which means that both concentrations would be equal. However, as it was said in lecture, if an acid or base was added and changed the concentrations of H3O+ or OH-, they would not be equal but Kw would remain the same.

Marley Magee 3A
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### Re: Kw

They just mean that in general, when we talk about water we mean that's it's neutral with a pH of 7. So if that's true then [OH-] and [H3O+] are equal. Knowing these things, you can often determine how adding an acid or base to water will affect your concentrations.

Chem_Mod
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### Re: Kw

In pure water, the concentrations of hydroxide and hydronium ions must be equal because they are only formed through water dissociation, when two molecules of water form one hydroxide and one hydronium ion:

$2H_{2}O_{(l)}\rightleftharpoons H_{3}O^{+}_{(aq)}+OH_{(aq)}^{-}$

If an acid or base is added, then the concentrations of the two will no longer be equal.

Daniel Huynh 2J
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Joined: Wed Jan 08, 2020 12:16 am

### Re: Kw

Kw is not always 1.0*10^-14. It changes according to temperature, like all other equilibrium constants. But for the purposes of most of the problems we encounter, Kw will be equal to 1.0*10^-14 since standard temperature is assumed.