## Using Kw

Mackenzie Fernandez 3G
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### Using Kw

hi!

I had a quick question about $K_{w}$
When do we need to use it? I know Dr. Lavelle used it for lectures this week, but could someone lay out the rules for when we use $K_{w}$ specifically?

thank you!

Chem_Mod
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### Re: Using Kw

Kw can be used in a lot of contexts. The most important thing to remember is that, for a conjugate acid/base pair, $K_{w}=K_{a}K_{b} = [H_{3}O^{+}][OH^{-}]$

Often in problems, you are given the Ka of a compound, but you really need the Kb of its conjugate base. Then you use the fact that $K_{b}=K_{w}/K_{a}$

Alternatively, in some problems you are given the concentration of hydronium or hydroxide in a system, and asked for the concentration of the other. You can use the equation for Kw similarly in this case.

Samudrala_Vaishnavi 3A
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### Re: Using Kw

We use Kw (1*10^-14) whenever we are dealing with the concentrations of hydronium and hydroxide ions, so basically when it comes to any reactions involving aqueous solutions. For instance, if we were asked for the hydronium concentration from the hydroxide concentration or if we were asked for their concentrations in general.

The general formulas to keep in mind (are mostly derived from each other):
pH + pOH = 14
[H3O+][OH-]= Kw
-log[H3O+] + -log[OH-] = -log [Kw]

Lucy_Balish_3G
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### Re: Using Kw

The most helpful for me was to think Kw constant allows us to find hydronium ion concentration or hydroxide concentration when only one is given.

Kamille Kibria 2A
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Joined: Wed Sep 30, 2020 9:52 pm

### Re: Using Kw

Kw is used to find the different concentrations of H3O+ and OH-, in that [H3O+][OH-]= Kw=10^-14. other formulas that are useful in this context are
pOH+pH=14
pH=-log[H3O+] & pOH=-log[OH-]

keely_bales_1f
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Joined: Wed Sep 30, 2020 9:41 pm

### Re: Using Kw

Depending on the context of the problem, Kw can be used in a multitude of ways. However, primarily it is used to find the H3O+ and the OH- concentrations. ( [H3O+][OH-]=Kw=1.0 x 10^-14 ) If you are given the pOH of the pH and need to find the other, you could used this equation: pH + pOH = 14. If you need to find the concentrations, you could do the negative logs of the pOH or the pH to find the [OH-] and [H3O+].

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