Using Kw

[Mackenzie Fernandez 3G]

hi!

I had a quick question about
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 specifically?

thank you!

[Chem_Mod]

K_w can be used in a lot of contexts. The most important thing to remember is that, for a conjugate acid/base pair,

Often in problems, you are given the K_a of a compound, but you really need the K_b of its conjugate base. Then you use the fact that

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 K_w similarly in this case.

[Samudrala_Vaishnavi 3A]

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]

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]

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]

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+].