Acid base and pH

Question

What is the molarity of OH^− in solutions with an H3O^+ concentration of 0.00262 mol/L?

Answer 1

Memorize this relationship:

[H₃O⁺][OH^-] = 1x10^-14 = Kw

[OH-] = 1x10^-14 / 2.62x10^-3

[OH-] = 3.82x10^-12 M

Answer 2

Take the negative log₁₀ of 0.262. Subtract this from 14 and multiply by (-1) to get the negative log₁₀ of the [OH^-]. Convert to mol/L.