pOH Calculations

Sometimes you are asked to calculate pOH rather than pH. Here's a review of the pOH definition and an example calculation..

Acids, Bases, pH and pOH

There are several ways to define acids and bases, but pH and pOH only refers to hydrogen ion concentration and hydroxide ion concentration, respectively. pH and pOH are only meaningful when applied to aqueous (water-based) solutions. When water dissociates it yields a hydrogen ion and a hydroxide.

H₂O <--> H⁺ + OH^-

When calculating pOH, remember that [] refers to molarity, M.

K_w = [H⁺][OH^-] = 1x10^-14 at 25°C
for pure water [H⁺] = [OH^-] = 1x10^-7
Acidic Solution: [H⁺] > 1x10^-7
Basic Solution: [H⁺] < 1x10^-7

pOH Calculations

pOH = -log₁₀[OH^-]
[OH^-] = 10^-pOH
pOH + pH = 14 for any aqueous solution

Example:

Find the [OH^-] given the pH or pOH. If you are given that the pH = 4.5 then,

pOH + pH =14
pOH + 4.5 = 14
pOH = 14 - 4.5
pOH = 9.5

[OH^-] = 10^-pOH
[OH^-] = 10^-9.5
[OH^-] = 3.2 x 10^-10 M