Ionic Product Constant of Water

H₂O and Kc are both constant and combining these constants together make up the ionic product constant for water, written K_W.

K_W = [H₃O⁺] [OH-]

The value of K_W is 1.0 x 10^-14 mol2 dm^-6 at 25°C. Like any equilibrium constant, K_W varies with temperature.

These ions are produced in equal numbers in pure water, so

[H⁺] = [OH] x 10^-14 = [H⁺]²

[H⁺] = [OH^–] = 1.0 x 10^-7 Mol.dm^-3

Experiments have shown that the value for [H⁺] x [OH-] always equals 1.0 x 10^-14 at 25°C even in solutions (such as acids or bases) where [H⁺] ≠ [OH-]. This means that if [H⁺] is known, then [OH-] can be calculated, or vice versa.

Example

Calculate the concentration of OH- ions in 0.10 M HCl

HC1 (aq) → H⁺ (aq) + Cl- (aq)

HC1 is a strong acid, so [H⁺ (aq)] will be 0.10 M.

Substituting [H⁺] = 0.10 into the K_W expression, we get:

1.0 x 10^-14 = (0.10) x [OH-]

[OH^–] = 1.0 x 10^-13 M