If the pH of a solution is 5, what is its OH ion concentration?

1.0 x 10⁻⁹ M.

Explanation:

∵ pH = - log[H⁺].

∴ 5.0 = - log[H⁺].

log[H⁺] = - 5.0.

∴ [H⁺] = 1.0 x 10⁻⁵ M.

∵ [H⁺][OH⁻] = 10⁻¹⁴.

∴ [OH⁻] = 10⁻¹⁴/[H⁺] = (10⁻¹⁴) / (1.0 x 10⁻⁵ M) = 1.0 x 10⁻⁹ M.