pOH

Meaning: pOH puts hydroxide concentration on a log scale, the same way pH does for H⁺. A low pOH means a high [OH⁻] and a basic solution. At 25 °C, pure water has pOH 7, a basic solution has pOH below 7, and an acidic one has pOH above 7. Each 1-unit change is a tenfold change in [OH⁻]. Taking −log of Kw = [H⁺][OH⁻] gives the link between pH and pOH.

Equation: pOH = −log[OH⁻], and pH + pOH = pKw = 14 at 25 °C. Here [OH⁻] is the hydroxide concentration in mol/L (M), pKw = −log Kw, and Kw = 1.0 × 10⁻¹⁴ at 25 °C. pH and pOH are unitless.

On the MCAT: You may be given a strong base such as 0.010 M NaOH, so pOH = 2 and pH = 12. For a weak base, use Kb to find [OH⁻] (about √(Kb × C) when ionization is small), then take the log. Estimation is common: [OH⁻] = 3 × 10⁻⁴ M gives a pOH between 3 and 4, close to 3.5. Titration curves of a base with an acid may also be described in pOH terms.

Common confusion: Students forget that pH + pOH = 14 holds only at 25 °C, because Kw changes with temperature. At other temperatures, neutral water still has [H⁺] = [OH⁻], but its pH is not 7. Also count hydroxides per formula unit: Ba(OH)₂ releases two OH⁻, so [OH⁻] is twice the salt concentration.