pKw

Meaning: Water autoionizes slightly: 2 H₂O ⇌ H₃O⁺ + OH⁻. Its equilibrium constant is Kw = [H₃O⁺][OH⁻], and pKw is the negative log of that value. At 25 °C, Kw = 1.0 × 10⁻¹⁴, so pKw = 14. Autoionization absorbs heat, so raising the temperature shifts the equilibrium right, which increases Kw and lowers pKw.

Equation: pKw = −log Kw = pH + pOH. Here Kw = [H₃O⁺][OH⁻], with concentrations in mol/L. Kw and pKw are dimensionless. For a conjugate acid-base pair, Ka × Kb = Kw, so pKa + pKb = pKw.

On the MCAT: Expect to convert between pH and pOH using pH = pKw − pOH, which is 14 − pOH at 25 °C. For a conjugate pair, you may be asked to find pKb from a given pKa. A passage may also give Kw or pKw for water at another temperature. You then predict that neutral pH moves away from 7 and that the pH of a neutral solution equals pKw/2.

Common confusion: Students treat pH 7 as neutral and pH + pOH = 14 as true at any temperature. Both hold only at 25 °C. A solution is neutral when [H₃O⁺] = [OH⁻]. In hot water, pKw is below 14, so neutral pH is below 7, but the water is not acidic.