25.1.3—Math definitions: pH, Ka, pKa and Kw and use

pH = −log[H⁺], Ka = [H⁺][A⁻]/[HA], pKa = −log Ka, and Kw = [H⁺][OH⁻] at a specified temperature. Concentrations are treated consistently with the syllabus approximation.

A larger Ka or smaller pKa means a stronger weak acid. Convert logs and powers carefully and state the temperature when using Kw.

If [H⁺]=1.0×10⁻³ mol dm⁻³, pH=3.00. If Ka=1.0×10⁻⁵, pKa=5.00.

pH is not [H⁺] itself, and pKa is not a concentration. Do not introduce Kb or Kw=KaKb when outside the assessed scope.