(A Level) Acids and bases: pH, Ka, Kw and buffer solutions

Learning objectives

Identify the conjugate acid–base pairs in a proton-transfer reaction, e.g. CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺

Define pH, Ka, pKa and Kw mathematically and calculate [H⁺] and pH for strong acids, strong alkalis and weak acids, using [H⁺] = √(Ka × c) for a weak acid

Explain what a buffer solution is, how acidic and basic buffers are made, and how they resist a change in pH when small amounts of acid or alkali are added, with equations

Calculate the pH of a buffer from Ka and the concentrations of the weak acid and its salt, using [H⁺] = Ka × [HA]/[A⁻]

Explain how the hydrogencarbonate/carbonic acid equilibrium keeps blood pH close to 7.4, and give other uses of buffer solutions