A level Chemistry · Edexcel 9CH0 · Topic 12B
pH calculations
Key points
- and . A change of one pH unit is a ten-fold change in [H⁺].
- Strong acid: [H⁺] = acid concentration (for monoprotic acids).
- Strong base: find [OH⁻], then with = 1.00 × 10⁻¹⁴ mol² dm⁻⁶ at 298 K.
- Weak acid: assume [H⁺] = [A⁻] and [HA] at equilibrium ≈ initial concentration, so .
- These assumptions fail for fairly strong weak acids or very dilute solutions.
- Pure water is neutral at all temperatures because [H⁺] = [OH⁻], even though its pH falls below 7 when warmed (ionisation is endothermic).
- After partial neutralisation, work out excess moles of acid or base, divide by the total volume, then find pH.
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Almost every acid–base question ends in a pH. This sub-topic gives you one method for each kind of solution: strong acid, strong base, weak acid and a mixture after partial neutralisation.
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