A-Level Chemistry: Buffers & pH Calculations — Weak Acids, Conjugate Bases, and the Henderson–Hasselbalch Shortcut

What is a buffer?

A buffer is a solution that resists large pH changes when small amounts of acid/alkali are added. Typically made from a weak acid + its conjugate base (e.g. CH₃COOH + CH₃COO⁻). In Singapore A-Level H2 Chemistry, you must say it works by “removing added H⁺/OH⁻ via equilibrium shift”.

Henderson–Hasselbalch

For acidic buffer: pH = pKa + log([A⁻]/[HA]). This comes from Ka = [H⁺][A⁻]/[HA]. As long as both weak acid and conjugate base are present in significant amounts, you can use this shortcut to get pH fast in MCQs / structured questions.

Common traps

After adding strong acid: [A⁻] decreases, [HA] increases. After adding strong base: [HA] decreases, [A⁻] increases. Students often forget to re‑calculate the new ratio before reusing Henderson–Hasselbalch. Also: never call buffer pH “constant”; say “approximately constant / resists large change”.

Exam habits

• State the equilibrium shift (“added H⁺ is removed by A⁻ to form HA”). Markers look for that language explicitly.
• Show moles first, then concentrations — Cambridge wants stoichiometry logic, not blind formula use.
• Check units when you quote Ka or pKa; always label them.