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3.1.16 (HL)—Buffer solutions

Syllabus
First assessment 2025
Objective
3.1.16
Level
HL

Buffer Solutions

HL only

An acidic buffer contains a weak acid and its conjugate base; a basic buffer contains a weak base and its conjugate acid. The pair resists pH change when small amounts of strong acid or base are added.

The conjugate base consumes added H+; the weak acid equilibrium supplies H+ when added OH− removes it. In both cases the conjugate equilibrium shifts to oppose the change.

In an ethanoic acid/ethanoate buffer, CH₃COO⁻ consumes added H⁺ and CH₃COOH consumes added OH⁻, so the conjugate ratio changes only slightly. A buffer resists small additions but has finite capacity; once one component is nearly exhausted, the pH can change sharply.

Explaining Buffer Action

HL only

Assessment in practice

Representative question

Question 1

[Maximum number: 2]

Write equations to show the action of the buffer solution when small amounts of a strong acid or a strong base are added.

Addition of strong acid:
Addition of strong base:

Proton Transfer Reactions Summary

Retrieve the route: track proton transfer and conjugates, calculate pH and Kw, distinguish strength, balance neutralization, read titration curves, use Ka/Kb and hydrolysis, select indicators, and explain and calculate buffer behaviour.

Check donor versus acceptor, one-proton differences, logarithm direction, ion comparison, strength versus concentration, equivalence versus endpoint, pKa landmarks, conjugate equations and dilution ratios.