Define and prepare buffer solutions
Recognise buffer composition and explain how to prepare one from a weak acid and its conjugate base.
Section 25 · Equilibria
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 25. Use the equilibrium expressions and data stated in the question; written practice is self-checked and human subject-teacher review remains pending.
Before you begin
- Use AS equilibrium expressions, mole calculations, logarithms, and Brønsted–Lowry acid–base ideas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which pair is suitable for an acidic buffer?
What you will learn
- Define a buffer by its resistance to pH change
- Identify the weak-acid/conjugate-base or weak-base/conjugate-acid pair
- Describe preparation by mixing or partial neutralisation
A buffer resists pH change
A buffer solution contains a conjugate acid–base pair in useful amounts and resists a large change in pH when a small amount of acid or alkali is added. A common acidic buffer contains a weak acid HA and its conjugate base A⁻.
The components remove added H⁺ or OH⁻ through reactions. A weak acid alone does not provide enough conjugate base for effective buffering, and a salt alone does not provide the matching acid component.
Prepare both members of the pair
Mix a weak acid with a soluble salt containing its conjugate base, such as ethanoic acid and sodium ethanoate. Alternatively, partially neutralise a weak acid with a measured amount of strong alkali so that some HA remains and some A⁻ is produced.
Avoid complete neutralisation: the solution must retain appreciable amounts of both members. Use a balanced neutralisation equation to track amounts before considering concentrations.
Pause and explain
Why should partial neutralisation stop before all the weak acid is consumed?
Worked example
Describe two ways to prepare an ethanoic acid/ethanoate buffer.
Show the worked solution
- Name a weak acid and its conjugate-base salt.
- Or partially react the acid with a strong alkali.
- Explain why both components must remain.
Answer Mix ethanoic acid with sodium ethanoate, or partially neutralise ethanoic acid with NaOH. In either method, retain both CH₃COOH and CH₃COO⁻ so the pair can respond to added acid or alkali.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Name a suitable conjugate-base salt to combine with ethanoic acid to make a buffer.
Give me a hint
The salt must supply CH₃COO⁻.
Compare my reasoning
- Write the conjugate base of CH₃COOH.
- Choose a soluble salt containing that ion.
- State the resulting buffer pair.
Sodium ethanoate supplies CH₃COO⁻; the buffer pair is CH₃COOH/CH₃COO⁻.
Look for these in your work
- Conjugate base identified
- Suitable salt named
- Pair stated
A student mixes 0.20 mol ethanoic acid with 0.05 mol NaOH. Assuming reaction goes to completion, state which buffer components remain and their amounts.
Give me a hint
Use CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O.
Compare my reasoning
- NaOH consumes 0.05 mol acid.
- Subtract consumed acid from 0.20 mol.
- The same amount of ethanoate forms.
0.15 mol CH₃COOH remains and 0.05 mol CH₃COO⁻ forms, so both buffer components are present.
Look for these in your work
- Neutralisation stoichiometry used
- Remaining acid calculated
- Conjugate base amount calculated
A proposed preparation adds 0.30 mol NaOH to 0.20 mol ethanoic acid. Explain why this does not leave the intended acidic buffer pair.
Give me a hint
Compare the reacting mole amounts before deciding what remains.
Compare my reasoning
- Only 0.20 mol OH⁻ is needed to consume the acid completely.
- Excess strong alkali remains after neutralisation.
- No ethanoic acid remains to buffer added OH⁻.
The acid is completely consumed and 0.10 mol OH⁻ remains. The mixture lacks the weak-acid/conjugate-base pair in useful amounts and is not the intended buffer.
Look for these in your work
- Limiting amount identified
- Excess alkali calculated
- Buffer composition assessed
Common mistakes
- Calling a solution a buffer when it contains only a strong acid or alkali
- Neutralising all of the weak acid
- Choosing a salt whose anion is not the weak acid's conjugate base
- Defining a buffer as having exactly pH 7
What can you explain now?
Which pair is suitable for an acidic buffer?
Compare with the explanation
A weak acid and its conjugate base
Both members can react with small additions of acid or alkali.
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Make it stick
What two components must an acidic buffer contain, and how can both be made in one vessel?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.