AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
A buffer solution maintains an approximately constant pH, despite dilution…
Full specification wording
Buffer action (A-level only): A buffer solution maintains an approximately constant pH, despite dilution or addition of small amounts of acid or base.
A buffer holds the pH nearly steady because it contains two things at once in substantial amounts: a weak acid HA and a large reservoir of its conjugate base A⁻.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A buffer holds the pH nearly steady because it contains two things at once in substantial amounts: a weak acid HA and a large reservoir of its conjugate base A⁻. Add acid and the A⁻ removes the H⁺: A⁻ + H⁺ → HA. Add alkali and the HA removes the OH⁻: HA + OH⁻ → A⁻ + H₂O. In each case one member of the pair is converted into the other rather than being left free in solution. Since [H⁺] = Ka × [HA]/[A⁻], only the ratio matters, and when both amounts are large a small addition barely changes that ratio. Dilution divides both concentrations by the same factor, so the ratio, and therefore the pH, is unchanged. The protection is approximate and finite: add enough acid to consume the A⁻ and the pH falls sharply.
What earns the marks
- Stating that a buffer contains a weak acid together with a reservoir of its conjugate base.
- An equation showing added H⁺ being removed by the conjugate base.
- An equation showing added OH⁻ being removed by the weak acid.
- Stating that [H⁺] depends on the ratio [HA]/[A⁻], which dilution does not change.
- Saying the pH changes only slightly rather than staying exactly constant.
Common mistakes and how to put them right
Writing that a buffer keeps the pH exactly constant.
Saying the buffer 'neutralises' the added acid without naming the species that reacts.
Claiming dilution changes the pH because both concentrations fall.
Using OH⁻ as the species that removes added acid in an acidic buffer.
Assuming a buffer works for any amount of added acid, ignoring its finite capacity.
Exam tips
- Every buffer explanation needs two equations, one for added acid and one for added alkali; one equation caps the marks.
- Use the word 'approximately' or 'only slightly' about the pH; 'constant' on its own is often refused.
Try the lesson
The start of the Buffer action (A-level only): A buffer solution maintains an approximately constant pH, despite dilution or addition of small amounts of acid or base. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes the behaviour of a buffer solution when a small amount of acid is added?
Listen
A buffer holds the pH nearly steady because it contains two things at once in substantial amounts: a weak acid HA and a large reservoir of its conjugate base A⁻. Add acid and the A⁻ removes the H⁺: A⁻ + H⁺ → HA. Add alkali and the HA removes the OH⁻: HA + OH⁻ → A⁻ + H₂O. In each case one member of the pair is converted into the other rather than being left free in solution. Since [H⁺] = Ka × [HA]/[A⁻], only the ratio matters, and when both amounts are large a small addition barely changes that ratio. Dilution divides both concentrations by the same factor, so the ratio, and therefore the pH, is unchanged. The protection is approximate and finite: add enough acid to consume the A⁻ and the pH falls sharply. A strong answer earns marks for these points. stating that a buffer contains a weak acid together with a reservoir of its conjugate base. an equation showing added H⁺ being removed by the conjugate base. an equation showing added OH⁻ being removed by the weak acid. stating that [H⁺] depends on the ratio [HA]/[A⁻], which dilution does not change. Exam tip: Every buffer explanation needs two equations, one for added acid and one for added alkali; one equation caps the marks.
Key words
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buffer definition
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conjugate pair reservoir
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removal of added acid and alkali
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ratio controls pH
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See every definition
- buffer definition
- A solution that maintains an approximately constant pH, despite dilution or addition of small amounts of acid or base.
- conjugate pair reservoir
- A weak acid HA together with a large reservoir of its conjugate base A⁻, both present in substantial amounts.
- removal of added acid and alkali
- Added H⁺ is removed by the conjugate base and added OH⁻ is removed by the weak acid, converting one member of the pair into the other.
- ratio controls pH
- Since [H⁺] = Ka × [HA]/[A⁻], only the ratio matters, and dilution divides both concentrations by the same factor so the pH is unchanged.
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