1. What does a buffer do?
- Resists small pH changes
- Keeps pH exactly fixed under any addition
- Always has pH 7
Check the answer and reasoning
Resists small pH changes
A finite reservoir of conjugate species consumes added acid or base.
CHEM / A2 level
Quantitative equilibria and buffers
Acid dissociation, ionic product of water and solubility products quantify equilibria. Approximations require checking against the starting concentration and competing effects.
Learning objectives
- Explain quantitative equilibria and buffers using a concrete example.
- Apply the worked method and justify each step.
- Recognise the limitation described in the final check.
The big idea
Acid dissociation, ionic product of water and solubility products quantify equilibria. Approximations require checking against the starting concentration and competing effects.
Buffers resist small additions of acid or base because they contain a conjugate pair in substantial amounts. A titration curve depends on acid/base strengths and shows where an indicator must change colour.
Equations and relationships
Ideal dilute-solution concentration form, with concentration in mol/dm³.
Worked example
Find pH when [H⁺]=1.0×10⁻³ mol/dm³.
Follow the solution
Substitute the hydrogen-ion concentration: −log₁₀(10⁻³)=3.
Investigate and explain
Predict the worked result before calculating. Change one input, record the new result, and explain which assumptions still hold.
Related interactive model. Compare its assumptions with the explanation; it is not a complete syllabus lesson.
Check your understanding
2. Find pH when [H⁺]=1.0×10⁻³ mol/dm³.
- −3
- 3
- 11
Check the answer and reasoning
3
Substitute the hydrogen-ion concentration: −log₁₀(10⁻³)=3.
3. Does a buffer have unlimited capacity?
- Yes
- Only if dilute
- No
Check the answer and reasoning
No
Large additions exhaust one component, so its resistance to pH change is limited.