Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- Buffer capacity is how much strong acid or strong base a buffer can neutralise while its pH stays nearly constant.
- Added acid reacts with the conjugate base: A⁻ + H₃O⁺ → HA + H₂O.
- Added base reacts with the weak acid: HA + OH⁻ → A⁻ + H₂O.
- Each reaction is 1 : 1 and goes to completion, so the moles of A⁻ (for acid) or HA (for base) set the upper limit.
- When the limiting component is used up, the excess H₃O⁺ or OH⁻ changes the pH sharply.
What sets what
| Change to the buffer | pH | Capacity |
|---|---|---|
| Both concentrations ×10, same ratio | same | ×10 (both directions) |
| Dilute with water | same | lower per litre |
| More HA than A⁻ | below pKa | greater for added base |
| More A⁻ than HA | above pKa | greater for added acid |
| Equal HA and A⁻ | = pKa | equal both ways |
Assumptions behind the reasoning
- The added acid or base is strong and reacts completely with the buffer component.
- Capacity comparisons use moles, not concentrations, when volumes differ. Concentration comparisons assume equal volumes.
- In this model, dilution does not change the ratio, so it does not change the pH.
Mistakes to avoid
- Confusing pH with capacity. Two buffers can share a pH and differ tenfold in capacity.
- Wrong component. Acid is absorbed by A⁻, not by HA.
- Using total amount. For one direction, only one component counts.
- Comparing concentrations, not moles. Multiply by volume first.
- “Buffers keep pH exactly constant.” The pH always shifts a little.
- Lab errors. An error that changes amounts but not the ratio (dilution, a wrong titrant concentration) affects capacity, not pH. A constant pH-meter offset cancels if you measure a change in pH.
Quick self-check
- A buffer holds 0.20 mol HA and 0.05 mol A⁻. Most HCl it can neutralise? Most NaOH? (0.05 mol; 0.20 mol)
- Which has more capacity: 100 mL of 0.30 M HA/0.30 M A⁻, or 1.00 L of 0.030 M HA/0.030 M A⁻? (Equal: 0.030 mol of each in both)
- A buffer is diluted to twice its volume. What happens to its pH and capacity per millilitre? (pH unchanged; capacity per millilitre halves)
Next: practice questions.