Short on time? This page is the recap. For explanations, the bar diagram and worked examples, use the full study guide.
Recap
- A buffer resists pH change when small amounts of acid or base are added.
- It contains large concentrations of both members of a conjugate pair: HA with A⁻, or B with BH⁺.
- Added H₃O⁺ is removed by the base member (A⁻ or B). Added OH⁻ is removed by the acid member (HA or BH⁺).
- These reactions go essentially to completion. The strong acid or base is swapped for a weak one, so the pH changes only slightly.
- Each addition converts some of one member into the other. The buffer works only while both remain in large amounts.
- A buffer’s pH sits near the pK_a of its weak acid, not necessarily near 7.
Key reactions
| Buffer | Added acid (H₃O⁺) | Added base (OH⁻) |
|---|---|---|
| HA / A⁻ | A⁻ + H₃O⁺ → HA + H₂O | HA + OH⁻ → A⁻ + H₂O |
| B / BH⁺ | B + H₃O⁺ → BH⁺ + H₂O | BH⁺ + OH⁻ → B + H₂O |
| Amounts after adding x mol | base member − x, acid member + x | acid member − x, base member + x |
Making a buffer
| Mixture | Buffer? |
|---|---|
| Weak acid + salt of its conjugate base | Yes |
| Weak base + salt of its conjugate acid | Yes |
| Excess weak acid + some strong base (or excess weak base + some strong acid) | Yes, after reaction |
| Weak acid + equal moles of strong base | No: only A⁻ remains |
| Strong acid + its salt (HCl + NaCl) | No: Cl⁻ cannot remove H₃O⁺ |
| Weak acid alone | No: almost no A⁻ |
Assumptions
- The added acid or base is small compared with the amounts of both buffer members.
- Spectator ions (Na⁺, K⁺, Cl⁻, NO₃⁻) take no part.
Mistakes to avoid
- Swapping the roles. The base member removes acid; the acid member removes base.
- Judging by labels. Do the strong acid/base reaction first, then list what remains.
- “Buffers keep pH at 7.” They keep it near the weak acid’s pK_a.
- “pH stays perfectly constant.” It changes, just much less than in water.
- Writing H⁺ + OH⁻ → H₂O as the buffer reaction. Show the buffer member reacting.
Quick self-check
- Which buffer member removes added OH⁻ in a CH₃COOH / CH₃COO⁻ buffer? (CH₃COOH)
- 0.30 mol HA and 0.20 mol A⁻; add 0.050 mol HCl. Amounts after? (HA 0.35 mol, A⁻ 0.15 mol)
- Is 0.10 mol HF + 0.10 mol NaOH a buffer? (No: only F⁻ remains)
Next: practice questions.