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Buffer action; pH after adding KOH to acetic acid buffer

Short answer

A buffer solution resists pH change. An acidic buffer neutralizes added H+ with CH3COO- and OH- with CH3COOH. Initial CH3COOH 0.040 mol, after 0.005 mol KOH, acid 0.035 mol, salt 0.005 mol. pH = 4.74 + log(0.005/0.035) = 3.90.

What is buffer solution? How acidic buffer resists the change in pH on the addition of small amount of acid and base? 0.005 mole of KOH is added to 400 ml of 0.1 M CH₃COOH. Calculate the pH of the mixture. Given pKa = 4.74.

Buffer solution

A solution which resists a change in pH on adding small amounts of acid or base. An acidic buffer is a weak acid with its salt, e.g. CH₃COOH + CH₃COONa.

Action of acidic buffer

The solution has much CH₃COOH (acid reserve) and CH₃COO⁻ from the salt (base reserve).

  • Small amount of acid (H⁺) added: H⁺ is taken up by acetate ions: CH₃COO⁻ + H⁺ → CH₃COOH. The weak acid is formed, so pH hardly falls.
  • Small amount of base (OH⁻) added: OH⁻ is neutralised by acetic acid: CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O. So pH hardly rises.

The ratio [salt]/[acid] changes little, so pH = pKa + log([salt]/[acid]) is almost constant.

Calculation

Moles CH₃COOH = 0.1 × 0.400 = 0.040 mol
KOH added = 0.005 mol
CH₃COOH + KOH → CH₃COOK + H₂O
Acid left = 0.040 − 0.005 = 0.035 mol
Salt formed = 0.005 mol

pH = 4.74 + log(0.005/0.035)
   = 4.74 − 0.845 = 3.895

Answer: pH ≈ 3.90

✓ Answer checked by Claude Opus 5.5 (high effort)

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