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A2 · Practice questions · Buffer solutions

Resist the change.

Six original Cambridge-style questions on buffer solutions and indicators.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Weak acid + conjugate base, both in large amounts.

01
Explain
[3 marks]

Explain, with equations, how a mixture of ethanoic acid and sodium ethanoate keeps its pH almost constant when small amounts of acid or alkali are added.

  • The mixture contains large amounts of CH3COOH and CH3COO− (from the fully ionised salt). ✓
  • Added H+: CH3COO− + H+ → CH3COOH. Added OH−: CH3COOH + OH− → CH3COO− + H2O. ✓
  • Both reservoirs are large, so the ratio [HA] ÷ [A−] and hence [H+] barely change. ✓
02
Calculation
[3 marks]

4.10 g of sodium ethanoate (Mr 82.0) is dissolved in 500 cm3 of 0.200 mol dm−3 ethanoic acid. Calculate the pH of the buffer (Ka = 1.74 × 10−5 mol dm−3).

  • n(salt) = 4.10 ÷ 82.0 = 0.0500 mol, so [A−] = 0.0500 ÷ 0.500 = 0.100 mol dm−3. ✓
  • [H+] = 1.74 × 10−5 × 0.200 ÷ 0.100 = 3.48 × 10−5 mol dm−3. ✓
  • pH = 4.46. ✓
03
Calculation
[2 marks]

Calculate the ratio [CH3COO−] ÷ [CH3COOH] needed to make a buffer of pH 5.00 (Ka = 1.74 × 10−5 mol dm−3).

  • [H+] = 10−5.00 = 1.00 × 10−5 mol dm−3. ✓
  • [A−] ÷ [HA] = Ka ÷ [H+] = 1.74 × 10−5 ÷ 1.00 × 10−5 = 1.74. ✓
04
Calculation
[3 marks]

A buffer contains 0.200 mol dm−3 NH3 and 0.100 mol dm−3 NH4Cl. For NH4+, Ka = 5.62 × 10−10 mol dm−3. Calculate the pH.

  • NH4+ is the acid and NH3 its conjugate base: [H+] = Ka × [NH4+] ÷ [NH3]. ✓
  • [H+] = 5.62 × 10−10 × 0.100 ÷ 0.200 = 2.81 × 10−10 mol dm−3. ✓
  • pH = 9.55. ✓
05
Explain
[2 marks]

Explain how the H2CO3 / HCO3− system controls the pH of blood.

  • H2CO3 ⇌ H+ + HCO3−; extra H+ is removed by HCO3− + H+ → H2CO3. ✓
  • Extra OH− is removed by H2CO3 + OH− → HCO3− + H2O, keeping blood near pH 7.4. ✓
06
Analysis
[2 marks]

Choose a suitable indicator for (a) ethanoic acid titrated with NaOH(aq) and (b) aqueous ammonia titrated with HCl(aq). Explain each choice.

  • (a) Phenolphthalein: the equivalence point is about pH 9 and the steep section lies in its range, 8.3 to 10.0. ✓
  • (b) Methyl orange: the equivalence point is about pH 5 and the steep section covers its range, 3.1 to 4.4. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.