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AS · Practice questions · pH, strong and weak acids, titration curves

Read the curve.

Six original Cambridge-style questions on pH and titration curves: pH calculations for strong acids, curve shapes, equivalence points and choosing 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

The indicator must change within the jump.

01
Calculation
[2 marks]

Calculate the pH of (a) 0.100 mol dm−3 HCl and (b) 0.00100 mol dm−3 HNO3.

  • (a) pH = −log(0.100) = 1.00. ✓
  • (b) pH = −log(0.00100) = 3.00. ✓
02
Calculation
[2 marks]

A solution of hydrochloric acid has pH 0.60. Calculate its concentration.

  • [H+] = 10−0.60 = 0.251 mol dm−3. ✓
  • HCl is fully dissociated, so [HCl] = 0.251 mol dm−3. ✓
03
Analysis
[3 marks]

Sketch the pH curve for adding 0.100 mol dm−3 NaOH to 25.0 cm3 of 0.100 mol dm−3 ethanoic acid. Label the starting pH, the equivalence volume and the approximate pH range of the vertical section.

  • Starts at about pH 3 and rises gradually. ✓
  • Vertical section at 25.0 cm3, from about pH 7 to 11 (equivalence above 7). ✓
  • Levels off at about pH 13 as excess NaOH is added. ✓
04
Explain
[2 marks]

Explain why phenolphthalein is suitable for titrating ethanoic acid with sodium hydroxide but methyl orange is not.

  • The vertical section is at about pH 7 to 11; phenolphthalein changes over 8.3 to 10.0, inside this range. ✓
  • Methyl orange changes at 3.1 to 4.4, where the pH is still rising slowly, so it would change colour gradually well before equivalence. ✓
05
Explain
[2 marks]

Explain why the pH at the equivalence point is below 7 when hydrochloric acid is titrated with aqueous ammonia.

  • At equivalence the solution contains ammonium chloride. ✓
  • NH4+ is a weak acid (it donates H+ to water), so the solution is slightly acidic. ✓
06
Explain
[2 marks]

Suggest how the end point of a titration between ethanoic acid and aqueous ammonia could be found, and why.

  • Use a pH meter (or a conductivity meter) and plot the curve. ✓
  • There is no sharp vertical section, so no indicator would change colour at a precise volume. ✓

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.