Bronsted-Lowry acids and bases and pHEdexcel A-Level Chemistry: Subtopic test

1. 1
   A student mixes aqueous ethanoic acid with aqueous ammonia: CH₃COOH(aq) + NH₃(aq) ⇌ CH₃COO⁻(aq) + NH₄⁺(aq).

   (a)

   Which species is the conjugate base of ethanoic acid in this equilibrium?

   [1 mark]

   • ANH₄⁺
   • BCH₃COO⁻
   • CNH₃
   • DCH₃COOH

   (b)

   Which statement describes the role of NH₃ in the forward reaction?

   [1 mark]

   • AIt is a Brønsted–Lowry base because it accepts a proton from CH₃COOH
   • BIt is a Brønsted–Lowry acid because it donates a proton to CH₃COOH
   • CIt is both an acid and a base because it is a molecule
   • DIt is a spectator because its oxidation state does not change

   (c)

   Identify the two conjugate acid–base pairs in this equilibrium, stating which species in each pair is the acid and which is the base.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A school technician prepares solutions of strong monoprotic acids for a practical. All measurements are made at 298 K.

   (a)

   What is the pH of 0.0500 mol dm⁻³ nitric acid, a strong monoprotic acid?

   [1 mark]

   • A1.70
   • B2.60
   • C1.30
   • D12.70

   (b)

   A sample of rainwater tested by the technician has a pH of 3.40. What is its hydrogen ion concentration?

   [1 mark]

   • A3.4 × 10⁻³ mol dm⁻³
   • B2.5 × 10⁻³ mol dm⁻³
   • C3.4 × 10⁻⁴ mol dm⁻³
   • D4.0 × 10⁻⁴ mol dm⁻³

   (c)

   The technician dilutes 25.0 cm³ of 0.200 mol dm⁻³ hydrochloric acid to a final volume of 500 cm³. Calculate the pH of the diluted solution.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student has equimolar 0.100 mol dm⁻³ solutions of nitric acid, a strong acid, and methanoic acid, a weak acid. At 298 K the pH of the nitric acid is 1.00 and the pH of the methanoic acid is 2.38.

   (a)

   Use the pH values to show that nitric acid is a strong acid and methanoic acid is a weak acid.

   [3 marks]

   (b)

   Each solution is diluted 100 times. The pH of the nitric acid becomes 3.00 and the pH of the methanoic acid becomes 3.47. Analyse these data to explain the different changes in pH.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student measures the pH of 0.100 mol dm⁻³ solutions at 298 K: hydrochloric acid 1.00; ethanoic acid 2.88; sodium hydroxide 13.00; ammonia solution 11.12; sodium chloride 7.00; ammonium chloride 5.13; sodium ethanoate 8.88. She then dilutes the hydrochloric acid and the ethanoic acid by factors of 10, 100 and 1000.

   (a)

   Analyse the pH values of the seven undiluted solutions to compare the strengths of the acids and of the bases, and to explain the pH of each of the three salts.

   [6 marks]

   (b)

   The pH of the diluted ethanoic acid is 3.39 (10 times), 3.91 (100 times) and 4.47 (1000 times). Calculate the pH of the diluted hydrochloric acid in each case and explain the difference in the way the pH of the two acids changes on dilution.

   [6 marks]

   Total for question 4: 12 marks

End of questions