1. 1
   The hydrogenphosphate ion, HPO₄²⁻, is used in some processed foods to control acidity. It takes part in the two reactions below.
   Reaction 1: HPO₄²⁻(aq) + H₃O⁺(aq) → H₂PO₄⁻(aq) + H₂O(l)
   Reaction 2: HPO₄²⁻(aq) + OH⁻(aq) → PO₄³⁻(aq) + H₂O(l)

   (a)

   Which species acts as a Brønsted–Lowry acid in reaction 2?

   [1 mark]

   • AOH⁻
   • BPO₄³⁻
   • CH₃O⁺
   • DHPO₄²⁻

   (b)

   What is the formula of the conjugate acid of HPO₄²⁻?

   [1 mark]

   • APO₄³⁻
   • BH₃PO₄
   • CH₂PO₄⁻
   • DH₃O⁺

   (c)

   Explain, with reference to reactions 1 and 2, why HPO₄²⁻ is described as amphiprotic.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student measured the pH of three aqueous solutions at 298 K. Solution P, 0.0100 mol dm⁻³ hydrochloric acid, had pH 2.00. Solution Q, 0.0100 mol dm⁻³ ethanoic acid, had pH 3.38. Solution R, a dilute household ammonia solution, had [OH⁻] = 2.0 × 10⁻³ mol dm⁻³. At 298 K, Kw = 1.00 × 10⁻¹⁴.

   (a)

   What is the concentration of H⁺ ions in solution Q?

   [1 mark]

   • A2.4 × 10⁻¹¹ mol dm⁻³
   • B4.2 × 10⁻⁴ mol dm⁻³
   • C1.00 × 10⁻² mol dm⁻³
   • D0.53 mol dm⁻³

   (b)

   What is the pH of solution R?

   [1 mark]

   • A11.30
   • B2.70
   • C5.0 × 10⁻¹²
   • D7.00

   (c)

   Deduce, using the data, which of solutions P and Q contains a weak acid, and justify your answer.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An antacid tablet contains calcium carbonate and magnesium hydroxide. A student added one crushed tablet to 50.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid, which is an excess. When the fizzing stopped, the unreacted acid was titrated with 0.500 mol dm⁻³ sodium hydroxide solution, and 42.0 cm³ was needed. A separate analysis showed that the tablet contained 0.500 g of calcium carbonate (Mr = 100.09). The Mr of magnesium hydroxide is 58.33.

   (a)

   Formulate balanced equations for the reactions of hydrochloric acid with calcium carbonate and with magnesium hydroxide, and state one observation that would occur with only one of the two solids.

   [3 marks]

   (b)

   Determine the mass, in g, of magnesium hydroxide in the tablet. Give your answer to three significant figures.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student titrated 25.0 cm³ of 0.100 mol dm⁻³ hydrochloric acid with 0.100 mol dm⁻³ sodium hydroxide at 298 K, recording the pH with a meter. The pH was 1.00 at the start and 1.60 after 15.0 cm³ of alkali had been added. It was 3.70 after 24.9 cm³, 10.30 after 25.1 cm³, 12.36 after 40.0 cm³ and 12.52 after 50.0 cm³.
   In a second experiment, 0.100 mol dm⁻³ ethanoic acid had a pH of 2.88 and conducted electricity much less well than 0.100 mol dm⁻³ hydrochloric acid. When excess magnesium ribbon was added to 25.0 cm³ of each acid, the hydrochloric acid fizzed vigorously and the ethanoic acid fizzed slowly, but both finally produced 31 cm³ of hydrogen. 25.0 cm³ of each acid needed 25.0 cm³ of the 0.100 mol dm⁻³ sodium hydroxide for neutralisation.

   (a)

   Using the data from the first experiment, interpret the shape of the pH curve for the titration of hydrochloric acid with sodium hydroxide.

   [6 marks]

   (b)

   A student claims: "Ethanoic acid is a weaker acid than hydrochloric acid, so it neutralises less sodium hydroxide and reacts less with magnesium." Evaluate this claim using all the data from the second experiment, and explain in terms of the position of equilibrium why ethanoic acid reacts more slowly with magnesium.

   [6 marks]

   Total for question 4: 12 marks

End of questions