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

1. 1
   Ethanoic acid dissolves in water and reaches the equilibrium CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺.

   (a)

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

   [1 mark]

   • AH₃O⁺
   • BCH₃COO⁻
   • CH₂O
   • DCH₃COOH

   (b)

   Which species acts as a Brønsted–Lowry base in the forward reaction?

   [1 mark]

   • ACH₃COOH
   • BCH₃COO⁻
   • CH₃O⁺
   • DH₂O

   (c)

   Identify the two conjugate acid–base pairs in this equilibrium.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A swimming-pool technician lowers the pH of pool water by dosing it with dilute hydrochloric acid. Treat hydrochloric acid as fully dissociated, and assume all solutions are at 25 °C.

   (a)

   The pool water is dosed to give a hydrochloric acid concentration of 0.0400 mol dm⁻³. What is its pH?

   [1 mark]

   • A1.40
   • B1.60
   • C0.40
   • D2.40

   (b)

   A sample from the dosing tank has a pH of 3.20. What is the hydrogen ion concentration in the sample?

   [1 mark]

   • A1.58 × 10³ mol dm⁻³
   • B3.20 × 10⁻³ mol dm⁻³
   • C6.31 × 10⁻⁴ mol dm⁻³
   • D1.58 × 10⁻¹¹ mol dm⁻³

   (c)

   A technician dilutes 25.0 cm³ of 0.0800 mol dm⁻³ hydrochloric acid with water to a total volume of 1.00 dm³. Calculate the pH of the diluted solution.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student compares two acid solutions at 25 °C, each of concentration 0.100 mol dm⁻³: hydrochloric acid and ethanoic acid. The pH of the ethanoic acid solution is measured as 2.88.

   (a)

   Calculate the pH of the hydrochloric acid and explain why the ethanoic acid has a higher pH at the same concentration.

   [3 marks]

   (b)

   Calculate the hydrogen ion concentration in the ethanoic acid solution and hence the percentage of the acid molecules that are dissociated.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician prepares solutions for an acid–base practical. She has hydrochloric acid, nitric acid and ammonia solution available, and the strong acids are fully dissociated.

   (a)

   Hydrogen chloride gas reacts with ammonia gas to form ammonium chloride. Explain this reaction in terms of the Brønsted–Lowry theory, identifying the acid, the base and the conjugate species.

   [6 marks]

   (b)

   The technician mixes 25.0 cm³ of 0.200 mol dm⁻³ hydrochloric acid with 75.0 cm³ of 0.0400 mol dm⁻³ nitric acid. Calculate the pH of the mixture.

   [6 marks]

   Total for question 4: 12 marks

End of questions