Weak acids and KaAQA A-Level Chemistry: Subtopic test

1. 1
   Methanoic acid, HCOOH, is a weak acid found in ant stings. In aqueous solution it dissociates slightly: HCOOH ⇌ H⁺ + HCOO⁻. At 298 K its acid dissociation constant, Ka, is 1.78 × 10⁻⁴ mol dm⁻³. A solution of methanoic acid has a concentration of 0.0500 mol dm⁻³.

   (a)

   Which is the correct expression for Ka for methanoic acid?

   [1 mark]

   • A[HCOOH] / [H⁺][HCOO⁻]
   • B[H⁺][HCOO⁻] / [HCOOH]
   • C[H⁺][HCOO⁻] / [H₂O][HCOOH]
   • D[HCOOH] / [H⁺]

   (b)

   What is the pKa of methanoic acid?

   [1 mark]

   • A4.25
   • B10.25
   • C3.75
   • D–3.75

   (c)

   Calculate the pH of the 0.0500 mol dm⁻³ solution of methanoic acid.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist measures the pH of a 0.150 mol dm⁻³ solution of a weak monoprotic acid, HA, at 298 K and finds it to be 2.85.

   (a)

   What is the hydrogen ion concentration in the solution?

   [1 mark]

   • A1.41 × 10⁻³ mol dm⁻³
   • B2.85 × 10⁻³ mol dm⁻³
   • C7.08 × 10⁻¹² mol dm⁻³
   • D2.85 mol dm⁻³

   (b)

   What is the value of Ka for HA, assuming [H⁺] = [A⁻] and that the equilibrium concentration of HA equals its initial concentration?

   [1 mark]

   • A2.00 × 10⁻⁶ mol dm⁻³
   • B9.42 × 10⁻³ mol dm⁻³
   • C1.33 × 10⁻³ mol dm⁻³
   • D1.33 × 10⁻⁵ mol dm⁻³

   (c)

   State two assumptions made when calculating Ka for HA from the pH of its solution.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Ethanoic acid, CH₃COOH, is the weak acid in vinegar. At 298 K its acid dissociation constant, Ka, is 1.74 × 10⁻⁵ mol dm⁻³.

   (a)

   Calculate the pH of a 0.250 mol dm⁻³ solution of ethanoic acid. State the assumptions that you make.

   [3 marks]

   (b)

   A diluted solution of ethanoic acid has a pH of 3.20. Calculate the concentration of ethanoic acid in this solution.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student compares 0.100 mol dm⁻³ solutions of hydrochloric acid and ethanoic acid at 298 K. The Ka of ethanoic acid at this temperature is 1.74 × 10⁻⁵ mol dm⁻³.

   (a)

   Calculate the pH of each solution and explain why the two pH values are different.

   [6 marks]

   (b)

   Calculate the pKa of ethanoic acid. A second weak acid, HX, has pKa = 3.80. Deduce which acid is stronger, and calculate the pH of a 0.100 mol dm⁻³ solution of HX.

   [6 marks]

   Total for question 4: 12 marks

End of questions