Weak acids, Ka, Kw and strong basesEdexcel International A Level Chemistry: Subtopic test

1. 1
   Benzoic acid, C₆H₅COOH, is a weak monoprotic acid used as a food preservative. At 298 K its acid dissociation constant, Ka, is 6.3 × 10⁻⁵ mol dm⁻³.

   (a)

   Which expression gives Ka for benzoic acid?

   [1 mark]

   • A[C₆H₅COOH] / ([H⁺][C₆H₅COO⁻])
   • B[H⁺][C₆H₅COOH] / [C₆H₅COO⁻]
   • C[H⁺][C₆H₅COO⁻] / [C₆H₅COOH]
   • D[H⁺][C₆H₅COO⁻] / ([C₆H₅COOH][H₂O])

   (b)

   What is the pKa of benzoic acid at 298 K?

   [1 mark]

   • A4.20
   • B9.80
   • C2.10
   • D6.30

   (c)

   Calculate the pH of a 0.0100 mol dm⁻³ solution of benzoic acid, stating the assumptions you make.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician works with aqueous solutions at 25 °C, at which the ionic product of water, Kw, is 1.00 × 10⁻¹⁴ mol² dm⁻⁶. At 50 °C the value of pKw is 13.26.

   (a)

   A sodium hydroxide solution has a concentration of 0.0500 mol dm⁻³ at 25 °C. What is its pH?

   [1 mark]

   • A1.30
   • B13.30
   • C12.30
   • D12.70

   (b)

   What is the pH of pure water at 50 °C?

   [1 mark]

   • A7.00
   • B6.63
   • C13.26
   • D14.00

   (c)

   Calculate the pH of 0.0150 mol dm⁻³ barium hydroxide solution, Ba(OH)₂, at 25 °C.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In Core Practical 11, a student dissolves 0.610 g of benzoic acid, C₆H₅COOH (M = 122.0 g mol⁻¹), in water and makes the solution up to 100 cm³ in a volumetric flask. The pH of the solution at 25 °C is measured as 2.75.

   (a)

   Calculate the value of Ka for benzoic acid from these data, stating the units.

   [3 marks]

   (b)

   The student dilutes the solution tenfold with water. Using your value of Ka, calculate the new pH, and explain why the pH rises by less than one unit.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A class measures the pH of a range of 0.100 mol dm⁻³ solutions at 25 °C. The results are: hydrochloric acid 1.00; ethanoic acid 2.88; sodium hydroxide 13.00; ammonia solution 11.12; sodium chloride 7.0; ammonium chloride 5.1; sodium ethanoate 8.9.

   (a)

   Use the data to explain the differences in pH between the strong and weak acids and bases, and the pH values of the three salt solutions.

   [6 marks]

   (b)

   Use the pH of the ethanoic acid to calculate its Ka and pKa. Then calculate the pH of the ethanoic acid after a tenfold dilution and compare the change with that of the hydrochloric acid.

   [6 marks]

   Total for question 4: 12 marks

End of questions