Buffer solutionsEdexcel A-Level Chemistry: Subtopic test

1. 1
   A food scientist wants to keep the pH of a sauce steady during storage. She uses a mixture of ethanoic acid, CH₃COOH, and sodium ethanoate, CH₃COONa, in water. For ethanoic acid, $K_a$ = 1.74 × 10⁻⁵ mol dm⁻³ at 298 K.

   (a)

   Which statement defines a buffer solution?

   [1 mark]

   • AA solution of a strong acid whose pH does not change at all
   • BA solution that always has a pH of exactly 7
   • CA solution whose pH does not change when it is diluted by any amount
   • DA solution that resists changes in pH when small amounts of acid or alkali are added

   (b)

   Which mixture would form a buffer solution?

   [1 mark]

   • A25.0 cm³ of 0.10 mol dm⁻³ HCl + 25.0 cm³ of 0.10 mol dm⁻³ NaOH
   • B50.0 cm³ of 0.10 mol dm⁻³ CH₃COOH + 25.0 cm³ of 0.10 mol dm⁻³ NaOH
   • C25.0 cm³ of 0.10 mol dm⁻³ CH₃COOH + 25.0 cm³ of 0.10 mol dm⁻³ NaOH
   • D50.0 cm³ of 0.10 mol dm⁻³ HCl + 25.0 cm³ of 0.10 mol dm⁻³ NaOH

   (c)

   Explain how this mixture resists a change in pH when a small amount of hydrochloric acid is added.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician prepares a buffer solution by dissolving sodium ethanoate in ethanoic acid. The final solution contains ethanoic acid at 0.250 mol dm⁻³ and sodium ethanoate at 0.150 mol dm⁻³. For ethanoic acid, $K_a$ = 1.74 × 10⁻⁵ mol dm⁻³ at 298 K.

   (a)

   Which expression gives $[H^+]$ in this buffer?

   [1 mark]

   • A$[H^+] = \frac{K_a[CH_3COOH]}{[CH_3COO^-]}$
   • B$[H^+] = \frac{K_a[CH_3COO^-]}{[CH_3COOH]}$
   • C$[H^+] = \sqrt{K_a \times [CH_3COOH]}$
   • D$[H^+] = K_a[CH_3COOH][CH_3COO^-]$

   (b)

   What is the pH of a buffer made from ethanoic acid and sodium ethanoate if the two concentrations are equal?

   [1 mark]

   • A7.00
   • B9.24
   • C4.76
   • D2.38

   (c)

   Calculate the pH of the buffer solution.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student prepares 100 cm³ of a buffer solution containing 0.0500 mol of ethanoic acid and 0.0500 mol of sodium ethanoate at 298 K. For ethanoic acid, $K_a$ = 1.74 × 10⁻⁵ mol dm⁻³. She then adds 0.00500 mol of solid sodium hydroxide. Assume that adding the solid does not change the volume.

   (a)

   Calculate the pH of the solution after the sodium hydroxide has been added.

   [3 marks]

   (b)

   Compare this change with the effect of adding the same 0.00500 mol of sodium hydroxide to 100 cm³ of pure water, and explain the difference.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A laboratory prepares buffer solutions from ethanoic acid ($K_a$ = 1.74 × 10⁻⁵ mol dm⁻³ at 298 K) and sodium ethanoate. Separately, physiologists study how the blood of a runner stays at pH 7.4 even though her muscles release lactic acid during a race. Blood contains dissolved carbon dioxide (as carbonic acid, H₂CO₃) and hydrogencarbonate ions, HCO₃⁻.

   (a)

   Calculate the volume of 0.500 mol dm⁻³ sodium ethanoate solution that must be added to 100 cm³ of 0.500 mol dm⁻³ ethanoic acid to make a buffer of pH 5.00.

   [6 marks]

   (b)

   Explain how the H₂CO₃/HCO₃⁻ system keeps the pH of the runner's blood almost constant as lactic acid enters the blood, and why the system also copes with a small increase in alkali.

   [6 marks]

   Total for question 4: 12 marks

End of questions