Buffer solutionsEdexcel International A Level Chemistry: Subtopic test

1. 1
   A technician prepares solutions containing ethanoic acid and sodium ethanoate. The Ka of ethanoic acid is 1.74 × 10⁻⁵ mol dm⁻³, so its pKa is 4.76.

   (a)

   Which mixture would act as an acidic buffer solution?

   [1 mark]

   • Aethanoic acid and sodium ethanoate
   • Bhydrochloric acid and sodium chloride
   • Cethanoic acid and sodium chloride
   • Dsodium hydroxide and sodium ethanoate

   (b)

   A buffer contains ethanoic acid at 0.200 mol dm⁻³ and sodium ethanoate at 0.100 mol dm⁻³. What is its pH?

   [1 mark]

   • A4.76
   • B5.06
   • C4.46
   • D4.16

   (c)

   Explain how a buffer made from ethanoic acid and sodium ethanoate resists a change in pH when a small amount of hydrochloric acid is added.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Human blood is kept at a pH of about 7.4. This is maintained largely by a buffer system based on carbonic acid, H₂CO₃, and hydrogencarbonate ions, HCO₃⁻.

   (a)

   Which pair of species forms the conjugate acid–base pair in this buffer?

   [1 mark]

   • AH₂CO₃ and CO₃²⁻
   • BH₂CO₃ and HCO₃⁻
   • CHCO₃⁻ and CO₂
   • DH₂O and H₂CO₃

   (b)

   Intense exercise releases lactic acid, a source of H⁺, into the blood. Which reaction in the buffer removes it?

   [1 mark]

   • AHCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O
   • BH₂CO₃ → H⁺ + HCO₃⁻
   • CH₂CO₃ + H⁺ → H₃CO₃⁺
   • DHCO₃⁻ + H⁺ → H₂CO₃

   (c)

   Explain why it is important that the pH of blood remains almost constant, and how the buffer achieves this.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A technician needs 500 cm³ of a buffer solution of pH 5.00. The final buffer must contain ethanoic acid at a concentration of 0.500 mol dm⁻³, and the conjugate base is supplied by solid sodium ethanoate, CH₃COONa (M = 82.0 g mol⁻¹). The Ka of ethanoic acid is 1.74 × 10⁻⁵ mol dm⁻³.

   (a)

   Calculate the mass of sodium ethanoate needed.

   [3 marks]

   (b)

   The technician adds 5.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid to the buffer. Calculate the new pH and comment on the change.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student titrates 25.0 cm³ of a 0.100 mol dm⁻³ solution of a weak monoprotic acid, HA, with 0.100 mol dm⁻³ sodium hydroxide solution, measuring the pH throughout. The pH is 4.20 after 12.5 cm³ of alkali has been added, and the equivalence point is reached at 25.0 cm³.

   (a)

   Explain how this titration curve demonstrates buffer action, and how the value of Ka of the acid is determined from it.

   [6 marks]

   (b)

   A food technologist wants a buffer of pH 4.50 for a fruit preserve, made by adding sodium hydroxide solution to 50.0 cm³ of the same acid solution. Calculate the volume of 0.100 mol dm⁻³ sodium hydroxide needed, and suggest why a buffer is important in foods.

   [6 marks]

   Total for question 4: 12 marks

End of questions