Buffer solutionsAQA A-Level Chemistry: Subtopic test

1. 1
   A technician prepares a buffer solution by dissolving solid sodium ethanoate in a solution of ethanoic acid.

   (a)

   Which species acts as the main reservoir of ethanoate ions in this buffer?

   [1 mark]

   • AEthanoic acid, because it is fully ionised in water
   • BWater, because it ionises to give ethanoate ions
   • CSodium ethanoate, because it is fully dissociated in solution
   • DSodium ions, because they react with added acid

   (b)

   A small amount of hydrochloric acid is added to the buffer. Which equation shows the reaction that removes the added H⁺ ions?

   [1 mark]

   • ACH₃COO⁻ + H⁺ → CH₃COOH
   • BCH₃COOH + H⁺ → CH₃COOH₂⁺
   • CNa⁺ + H⁺ → NaH²⁺
   • DCH₃COOH + OH⁻ → CH₃COO⁻ + H₂O

   (c)

   Explain how this buffer solution resists a change in pH when a small amount of sodium hydroxide solution is added.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A laboratory needs a buffer to hold a solution at an alkaline pH. The technician mixes aqueous ammonia with a solution of ammonium chloride.

   (a)

   Which statement about this type of buffer is correct?

   [1 mark]

   • AIt contains a strong base and a strong acid
   • BIt contains a weak acid and its salt
   • CIt contains a strong base and its salt
   • DIt contains a weak base and the salt of that weak base

   (b)

   A small amount of hydrochloric acid is added to this buffer. Which species mainly reacts with the added H⁺ ions?

   [1 mark]

   • ANH₄⁺ ions, which accept the H⁺
   • BNH₃ molecules, which accept the H⁺
   • CCl⁻ ions, which accept the H⁺
   • DNH₄⁺ ions, which donate H⁺ to it

   (c)

   Explain how this buffer resists a change in pH when a small amount of sodium hydroxide solution is added.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student prepares a buffer by mixing 50.0 cm³ of 0.200 mol dm⁻³ methanoic acid, HCOOH, with 25.0 cm³ of 0.100 mol dm⁻³ sodium methanoate, HCOONa. For methanoic acid, $K_a = 1.78 \times 10^{-4}$ mol dm⁻³ at 298 K.

   (a)

   Calculate the pH of the buffer solution. Give your answer to 3 significant figures.

   [3 marks]

   (b)

   5.00 cm³ of 0.100 mol dm⁻³ sodium hydroxide solution is added to the buffer. Calculate the new pH and the change in pH.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Blood plasma is kept at about pH 7.4 mainly by the H₂CO₃/HCO₃⁻ buffer system. During intense exercise, muscle cells release lactic acid into the blood. For H₂CO₃ in blood, take $K_a = 4.5 \times 10^{-7}$ mol dm⁻³.

   (a)

   Explain how the buffer system in blood limits the change in pH when lactic acid enters the blood during exercise, and why this is important.

   [6 marks]

   (b)

   Calculate the ratio [HCO₃⁻]/[H₂CO₃] in blood at pH 7.4. Hence suggest why this buffer is better at removing added acid than added alkali.

   [6 marks]

   Total for question 4: 12 marks

End of questions