Lesson 3.1.12.6

3.1.12.6 Buffer action Quiz: AQA Chemistry, Unit 1

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 3.1.12.6, Buffer action: 20 multiple choice questions for the AQA Chemistry (7405), Unit 1: Physical chemistry, written with Revision Ninja.

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The 20 questions

  1. What is a buffer solution?

    • A solution that always has a pH of exactly 7 at 298 K, whatever acid or base is added to it during the test
    • A solution that maintains an approximately constant pH when it is diluted or a small amount of acid or base is added
    • A solution of a strong acid and its salt, which fully dissociates on dilution and keeps the pH constant at all times
    • A solution that contains only a strong base and an indicator, which changes colour when the pH is kept constant

  2. What is an acidic buffer solution made from?

    • A weak acid and the salt of that weak acid
    • A weak base and the salt of that weak base
    • A strong acid and the salt of that strong acid
    • A strong acid and a strong base in equal amounts

  3. What is a basic buffer solution made from?

    • A weak base and the salt of that weak base
    • A strong base and its salt, which fully dissociates
    • A strong acid and a strong base mixed in equal amounts
    • A weak acid and the salt of that weak acid

  4. Which pair makes an acidic buffer?

    • Ammonia and hydrochloric acid in equal moles
    • Ethanoic acid and sodium ethanoate
    • Sodium hydroxide and sodium chloride
    • Hydrochloric acid and sodium chloride

  5. Which pair makes a basic buffer?

    • Ethanoic acid and sodium ethanoate
    • Hydrochloric acid and sodium hydroxide
    • Sodium chloride and sodium nitrate
    • Ammonia and ammonium chloride

  6. When a small amount of hydrochloric acid is added to an ethanoic acid and sodium ethanoate buffer, what happens?

    • The buffer stops working, because the acid and the salt cancel each other out and the pH jumps to the value of the acid
    • The sodium ions react with the H+, removing it from solution completely and leaving the pH unchanged after addition
    • The ethanoate ions react with the added H+ to form ethanoic acid, so the pH changes only slightly
    • The ethanoic acid dissociates completely, so the pH falls to 1 almost at once when the acid is added to it

  7. When a small amount of sodium hydroxide is added to an ethanoic acid and sodium ethanoate buffer, what happens?

    • The OH- ions remain free in solution, so the pH rises to 14 immediately, as with any strong base added at once
    • The buffer stops working, because the OH- ions cancel the salt, so the mixture behaves as pure water at pH 7
    • The ethanoic acid reacts with the added OH- to form ethanoate ions and water, so the pH changes only slightly
    • The sodium ethanoate reacts with the OH- to form ethanoic acid, which raises the pH sharply to a high value at once

  8. An acidic buffer has pKa = 4.74. What is the pH when the acid and salt concentrations are equal?

    • 4.74
    • 7.00
    • 2.87
    • 9.26

  9. An acidic buffer has pKa = 4.74. The acid is 0.30 mol dm^-3 and the salt is 0.10 mol dm^-3. What is the pH?

    • 4.26
    • 5.22
    • 4.74
    • 3.56

  10. An acidic buffer has pKa = 4.74. The acid is 0.10 mol dm^-3 and the salt is 0.40 mol dm^-3. What is the pH?

    • 4.74
    • 6.40
    • 4.14
    • 5.34

  11. An acidic buffer has pKa = 4.74. A 1 dm^3 buffer contains 0.50 mol of acid and 0.50 mol of salt, and 0.25 mol of extra salt is added with no change in acid amount. What is the new pH, to two decimal places?

    • 5.22
    • 4.74
    • 4.92
    • 4.44

  12. Why does dilution of a buffer solution have little effect on its pH?

    • The buffer contains a strong acid, which does not dissociate on dilution
    • The buffer contains only water, which is neutral at all concentrations
    • The ratio of weak acid to its salt, which sets the pH, stays the same on dilution
    • Dilution increases the Ka of the weak acid, which exactly cancels the change

  13. Where in biology is buffer action important, with a buffer system involving hydrogencarbonate ions?

    • In sugar solutions, where the pH is unaffected by any acid or base
    • In blood, which must be kept at a nearly constant pH
    • In the air, where carbon dioxide is absorbed into the sea
    • In the stomach only, where the pH is always above 7

  14. Why are buffer solutions important in living systems?

    • They raise the pH of body fluids to exactly 14 so that reactions are faster
    • They remove all acids and bases from the body completely
    • They supply the oxygen needed for respiration in all tissues
    • They keep the pH within a narrow range that enzymes and cell processes need to work properly

  15. Why does a buffer need both a weak acid and its salt, rather than just a strong acid?

    • A strong acid is fully dissociated, so its salt and acid cannot form a stable pH-controlling equilibrium
    • A strong acid has no conjugate base, so it cannot give any pH control at all in a solution with salt present
    • A strong acid produces too much heat when it is mixed with its salt, which destroys the salt in the solution
    • A strong acid cannot react with a base, so it cannot be part of any buffer and cannot resist any change in pH

  16. Which change increases the buffer capacity of an acidic buffer?

    • Diluting the buffer to a very small volume with water
    • Using higher concentrations of the weak acid and its salt in the same ratio
    • Using a strong acid instead of the weak acid, at the same concentration
    • Removing the salt, so that only the weak acid is present

  17. Which mixture would NOT act as a buffer?

    • Sodium hydroxide and hydrochloric acid in equal moles
    • Ethanoic acid and sodium ethanoate in equal moles
    • Ammonia and ammonium chloride in equal moles
    • Propanoic acid and sodium propanoate in equal moles

  18. A buffer has pKa = 4.74 and [salt]/[acid] = 10. What is its pH?

    • 6.74
    • 5.74
    • 3.74
    • 4.74

  19. What does the weak acid component of an acidic buffer do when OH^- ions are added?

    • It does nothing, because weak acids cannot react with bases at all, so the pH rises as with pure water
    • It reacts with the OH^- to form its conjugate base and water, so the pH changes only slightly
    • It releases more OH^- ions, so the pH rises sharply to a value of 14 at once, as with any strong base
    • It forms a precipitate with the OH^- ions, which are removed from the solution completely at once

  20. Why does a basic buffer resist a change in pH when acid is added?

    • Its weak base accepts the added H^+ ions, forming its conjugate acid
    • Its salt releases H^+ ions, which make the solution more acidic
    • Its weak base releases OH^- ions, which neutralise the added acid to water
    • It does not react with acid, so the pH of the buffer is unchanged by any addition

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