Lesson 3.1.12.4

3.1.12.4 Weak acids and bases Ka for weak acids Quiz: AQA Chemistry, Unit 1

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 3.1.12.4, Weak acids and bases Ka for weak acids: 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. For a weak acid HA dissociating as HA <=> H^+ + A^-, which is the correct expression for Ka?

    • Ka = [H^+] + [A^-]/[HA]
    • Ka = [HA]/([H^+][A^-])
    • Ka = [H^+][HA]/[A^-]
    • Ka = [H^+][A^-]/[HA]

  2. What is pKa defined as?

    • pKa = -Ka/10
    • pKa = -log10 Ka
    • pKa = log10 Ka
    • pKa = 1/Ka

  3. A weak acid has Ka = 1.0 x 10^-5 mol dm^-3. What is its pKa?

    • 1.00
    • 10.00
    • 5.00
    • -5.00

  4. A weak acid has pKa = 4.74. What is its Ka in mol dm^-3?

    • 1.8 x 10^-5
    • 4.7 x 10^-4
    • 4.74 x 10^-1
    • 5.5 x 10^-5

  5. What are the units of Ka for a weak acid?

    • s^-1
    • mol dm^-3
    • dm^3 mol^-1
    • No units, because Ka is a ratio

  6. Which acid is stronger, one with pKa = 3.0 or one with pKa = 5.0?

    • The acid with pKa = 3.0, because it has the larger Ka
    • The acid with pKa = 5.0, because it dissociates more
    • They are equally strong, because both are weak acids
    • The acid with pKa = 5.0, because it has the larger pKa

  7. Ethanoic acid has Ka = 1.8 x 10^-5 mol dm^-3. What is the approximate pH of a 0.050 mol dm^-3 solution?

    • 2.87
    • 4.74
    • 1.30
    • 3.02

  8. A weak acid with Ka = 1.8 x 10^-5 mol dm^-3 is at 0.10 mol dm^-3. What is its pH, to two decimal places?

    • 1.34
    • 4.74
    • 2.87
    • 1.00

  9. A 0.10 mol dm^-3 solution of a weak acid has pH 3.0. What is its Ka?

    • 1.0 x 10^-6 mol dm^-3
    • 1.0 x 10^-5 mol dm^-3
    • 1.0 x 10^-3 mol dm^-3
    • 3.0 x 10^-4 mol dm^-3

  10. A 0.50 mol dm^-3 solution of a weak acid has pH 4.0. What is its Ka?

    • 2.0 x 10^-8 mol dm^-3
    • 2.0 x 10^-10 mol dm^-3
    • 8.0 x 10^-5 mol dm^-3
    • 2.0 x 10^-4 mol dm^-3

  11. A strong acid and a weak acid have the same concentration. Which has the higher pH?

    • They have the same pH, because they have the same concentration
    • The strong acid, because it is fully dissociated and gives more H+ ions
    • The strong acid, because its pH is always above 7
    • The weak acid, because it is only partly dissociated and gives fewer H+ ions

  12. Hydrofluoric acid has Ka = 6.8 x 10^-4 mol dm^-3 and ethanoic acid has Ka = 1.8 x 10^-5 mol dm^-3. Which statement is correct at equal concentrations?

    • Ethanoic acid has the higher Ka and so is the stronger acid
    • Hydrofluoric acid has the higher [H+] and lower pH
    • Ethanoic acid has the higher [H+] and lower pH
    • Both have the same pH, because both are weak acids

  13. What is the pH of a weak acid at half-neutralisation by a strong base?

    • Equal to 7, because half-neutralisation always gives a neutral solution
    • Equal to the pOH of the strong base
    • Equal to 14 minus the pKa of the weak acid
    • Equal to the pKa of the weak acid

  14. A weak acid has pKa = 4.0 and is at 0.20 mol dm^-3. What is the approximate pH?

    • 0.70
    • 4.00
    • 3.35
    • 2.35

  15. When a weak acid is diluted ten times, its pH rises by less than one unit. Why?

    • On dilution the Ka of the acid increases by a factor of ten, so [H+] stays the same
    • On dilution the acid dissociates to a greater extent, so [H+] falls by less than tenfold
    • On dilution the water molecules stop the acid dissociating at all
    • On dilution the acid becomes a strong acid, so [H+] falls by exactly tenfold

  16. A weak acid at 0.10 mol dm^-3 has pH 2.87. What is its pKa?

    • 5.00
    • 1.00
    • 4.74
    • 2.87

  17. Which expression is an approximation for Ka of a weak acid when [H+] and c are known?

    • Ka = c/[H^+]^2
    • Ka = [H^+]/c
    • Ka = [H^+]^2/c
    • Ka = c x [H^+]

  18. A weak acid with pKa = 3.8 is at 0.40 mol dm^-3. What is its Ka?

    • 3.8 x 10^-1 mol dm^-3
    • 1.6 x 10^-4 mol dm^-3
    • 6.3 x 10^-4 mol dm^-3
    • 2.5 x 10^-5 mol dm^-3

  19. Which is the pKa of a weak acid with Ka = 6.3 x 10^-4 mol dm^-3?

    • -3.20
    • 6.30
    • 3.20
    • 4.20

  20. Why is the approximation [H^+] = sqrt(Ka x c) acceptable for a weak acid?

    • The acid is only slightly dissociated, so the undissociated acid is close to the starting concentration
    • The acid dissociates completely, so the H+ concentration equals the starting concentration
    • The pH is always seven, so the concentration of H+ is fixed
    • The Ka value is always one, so the square root cancels the dissociation

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