Lesson 12.2.2

12.2.2 Weak acids, Ka and pKa Quiz: Pearson Edexcel Chemistry, Unit 12

20 questions

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Lesson 12.2.2, Weak acids, Ka and pKa: 20 multiple choice questions for the Pearson Edexcel Chemistry (9CH0), Unit 12: Acid-base Equilibria, written with Revision Ninja.

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The 20 questions

  1. Which statement best describes a weak acid in aqueous solution?

    • It dissociates completely, releasing one mole of hydrogen ions for every mole of acid dissolved
    • It donates protons to water only in solutions more concentrated than 1.0 mol dm-3
    • It has no conjugate base because it does not donate protons to water molecules
    • It dissociates only partially, so an equilibrium is set up between the undissociated acid and its ions

  2. Which expression is the correct acid dissociation constant Ka for the equilibrium HA dissociating into H+ and A-?

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

  3. What is pKa defined as?

    • pKa = -log10(Ka)
    • pKa = 10^-Ka
    • pKa = 1/Ka
    • pKa = log10(Ka) + 14

  4. Which statement about pKa values is correct?

    • A smaller pKa means a weaker acid because fewer protons are released per molecule
    • A smaller pKa means a larger Ka and therefore a stronger acid
    • A larger pKa means the acid is fully dissociated in aqueous solution
    • A pKa of 7 means the acid is as strong as hydrochloric acid at 298 K

  5. Which of these is a weak acid?

    • Ethanoic acid
    • Hydrobromic acid
    • Hydrochloric acid
    • Nitric acid

  6. What is the value of the ionic product of water, Kw, at 298 K?

    • 1.00 x 10^-7 mol2 dm-6
    • 1.00 x 10^-14 mol2 dm-6
    • 1.00 x 10^-14 mol dm-3
    • 7.00 x 10^-14 mol2 dm-6

  7. Which expression correctly defines pH?

    • pH = -log10[H+]
    • pH = log10[H+]
    • pH = -[H+] / 14
    • pH = 10^-[H+]

  8. What is the pH of a 0.0100 mol dm-3 solution of hydrochloric acid, which is fully dissociated?

    • 12.00
    • 1.00
    • 2.00
    • 10.00

  9. A weak acid has Ka = 1.8 x 10^-5 mol dm-3. Assuming the dissociation is small, what is the approximate pH of a 0.100 mol dm-3 solution?

    • 1.00
    • 3.37
    • 4.74
    • 2.87

  10. A 0.100 mol dm-3 weak monobasic acid has pH 2.87. Which value is the best estimate of its Ka?

    • 2.9 x 10^-3 mol dm-3
    • 1.8 x 10^-3 mol dm-3
    • 1.8 x 10^-7 mol dm-3
    • 1.8 x 10^-5 mol dm-3

  11. What is the pKa of an acid with Ka = 6.3 x 10^-5 mol dm-3?

    • 3.20
    • 4.20
    • 5.20
    • 4.80

  12. The pKa of a weak acid is 4.00. What is its Ka?

    • 1.0 x 10^4 mol dm-3
    • 1.0 x 10^-4 mol dm-3
    • 4.0 x 10^-4 mol dm-3
    • 1.0 x 10^-10 mol dm-3

  13. A 0.50 mol dm-3 solution of a weak monobasic acid has Ka = 2.0 x 10^-4 mol dm-3. What is its pH?

    • 1.70
    • 2.00
    • 3.70
    • 4.00

  14. A strong acid solution is diluted 100 times, and its pH rises from 2.00 to 4.00. A weak acid of the same starting pH is diluted 100 times. What happens to its pH?

    • Its pH falls by 2 units because dilution makes an acid more acidic
    • Its pH rises by exactly 2 units, the same as the strong acid
    • Its pH rises by less than 2 units because more of the weak acid dissociates as it is diluted
    • Its pH stays exactly the same because weak acids do not dissociate on dilution

  15. A weak monobasic acid HA has Ka = 1.6 x 10^-5 mol dm-3. What is the pH of a 0.20 mol dm-3 solution?

    • 3.20
    • 2.20
    • 4.80
    • 2.75

  16. A 0.80 g sample of a weak monobasic acid (Mr = 80.0) is dissolved to make 500 cm3 of solution, and its pH is 3.00. Assuming [HA] is approximately 0.0200 mol dm-3, what is Ka?

    • 5.0 x 10^-5 mol dm-3
    • 1.5 x 10^-4 mol dm-3
    • 5.0 x 10^-3 mol dm-3
    • 2.0 x 10^-6 mol dm-3

  17. What is the pH of a 0.0010 mol dm-3 solution of sodium hydroxide at 298 K?

    • 14.00
    • 3.00
    • 11.00
    • 7.00

  18. At 350 K, Kw for water is 2.5 x 10^-14 mol2 dm-6. What is the pH of pure water at this temperature?

    • 7.60
    • 7.00
    • 6.80
    • 6.60

  19. Which assumption is needed to use [H+] = sqrt(Ka x c) for a weak acid?

    • The acid dissociates only slightly
    • The acid is fully dissociated, so [H+] equals the acid concentration
    • The Ka value is greater than 1 mol dm-3, so the equilibrium lies to the right
    • Water is the only source of hydrogen ions, so [H+] is 1.0 x 10^-7 mol dm-3

  20. Two 0.10 mol dm-3 solutions are compared: hydrochloric acid, and a weak monobasic acid with Ka = 1.7 x 10^-5 mol dm-3. Which statement is correct?

    • The hydrochloric acid has the lower pH, about 1.00
    • The weak acid has the lower pH because its Ka is larger than the strength of hydrochloric acid
    • Both solutions have pH 1.00 because both are monobasic acids at the same concentration
    • The weak acid has the lower pH because a weak acid releases more hydrogen ions per molecule

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