Lesson 3.1.12.3

3.1.12.3 The ionic product of water, Kw Quiz: AQA Chemistry, Unit 1

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 3.1.12.3, The ionic product of water, Kw: 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 the ionic product of water, Kw, in terms of the ion concentrations?

    • Kw = [H^+] + [OH^-]
    • Kw = [H^+][OH^-]
    • Kw = [H2O]/[H^+][OH^-]
    • Kw = [H^+]/[OH^-]

  2. Why is water described as slightly dissociated?

    • Water is a strong electrolyte that fully ionises in all conditions
    • Water dissociates completely into hydrogen and oxygen gas
    • Only a very small fraction of water molecules ionise into H^+ and OH^- ions
    • Water dissociates into ions only at temperatures above 100 C

  3. At 298 K, Kw = 1.0 x 10^-14 mol^2 dm^-6. What is [H^+] in pure water?

    • 1.0 x 10^-2 mol dm^-3
    • 1.0 x 10^-12 mol dm^-3
    • 1.0 x 10^-14 mol dm^-3
    • 1.0 x 10^-7 mol dm^-3

  4. What is the pH of a 0.010 mol dm^-3 solution of sodium hydroxide, a strong base, at 298 K?

    • 7.00
    • 10.00
    • 2.00
    • 12.00

  5. A solution of a strong base has [OH^-] = 0.10 mol dm^-3 at 298 K. What is its pH?

    • 13.00
    • 0.10
    • 7.00
    • 1.00

  6. The value of Kw varies with temperature. What happens to Kw as the temperature rises?

    • It is unchanged, because Kw is a fixed constant for all substances
    • It increases, because water ionises to a greater extent at higher temperature
    • It decreases, because water ionises to a smaller extent at higher temperature
    • It becomes zero at high temperature, because all the water is converted to ice

  7. Why does pure water remain neutral even though its pH is not exactly 7 at every temperature?

    • Its [OH^-] is always zero, so it is neutral by definition at all temperatures
    • Its [H^+] equals its [OH^-] at any temperature, so it is neutral whatever its pH value
    • Its pH is always exactly 7 because Kw is always 1.0 x 10^-14, whatever the temperature
    • Its [H^+] is always greater than its [OH^-], so it is slightly acidic at all temperatures

  8. What is the approximate value of [OH^-] in a solution of pH 3 at 298 K?

    • 1.0 x 10^-7 mol dm^-3
    • 1.0 x 10^-14 mol dm^-3
    • 1.0 x 10^-3 mol dm^-3
    • 1.0 x 10^-11 mol dm^-3

  9. A solution of a strong base has pH 11.0 at 298 K. What is [OH^-] in mol dm^-3?

    • 1.0 x 10^-11
    • 1.0 x 10^-3
    • 1.0 x 10^-1
    • 1.0 x 10^-14

  10. What is the pH of 0.0050 mol dm^-3 potassium hydroxide, a strong base, at 298 K?

    • 12.30
    • 11.70
    • 7.00
    • 2.30

  11. Why is Kw described as the ionic product of water and not as a pH value?

    • It is the concentration of H2O molecules, which never changes in any solution and so is written as a constant
    • It is a constant for the equilibrium H2O <=> H+ + OH-, from which pH is calculated for any solution
    • It is the pressure of hydrogen gas formed when water ionises at 298 K, which is measured in kPa in a closed vessel
    • It is the pH value of pure water at 298 K, which is always a fixed number that is the same for all solutions

  12. A solution of pH 9.0 is at 298 K. What is its [OH^-]?

    • 1.0 x 10^-3 mol dm^-3
    • 1.0 x 10^-9 mol dm^-3
    • 1.0 x 10^-5 mol dm^-3
    • 1.0 x 10^-14 mol dm^-3

  13. Using Kw = [H^+][OH^-] = 1.0 x 10^-14 at 298 K, which statement about a neutral solution is correct?

    • A neutral solution has [H^+] = 1.0 x 10^-14 and [OH^-] = 1.0 x 10^-14 mol dm^-3
    • A neutral solution has [H^+] = 0 and [OH^-] = 0 mol dm^-3
    • A neutral solution has [OH^-] = 1.0 and [H^+] = 1.0 x 10^-14 mol dm^-3
    • A neutral solution has [H^+] = [OH^-] = 1.0 x 10^-7 mol dm^-3 at 298 K

  14. Which expression gives the pH of a strong base when its hydroxide concentration is known?

    • pH = -log10[OH^-] at 298 K
    • pH = log10[OH^-] - 14 at 298 K
    • pH = 14 - [OH^-] at 298 K
    • pH = 14 + log10[OH^-] at 298 K

  15. A solution has [OH^-] = 1.0 x 10^-4 mol dm^-3 at 298 K. What is its pH?

    • 10.00
    • 4.00
    • 7.00
    • 14.00

  16. At 298 K, Kw = 1.0 x 10^-14. In a solution with [H^+] = 1.0 x 10^-5 mol dm^-3, what is [OH^-]?

    • 1.0 x 10^-9 mol dm^-3
    • 1.0 x 10^-14 mol dm^-3
    • 1.0 x 10^-5 mol dm^-3
    • 1.0 x 10^-19 mol dm^-3

  17. What are the units of Kw for the ionisation of water?

    • mol dm^-3
    • No units, because Kw is a pure number
    • mol^2 dm^-6
    • mol dm^-3 s^-1

  18. Pure water at 373 K has a higher Kw than at 298 K. Which statement about its pH is correct?

    • Its pH is below 7, but the water is still neutral because [H^+] = [OH^-]
    • Its pH is exactly 7 at every temperature, because neutrality always means pH 7
    • Its pH is 14, because heating raises the pH of all water to the maximum value
    • Its pH is above 7, because the water becomes alkaline when heated

  19. A student says a solution with pH 7.5 at 298 K is acidic. Which evaluation is correct?

    • Partly correct, because the solution is acidic only if it is diluted
    • Correct, because any pH above 6 is an acidic solution at 298 K
    • Incorrect, because pH 7.5 is above 7, so [OH^-] is greater than [H^+] and the solution is alkaline
    • Correct, because a pH of 7.5 means the hydrogen ion concentration is greater than the hydroxide ion concentration

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

    • 14.00
    • 3.00
    • 7.00
    • 11.00

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