A Level Chemistry (A2) chapter 3: Equilibria MCQs with answers

By Team Megalecture · Updated 4 Nov 2026

53 exam-style questions on Equilibria, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.

53 questions · 2 topics · 10 per test

Test this chapter freeAll A Level Chemistry (A2) chapters

What chapter 3 covers

Ten Equilibria questions with answers

Try each one, then open the answer. These ten stay the same; the test draws new ones every time.

1. During hard exercise, lactic acid releases H+ ions into the blood. How do hydrogencarbonate ions help to keep the pH of blood close to 7.4?

  1. A
    HCO3− → CO32− + H+, which neutralises the lactic acid
  2. B
    HCO3− + OH− → CO32− + H2O, which removes the lactic acid
  3. C
    HCO3− + H+ → H2CO3, which removes most of the added H+
  4. D
    H2CO3 + H+ → H3CO3+, which stores the extra acid
Show answer

Answer: C. Blood is buffered by the H2CO3/HCO3− system. Added H+ combines with HCO3− to form carbonic acid, which is removed as CO2 in the lungs, so [H+] changes very little.

2. The solubility of magnesium hydroxide, Mg(OH)2, in water at 25 °C is 9.6 × 10−3 g dm−3. What is its solubility product? (Mr of Mg(OH)2 = 58.3)

  1. A
    4.5 × 10−12 mol3 dm−9
  2. B
    8.9 × 10−12 mol3 dm−9
  3. C
    1.8 × 10−11 mol3 dm−9
  4. D
    2.7 × 10−8 mol3 dm−9
Show answer

Answer: C. s = 9.6 × 10−3 / 58.3 = 1.65 × 10−4 mol dm−3; [Mg2+] = s and [OH−] = 2s, so Ksp = 4s3 = 1.8 × 10−11. 4.5 × 10−12 forgets to double [OH−]; 8.9 × 10−12 doubles but does not square it.

3. What are the units of a partition coefficient calculated from concentrations in mol dm−3 in both solvents?

  1. A
    mol dm−3
  2. B
    dm3 mol−1
  3. C
    mol2 dm−6
  4. D
    no units
Show answer

Answer: D. It is a ratio of two concentrations in the same units, so the units cancel.

4. The solubility of calcium fluoride, CaF2, in water at 25 °C is 2.1 × 10−4 mol dm−3. What is its solubility product?

  1. A
    3.7 × 10−11 mol3 dm−9
  2. B
    9.3 × 10−12 mol3 dm−9
  3. C
    4.4 × 10−8 mol2 dm−6
  4. D
    8.8 × 10−8 mol2 dm−6
Show answer

Answer: A. [Ca2+] = 2.1 × 10−4 and [F−] = 4.2 × 10−4 mol dm−3. Ksp = [Ca2+][F−]2 = 2.1 × 10−4 × (4.2 × 10−4)2 = 3.7 × 10−11 mol3 dm−9. Forgetting to double [F−] gives 9.3 × 10−12.

5. For solute T, the partition coefficient water / ethoxyethane is 0.20. 3.00 g of T is shaken with 40 cm3 of ethoxyethane and 60 cm3 of water until equilibrium is reached. What mass of T is in the ethoxyethane layer?

  1. A
    0.35 g
  2. B
    0.69 g
  3. C
    2.31 g
  4. D
    2.50 g
Show answer

Answer: C. K(ethoxyethane / water) = 1 / 0.20 = 5.0. (x / 40) / ((3.00 − x) / 60) = 5.0 gives 60x = 200(3.00 − x), x = 2.31 g. 0.35 g uses 0.20 the wrong way round; 0.69 g is the mass left in the water.

6. A buffer is made by mixing 40.0 cm3 of 0.100 mol dm−3 weak acid HX (Ka = 6.3 × 10−5 mol dm−3) with 10.0 cm3 of 0.100 mol dm−3 NaOH. What is the pH of the buffer?

  1. A
    3.60
  2. B
    4.68
  3. C
    3.72
  4. D
    4.20
Show answer

Answer: C. HX starts at 4.00 × 10−3 mol; NaOH converts 1.00 × 10−3 mol of it to X−, leaving 3.00 × 10−3 mol of HX. [H+] = 6.3 × 10−5 × 3.00/1.00 = 1.89 × 10−4 mol dm−3, pH = 3.72. Not subtracting the acid used up gives 3.60.

7. What is the pH of 0.025 mol dm−3 nitric acid?

  1. A
    1.30
  2. B
    12.40
  3. C
    3.69
  4. D
    1.60
Show answer

Answer: D. Nitric acid is a strong monobasic acid, fully dissociated, so [H+] = 0.025 mol dm−3 and pH = −log 0.025 = 1.60. The value 3.69 comes from using ln instead of log.

8. A weak acid HA is partitioned between octan-1-ol and water. Aqueous sodium hydroxide is added to the aqueous layer and the mixture is shaken again. What happens to the amount of acid in the octan-1-ol layer, and why?

  1. A
    it increases, because the water layer is now more polar
  2. B
    it decreases, because HA is converted to A− ions which stay in the polar water layer
  3. C
    it is unchanged, because the partition coefficient is constant at constant temperature
  4. D
    it decreases, because NaOH dissolves in octan-1-ol and reacts there
Show answer

Answer: B. The ions A− are strongly hydrated and essentially insoluble in octan-1-ol. Removing HA from the water as A− draws more HA out of the organic layer, even though Kpc for HA itself is unchanged.

9. Hexan-1-ol has a much larger partition coefficient between octan-1-ol and water than methanol does. Which explanation is correct?

  1. A
    it forms more hydrogen bonds with water molecules than methanol does
  2. B
    its long non-polar hydrocarbon chain makes it much less soluble in water
  3. C
    methanol cannot form hydrogen bonds with octan-1-ol molecules
  4. D
    it has a smaller Mr than methanol, so it moves into the organic layer
Show answer

Answer: B. Both alcohols have one OH group, but hexan-1-ol also has a large non-polar C6 chain that mixes well with the hydrocarbon part of octan-1-ol and disrupts hydrogen bonding in water. Methanol is dominated by its OH group, so it stays mostly in the water.

10. A buffer contains 0.20 mol dm−3 ethanoic acid and 0.10 mol dm−3 sodium ethanoate. Ka for ethanoic acid is 1.74 × 10−5 mol dm−3. What is the pH of the buffer?

  1. A
    4.46
  2. B
    5.06
  3. C
    4.76
  4. D
    2.73
Show answer

Answer: A. [H+] = Ka × [acid]/[salt] = 1.74 × 10−5 × 0.20/0.10 = 3.48 × 10−5 mol dm−3, so pH = 4.46. Inverting the ratio gives 5.06; 4.76 is correct only when the two concentrations are equal.

Start the chapter testAbout the A Level Chemistry (A2) test

Read the notes for this chapter

Ways to practise this chapter

Equilibria: questions students ask

How many Equilibria MCQs are there?

53, each with the answer and a short explanation, across 2 topics: Acids and bases, Partition coefficients. Every chapter test draws ten of them, so you can take it many times.

Is the A Level Chemistry (A2) chapter 3 test free?

Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.

How is the chapter test marked?

The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.

Keep going

← Chapter 2: ElectrochemistryChapter 4: Reaction kinetics →All chaptersStuck? Ask a question