Acid–base & solubility equilibria: A Level Chemistry questions with answers
Free Acid–base & solubility equilibria questions for A Level Chemistry (9701), part of Physical chemistry. Each question has the answer and the marking points an examiner would look for.
Acid–base & solubility equilibria practice questions
Question 1Structured · 11 marksA* challenge
A weak acid HA has Ka = 1.74 × 10−5 mol dm−3.
(a)
Calculate the pH of a 0.200 mol dm−3 solution of HA. State one assumption.
[3]
(b)
A buffer contains 0.03 mol HA and 0.015 mol NaA in 1.00 dm3. Calculate its pH, and explain how it resists a change in pH when a small amount of acid is added.
[5]
(c)
A sparingly soluble salt MX has Ksp = 1.8 × 10−10 mol2 dm−6. Calculate its solubility in water and in 0.10 mol dm−3 NaX.
[3]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
2.73; [H+] = [A−] and dissociation is negligible
[H+] = √(Kac) C1 2.73 A1 assumption stated B1
3
(b)
4.46; A− + H+ → HA, large reservoir of A−
[H+] = Ka[HA]/[A−] C1 4.46 A1 added H+ removed by A−B1 equation A− + H+ → HA B1 ratio [HA]/[A−] changes little B1
5
(c)
1.34 × 10−5, 1.80 × 10−9
s = √KspC1 1.34 × 10−5A1 common ion: s = Ksp ÷ 0.10 = 1.80 × 10−9A1
3
Question 2Structured · 3 marksA* challenge
Solubility product applies to sparingly soluble salts.
(a)
Write the Ksp expression for calcium fluoride, CaF₂.
[1]
(b)
Explain the common ion effect when sodium fluoride is added to a saturated CaF₂ solution.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
Ksp = [Ca²⁺][F⁻]²
Ksp = [Ca²⁺][F⁻]2B1
1
(b)
equilibrium shifts left; precipitate
[F⁻] increases B1 equilibrium shifts to form more CaF₂ precipitate B1
2
Question 3Structured · 9 marks
The solubility product of CaF₂ is 3.90 × 10−11 mol3 dm−9 at 298 K.
(a)
Write the expression for Ksp of this salt.
[1]
(b)
Calculate the solubility of the salt in mol dm−3.
[3]
(c)
Explain the common ion effect and predict how solubility changes in a solution already containing the anion at 0.10 mol dm−3.
[3]
(d)
State whether a precipitate forms when equal volumes of solutions containing 2.0 × 10−3 mol dm−3 of each ion are mixed. Explain.
added H⁺ reacts with ethanoate ions B1 CH₃COO⁻ + H⁺ → CH₃COOH B1 large reservoir of ethanoate, so [H⁺] changes little B1
3
Question 6Structured · 11 marks
1.00 mol of dinitrogen tetroxide is allowed to reach equilibrium at a total pressure of 150 kPa: N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57 kJ mol⁻¹. At equilibrium, 25% of the N₂O₄ has dissociated.
(a)
Write the expression for Kp for this equilibrium.
[1]
(b)
Calculate the mole fraction of each gas at equilibrium.
[2]
(c)
Use your answers to (b) to calculate Kp, giving its units.
[3]
(d)
State and explain the effect on the position of equilibrium of increasing the total pressure at constant temperature.
[2]
(e)
State and explain the effect on the value of Kp of increasing the temperature.
[2]
(f)
Suggest why the colour of the mixture becomes darker when it is heated at constant volume.
[1]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
Kp = p(NO₂)²/p(N₂O₄)
Kp = pNO₂2 ÷ pN₂O₄B1
1
(b)
N₂O₄ 0.600; NO₂ 0.400
amounts 0.75 mol N₂O₄ and 0.50 mol NO₂, total 1.25 mol C1 x(N₂O₄) = 0.600, x(NO₂) = 0.400 A1
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Original Cambridge-style practice questions written for mentorriyaz.in. These are not Cambridge past paper questions, and this site is not affiliated with or endorsed by Cambridge University Press & Assessment. Updated 2026-11-04. Paper generator · mentorriyaz.in