6 exam-style question sets on Acids & bases (Paper 2), each with a hint and a fully worked answer. The app holds 30 questions on this section in total, including variants of every set below, and lets you mark yourself part by part.
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Question 1
Washing soda is hydrated sodium carbonate, Na₂CO₃·xH₂O. Because the carbonate ion is a WEAK BASE, a washing-soda solution is basic. A learner titrates dilute nitric acid with a washing-soda solution of unknown concentration.
The balanced equation for the reaction is:
Na₂CO₃(aq) + 2HNO₃(aq) → 2NaNO₃(aq) + CO₂(g) + H₂O(ℓ)
In each run, 25,0 cm³ of nitric acid of concentration 0,15 mol·dm⁻³ is pipetted into a conical flask and the washing-soda solution is added from a burette. The burette readings for two runs are recorded below.
| Run | Initial burette reading (cm³) | Final burette reading (cm³) | Volume of Na₂CO₃(aq) used (cm³) |
|---|---|---|---|
| 1 | p | 21,85 | 18,65 |
| 2 | 27,15 | q | 18,75 |
- aDefine the term weak base. (2)
- bWrite down the FORMULA of the conjugate base of HNO₃. (1)
- cDetermine the value of p. (1)
- dDetermine the value of q. (1)
- eMethyl orange (pH transition range 3,1–4,4) is used as the indicator. Explain why methyl orange is suitable for this titration by referring to the pH at the equivalence point. (2)
- fCalculate the concentration of the washing-soda solution. (5)
- gThe washing-soda solution was prepared by dissolving 7,17 g of Na₂CO₃·xH₂O in water and making the solution up to exactly 250 cm³. Calculate the value of x. (5)
Hint
Average the two burette volumes, use the 1 : 2 mole ratio to find c(Na₂CO₃), then n = cV and M = m/n unlock x.
Worked answer
a) A weak base ionises (dissociates) only partially / incompletely in water ✓✓ (only a small fraction of the dissolved base forms OH⁻ ions).
b) NO₃⁻ ✓
c) p = 21,85 − 18,65 = 3,20 cm³ ✓
d) q = 27,15 + 18,75 = 45,90 cm³ ✓
e) At the equivalence point the flask contains the neutral salt NaNO₃ and dissolved CO₂, which is weakly acidic, so the equivalence pH is below 7 (about 4). ✓ This pH lies inside the methyl orange transition range (3,1–4,4), so the indicator changes colour at the equivalence point. ✓
f) Average volume of Na₂CO₃(aq) = (18,65 + 18,75)/2 = 18,70 cm³ ✓
n(HNO₃) = cV = 0,15 × 0,025 = 3,75 × 10⁻³ mol ✓
n(Na₂CO₃) = 3,75 × 10⁻³ ÷ 2 = 1,875 × 10⁻³ mol (mole ratio 1 : 2) ✓
c(Na₂CO₃) = n/V = 1,875 × 10⁻³ ÷ 0,0187 ✓ = 0,10 mol·dm⁻³ ✓
g) n(Na₂CO₃) in 250 cm³ = cV = 0,10 × 0,250 = 0,025 mol ✓
M(Na₂CO₃·xH₂O) = m/n = 7,17 ÷ 0,025 ✓ = 286,80 ≈ 286 g·mol⁻¹ ✓
M(xH₂O) = 286 − 106 = 180 g·mol⁻¹ ✓
x = 180 ÷ 18 = 10 ✓ (washing soda is Na₂CO₃·10H₂O)
Modelled on Nov 2024 Paper 2, Question 7 — same skills, new scenario.
Question 2
The table shows the ionisation constants, Ka, of two acids at 25 °C.
| ACID | Ka |
|---|---|
| Methanoic acid | 1,8 × 10⁻⁴ |
| Propanoic acid | 1,3 × 10⁻⁵ |
Consider the following statements about solutions of these acids of EQUAL concentration at 25 °C:
(i) Both acids are weak acids.
(ii) Methanoic acid is a stronger acid than propanoic acid.
(iii) The propanoic acid solution has a HIGHER concentration of H₃O⁺(aq) than the methanoic acid solution.
Which of the statements above are TRUE?
Hint
A larger Ka means a greater degree of ionisation.
Worked answer
A. (i) is true: both Ka values are far smaller than 1, so both acids ionise only partially — they are weak. (ii) is true: methanoic acid has the larger Ka, so it ionises to a greater extent and is the stronger of the two. (iii) is false: the WEAKER propanoic acid ionises less, giving a LOWER [H₃O⁺] at equal concentration. B and C are wrong because each includes the false statement (iii); D is wrong for the same reason.
Question 3
Which ONE of the following pairs of acids and bases, all of the same concentration, reacts to give the LOWEST pH at the equivalence point in a titration at 25 °C?
Hint
At the equivalence point only the salt is present — which salt hydrolyses to give an acidic solution?
Worked answer
A. A strong acid + weak base gives the salt NH₄Cl; the NH₄⁺ ion hydrolyses (NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺), making the solution acidic — pH below 7, the lowest of the four. B is wrong: weak acid + strong base gives the basic ethanoate ion, pH above 7. C and D are wrong: strong acid + strong base gives neutral salts (pH = 7), which is higher than the pH in A.
Question 4
Sulphurous acid, H₂SO₃(aq), ionises in two steps in water:
Step 1: H₂SO₃(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HSO₃⁻(aq) Ka₁ = 1,3 × 10⁻²
Step 2: HSO₃⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + SO₃²⁻(aq) Ka₂ = 6,3 × 10⁻⁸
In a separate experiment, 2 g of sodium hydroxide, NaOH(s), is dissolved in water to make 500 cm³ of solution at 25 °C. Take M(NaOH) = 40 g·mol⁻¹.
- aDefine the term weak acid. (2)
- bWhich one is the stronger acid, H₂SO₃ or HSO₃⁻? Give a reason for the answer by referring to the data above. (2)
- cWrite down the FORMULA of the conjugate base of HSO₃⁻(aq). (1)
- dIdentify a substance that behaves as an ampholyte in the two steps above. (1)
- eCalculate the concentration of the NaOH solution. (3)
- fCalculate the pH of the NaOH solution at 25 °C. (4)
- gSodium sulphite, Na₂SO₃(s), is now dissolved in water. Will the resulting solution be ACIDIC or BASIC? (1)
- hWrite down a balanced equation to explain the answer to QUESTION g. (3)
Hint
Compare the two Ka values for the strength question. For the pH: NaOH is a strong base, so [OH⁻] equals its concentration; then use Kw to find [H₃O⁺].
Worked answer
a) A weak acid ionises only partially (incompletely) in water. ✓✓
b) H₂SO₃ ✓. Its Ka (1,3 × 10⁻²) is much larger than that of HSO₃⁻ (6,3 × 10⁻⁸), so it ionises to a greater extent. ✓
c) SO₃²⁻ ✓
d) HSO₃⁻ ✓ (it accepts a proton in the reverse of Step 2 and donates a proton in Step 2 — acts as base in Step 1's reverse and acid in Step 2).
e) mol ✓
✓ mol·dm⁻³ ✓
f) NaOH is a strong base and ionises completely: [OH⁻] = 0,10 mol·dm⁻³ ✓
mol·dm⁻³ ✓
✓ ✓
g) BASIC ✓
h) SO₃²⁻(aq) + H₂O(l) ⇌ HSO₃⁻(aq) + OH⁻(aq) ✓✓✓ (the sulphite ion hydrolyses and produces OH⁻ ions; 1 mark for reactants, 1 for products, 1 for balance/⇌)
Modelled on Nov 2025 Paper 2, Question 7 — same skills, new scenario.
Question 5
Dilute hydrochloric acid is added to sodium carbonate. The products of this reaction are …
Hint
Acid + metal carbonate gives three products.
Worked answer
D. An acid reacts with a metal carbonate to give a salt, water AND carbon dioxide: 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂. A and C are incomplete. B: hydrogen gas forms when acids react with active METALS, not with carbonates.
Modelled on Nov 2025 Paper 2, Question 1.7 — same skills, new scenario.
Question 6
Four solutions, each of concentration 0,2 mol·dm⁻³, are available:
HCl(aq), CH₃COOH(aq), NH₃(aq) and KOH(aq)
Which ONE of the following arranges the solutions in order of INCREASING pH?
Hint
Work from strong acid, through weak acid and weak base, up to strong base.
Worked answer
C. At equal concentration the strong acid HCl ionises completely (lowest pH), the weak acid CH₃COOH ionises partially (pH still below 7), the weak base NH₃ lies above 7, and the strong base KOH has the highest pH. A is decreasing order. B swaps the strong and weak acids. D swaps the strong and weak bases.
Modelled on Nov 2025 Paper 2, Question 1.8 — same skills, new scenario.