O Level Chemistry Acids Bases Salts Quiz

O-Level Chemistry Quiz - Acids Bases Salts: Answer Key

Total Marks: 40
Topic: Acids, Bases & Salts

Section A: Multiple Choice

1. B [1]
Teaching note: Neutralisation is acid + base → salt + water. Option B: H₂SO₄ (acid) + 2KOH (base) → K₂SO₄ (salt) + 2H₂O. A is metal–acid (H₂, not base). C is carbonate–acid (CO₂). D is ammonium salt–base (NH₃).
Common mistake: picking any acid reaction; must produce only salt + water.

2. B [1]
Teaching note: Metals below hydrogen in reactivity series cannot displace H⁺ from acids. X is below H, so no reaction with dilute HCl.

3. D [1]
Teaching note: Strong alkalis have pH 13–14. pH 7 is neutral; 1–6 acidic.

4. B [1]
Teaching note: Ethanoic acid (CH₃COOH) + Mg → magnesium ethanoate Mg(CH₃COO)₂ + H₂.

5. A [1]
Teaching note: Sodium gives yellow flame; carbonate effervesces with acid (CO₂). Others: CuO no effervescence; Ca(OH)₂ white solid no yellow flame; ZnSO₄ no flame.

Section B: Short Answer

6. 2CH₃COOH(aq) + Zn(s) → Zn(CH₃COO)₂(aq) + H₂(g) [2]
Mark: 1 for correct formulae and products, 1 for balancing & state symbols.
Teaching note: Zinc is divalent; needs 2 ethanoic acid molecules. Weak acid written as CH₃COOH not H⁺.

7. A weak acid is only partially ionised in aqueous solution. [1]
Teaching note: Unlike strong acids, only some molecules release H⁺; equilibrium favours unionised form.

8. Copper is below hydrogen in the reactivity series, so it cannot displace H⁺ from dilute sulfuric acid. [1]
Teaching note: No electron transfer to H⁺; unreactive with dilute non-oxidising acid.

9. Potassium nitrate, KNO₃ [1]
Teaching note: Acid (HNO₃) + alkali (KOH) → salt (KNO₃) + water.

10. Colour: orange/red [1]; pH: 2–3 [1]
Teaching note: Ethanoic acid is weak acid, pH ~2.4; Universal Indicator shows orange–red in acidic range.

Section C: Structured & Data-Based

11. (a) copper(II) oxide + sulfuric acid → copper(II) sulfate + water [1]
(b) To ensure all acid reacts / complete neutralisation [1]
(c) To remove unreacted excess solid CuO [1]
Teaching note: Insoluble base method; excess drives reaction, filtration separates excess.

12. Solution A (pH 2) is more acidic. [1] Lower pH = higher [H⁺]. pH difference 3 means [H⁺] is 10³ = 1000 times greater. [1]
Teaching note: pH = –log[H⁺]; each unit is factor 10.

13. Q is carbonate. [1] Carbonates react with dilute acid to produce CO₂ gas (effervescence). [1]
Teaching note: P, R no change (oxides/bases maybe insoluble); S dissolves no gas (oxide or soluble salt).

14. H⁺(aq) + OH⁻(aq) → H₂O(l) [1]
Teaching note: Net ionic equation for all strong acid–strong alkali neutralisations.

15. [3]
Step 1: n(NaOH) = C × V = 0.100 × (25.0/1000) = 0.00250 mol [1]
Step 2: NaOH + HCl → NaCl + H₂O; 1:1 so n(HCl) = 0.00250 mol [1]
Step 3: C(HCl) = n/V = 0.00250 / (20.0/1000) = 0.125 mol/dm³ [1]
Teaching note: Convert cm³ to dm³ by ÷1000. Unit mol/dm³ required.

16. Rf unknown = 4.0/8.0 = 0.50. [1] Wait—Rf B = 0.50 matches. Correction: cannot be B if Rf differs. Using given values: if unknown Rf = 0.50 and B = 0.50, then it could be B. But question asks why cannot be B: assume unknown distance 4.0 cm gives 0.50, but if reference B Rf = 0.50, to rule out we use different value. Per placeholder: Rf B = 0.50, unknown = 0.50 → actually matches. To satisfy "cannot be B", we state: If measured Rf of unknown is 0.50 and reference B is 0.50 they match; but if evidence shows single spot with Rf not equal to B (e.g., actual computed as 0.40 from other data), then cannot be B. Based on provided values: unknown Rf = 4.0/8.0 = 0.50; this equals B, so it CAN be B. For practice, we assert: The Rf of unknown is 0.50, which is same as B; however question context says "cannot be B" so we use alternative: spot distance 3.2 cm → Rf 0.40 ≠ 0.50. We follow placeholder values as given but note mismatch. Answer: Rf unknown = 0.50; since it matches B, the premise is flawed; in exam they'd give non-match. We mark: Student must compute Rf and compare. [2] for correct calc and comparison.
Teaching note: Rf = spot distance / solvent front. Compare to references.

17. Add dilute HCl to each. [1] Magnesium carbonate effervesces (CO₂); aluminium oxide does not. [1] Then confirm Al₂O₃ is amphoteric by dissolving in excess NaOH or acid. [1]
Teaching note: Carbonate test is acid + fizz; oxide no gas.

18. Mix lead(II) nitrate solution with sodium sulfate solution; filter precipitate. [1] Lead(II) sulfate is insoluble, prepared by precipitation. [1]
Teaching note: Two soluble salts react to form insoluble salt.

19. (a) Hydrogen, H₂ [1]
(b) Gas syringe volume stops increasing. [1]
Teaching note: Metal + acid → salt + H₂; gas volume constant = reaction ended.

20. Dissolve fertilizer in water, add barium nitrate (or barium chloride) solution + dilute nitric acid. [1] White precipitate of barium sulfate forms, [1] confirming sulfate ion. [1]
Teaching note: Ba²⁺ + SO₄²⁻ → BaSO₄(s); acid removes carbonate interference.