IJMB CHEMISTRY 2025 PAPER 3


QUESTION 1 — TITRATION PRACTICAL

Given Data:
Solution A: H₂SO₄ = 2.45 g dm⁻³
Solution B: Na₂CO₃.xH₂O = 1.43 g dm⁻³
Volume of B pipetted = 25 cm³
Indicator: Methyl orange

Reaction:
Na₂CO₃ + H₂SO₄ → Na₂SO₄ + CO₂ + H₂O

(b) CALCULATIONS

(i) Mass concentration of A
= 2.45 g dm⁻³

(ii) Molar concentration of A
Molar mass of H₂SO₄ = 98 g/mol

C = 2.45 / 98
= 0.025 mol dm⁻³

(iii) Moles of H₂SO₄ used
Volume = 25 cm³ = 0.025 dm³

n = C × V
= 0.025 × 0.025
= 6.25 × 10⁻⁴ mol

(iv) Moles of Na₂CO₃.xH₂O used
Ratio = 1 : 1

= 6.25 × 10⁻⁴ mol

(v) Molar concentration of B
C = n / V
= (6.25 × 10⁻⁴) / 0.025
= 0.025 mol dm⁻³

(vi) Molar mass of Na₂CO₃.xH₂O

Mass in 100 cm³ = 1.43 × 0.1 = 0.143 g

M = 0.143 / (6.25 × 10⁻⁴)
= 228.8 g/mol

(vii) Value of x

106 + 18x = 228.8
18x = 122.8
x = 6.82 ≈ 7

Na₂CO₃.7H₂O

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(c) WHY METHYL ORANGE IS USED

  • The reaction is between a strong acid (H₂SO₄) and a weak base (Na₂CO₃)
  • The equivalence point lies in the acidic range (pH 3–5)
  • Methyl orange changes colour within pH 3.1–4.4, suitable for endpoint
  • Phenolphthalein is unsuitable because it changes in alkaline range (8.3–10) and gives a false endpoint

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QUESTION 2 — QUALITATIVE ANALYSIS

A1: LEAD TRIOXONITRATE(V) Pb(NO₃)₂

(a) Confirmatory tests for Pb²⁺

  1. Add dilute H₂SO₄
    Observation: White precipitate (PbSO₄)
    Inference: Pb²⁺ present

  2. Add dilute HCl
    Observation: White precipitate (PbCl₂), soluble on heating and reappears on cooling
    Inference: Pb²⁺ confirmed

Alternative strong test:
Add KI → yellow precipitate of PbI₂

(b) Tests for NO₃⁻

  1. Brown ring test
    Observation: Brown ring at interface
    Inference: Nitrate ion present

  2. Copper + conc H₂SO₄ test
    Observation: Brown gas (NO₂) formed
    Inference: Nitrate confirmed

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A2: ACETOPHENONE (C₆H₅COCH₃)

(a) Physical properties

  • Colourless liquid
  • Sweet aromatic smell
  • Slightly oily

(b) Solubility

(i) Water
Slightly soluble, cloudy mixture

(ii) Dilute HCl
Slight solubility, no major reaction

(iii) Dilute NaOH
Insoluble, no reaction

(c) Functional group tests

  1. Flame test
    Observation: Smoky/sooty flame
    Inference: Aromatic compound present

  2. NaHCO₃ test
    Observation: No gas evolved
    Inference: No carboxylic acid group

  3. 2,4-DNPH test
    Observation: Orange/yellow precipitate
    Inference: Carbonyl group (C=O) present

  4. Iodoform test (IMPORTANT)
    Observation: Yellow precipitate of CHI₃
    Inference: Methyl ketone present (acetophenone confirmed)

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FINAL CONCLUSION

  • Solution A is a strong acid (H₂SO₄)
  • Solution B is hydrated sodium carbonate (Na₂CO₃.7H₂O)
  • Acetophenone is an aromatic methyl ketone confirmed by DNPH and iodoform tests