QUESTION 1
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
© 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⁺
-
Add dilute H₂SO₄
Observation: White precipitate (PbSO₄)
Inference: Pb²⁺ present -
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⁻
-
Brown ring test
Observation: Brown ring at interface
Inference: Nitrate ion present -
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
© Functional group tests
-
Flame test
Observation: Smoky/sooty flame
Inference: Aromatic compound present -
NaHCO₃ test
Observation: No gas evolved
Inference: No carboxylic acid group -
2,4-DNPH test
Observation: Orange/yellow precipitate
Inference: Carbonyl group (C=O) present -
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