IQ1 Consolidation — Classification and Separation

️ Activity 1 — Compare

A: Crystallisation would recover NaCl crystals from the water but would not tell you the exact mass of Ca²⁺ — the dissolved calcium would remain in solution. Gravimetric analysis is more appropriate: add excess Na₂SO₄ (or Na₂CO₃) to precipitate all Ca²⁺ as CaSO₄ (or CaCO₃), filter, dry to constant mass, weigh, and use stoichiometry to calculate m(Ca²⁺). It is the only technique that provides a quantitative measurement of a specific dissolved ion.

B: Filtration could physically separate the BaSO₄ precipitate from the solution (it is already insoluble). Gravimetric analysis in this context would involve the same filtration step — but also includes washing, drying to constant mass, and weighing to obtain a quantitative result. The difference: filtration alone gives you the solid; gravimetric analysis gives you the solid AND its mass, which is required for any quantitative determination. If you only need the solid and not a precise mass, filtration alone is sufficient.

Activity 2 — Apply to Novel Context

Scenario 1: Gravimetric analysis. Key property: Pb²⁺ forms an insoluble precipitate with specific reagents (e.g. add Na₂SO₄ → PbSO₄(s) precipitates). Procedure: add excess Na₂SO₄ to the water sample → PbSO₄ precipitates → filter with ashless paper → wash → dry to constant mass → weigh → use stoichiometry to calculate m(Pb²⁺). This gives both confirmation (precipitate forms) and quantification (mass calculated).

Scenario 2: Chromatography (paper or TLC). Key property: differential attraction of each amino acid to stationary/mobile phases gives different Rf values. Procedure: spot all three amino acids on the baseline along with known standards → develop with appropriate solvent → measure Rf values for each spot → compare to standard Rf values for identification. No quantitative measurement needed — separation and identification only.

Scenario 3: Remove CaF₂ by filtration — it is an insoluble precipitate. Pour the treated water through filter paper (or a membrane filter for industrial scale); CaF₂ is retained as residue; the treated water (filtrate) has reduced F⁻. To confirm fluoride level: use gravimetric analysis on a sample of the treated water — add excess CaCl₂ to precipitate any remaining F⁻ as CaF₂, dry, weigh, and calculate m(F⁻) to verify it is below 1.5 mg/L.

❓ Multiple Choice

1. B — Excess reagent ensures complete precipitation of all target ions. Insufficient reagent leaves some ions in solution, giving an underestimate.

2. D — n(AgCl) = 0.286 ÷ 143.3 = 0.001996 mol. n(Cl⁻) = 0.001996 mol (1:1 ratio). m(Cl⁻) = 0.001996 × 35.5 = 0.0709 g.

3. C — Residual moisture adds to the measured mass, giving an overestimate. A (insufficient reagent) and B (precipitate loss) both cause underestimates.

4. A — BaSO₄ is insoluble → filtration separates it as residue. KI is dissolved → passes through in filtrate. Crystallisation, distillation, and chromatography are not suited to separating an insoluble solid from a solution in this way.

5. B — Gravimetric analysis is a quantitative measurement technique for specific ions that form insoluble precipitates. It cannot separate miscible liquids, volatile components, or immiscible liquids. D is wrong (it is still widely used); A and C are incorrect overstatements.

Short Answer Model Answers

Q6 (3 marks): Step 1 — Add excess Na₂SO₄ to the solution: the excess ensures all Ba²⁺ ions react and precipitate as BaSO₄; without excess, some Ba²⁺ would remain dissolved and be unmeasured (1 mark). Step 2 — Filter the precipitate, wash with distilled water: filtration separates the insoluble BaSO₄ from the solution; washing removes soluble impurities that could add to the mass and cause overestimation (1 mark). Step 3 — Dry the precipitate to constant mass, then weigh: drying removes all residual water which would add falsely to the measured mass; drying to constant mass confirms no water remains; the final dry mass is used with stoichiometry to calculate m(Ba²⁺) (1 mark).

Q7 (4 marks): Reaction: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) [1:1 molar ratio] (1 mark). n(BaSO₄) = 0.932 ÷ 233.4 = 0.003993 mol (1 mark). n(SO₄²⁻) = 0.003993 mol (1:1 ratio from balanced equation) (1 mark). m(SO₄²⁻) = 0.003993 × 96.1 = 0.384 g (1 mark).

Q8 (5 marks): Gravimetric analysis is effective for determining Cl⁻ concentration in sea water because it can precisely quantify very small amounts of an ion, and AgCl is a highly insoluble precipitate that forms reliably and quantitatively (1 mark). Strength: it is a direct mass measurement — does not depend on colour, electronic signals, or calibration curves, making it highly accurate and reproducible (1 mark). Limitation 1: sea water contains multiple anions (SO₄²⁻, Br⁻, I⁻) that may also precipitate with AgNO₃, leading to co-precipitation and overestimation of the Cl⁻ content; additional steps to remove interfering ions are needed (1 mark). Limitation 2: the procedure is time-consuming — drying to constant mass can take hours to days; modern methods like ion chromatography or potentiometric titration are faster for routine analysis (1 mark). Overall: gravimetric analysis is highly accurate for Cl⁻ in sea water if interfering ions are controlled, but is not ideal for high-throughput or field testing due to time requirements (1 mark).