State the oxidation half-equation.

Deduce the overall redox equation for the reaction between acidic Sn²⁺(aq) and Cr₂O₇^2–(aq), using section 24 of the data booklet.

Calculate the percentage uncertainty for the mass of K₂Cr₂O₇(s) from the given data.

The sample of K₂Cr₂O₇(s) in (i) was dissolved in distilled water to form 0.100 dm³ solution. Calculate its molar concentration.

10.0 cm³ of the waste sample required 13.24 cm³ of the K₂Cr₂O₇ solution. Calculate the molar concentration of Sn²⁺(aq) in the waste sample.

Sn²⁺(aq) → Sn⁴⁺(aq) + 2e^–

Accept equilibrium sign.

Accept Sn²⁺(aq) – 2e^– → Sn⁴⁺(aq).

[1 mark]

Cr₂O₇^2–(aq) + 14H⁺(aq) + 3Sn²⁺(aq) → 2Cr³⁺(aq) + 7H₂O(l) + 3Sn⁴⁺(aq)

Accept equilibrium sign.

[1 mark]

«13.239 g ± 0.002 g so percentage uncertainty» 0.02 «%»

Accept answers given to greater precision, such as 0.0151%.

[1 mark]

« [K₂Cr₂O₇] = \(\frac{{13.239{\text{ g}}}}{{294.20{\text{ g}}\,{\text{mo}}{{\text{l}}^{ - 1}} \times 0.100{\text{ d}}{{\text{m}}^3}}}\) =» 0.450 «mol\(\,\)dm^–3»

[1 mark]

n(Sn²⁺) = «0.450 mol\(\,\)dm^–3 x 0.01324 dm³ x \(\frac{{3\,mol}}{{1\,mol}}\) =» 0.0179 «mol»

«[Sn²⁺] = \(\frac{{0.0179\,mol}}{{0.0100\,mol}}\) =» 1.79 «mol\(\,\)dm^–3»

Award [2] for correct final answer.

[2 marks]