Solid sodium sulfite is often provided as the hydrated salt, Na2SO3•xH2O, where x is an integer. You will determine x by using a solution of this sodium sulfite and reacting it with an excess of aqueous iodine.
Na2SO3(aq) + I2(aq) + H2O(l) → Na2SO4(aq) + 2I⁻(aq) + 2H⁺(aq)
The amount of iodine remaining will be determined by titration using a known concentration of sodium thiosulfate, Na2S2O3.
I2(aq) + 2S2O3²⁻(aq) → 2I⁻(aq) + S4O6²⁻(aq)
FA 3 is a solution containing 31.50 g dm⁻³ of hydrated sodium sulfite, Na2SO3•xH2O.
FA 4 is 0.100 mol dm⁻³ iodine, I2.
FA 5 is 0.100 mol dm⁻³ sodium thiosulfate, Na2S2O3.
FA 6 is starch indicator.

Q: What topic does this question test?

This question tests Atoms, molecules and stoichiometry in A-Level Chemistry (syllabus code 9701). It is worth 15 marks.

Q: Which exam paper is this question from?

This question appeared in the Cambridge A-Level Chemistry Feb/Mar 2022 examination, Paper 3 Variant 3 (9701_m22_qp_33.pdf).

Question

Solid sodium sulfite is often provided as the hydrated salt, Na2SO3•xH2O, where x is an integer. You will determine x by using a solution of this sodium sulfite and reacting it with an excess of aqueous iodine.
Na2SO3(aq) + I2(aq) + H2O(l) → Na2SO4(aq) + 2I⁻(aq) + 2H⁺(aq)
The amount of iodine remaining will be determined by titration using a known concentration of sodium thiosulfate, Na2S2O3.
I2(aq) + 2S2O3²⁻(aq) → 2I⁻(aq) + S4O6²⁻(aq)
FA 3 is a solution containing 31.50 g dm⁻³ of hydrated sodium sulfite, Na2SO3•xH2O.
FA 4 is 0.100 mol dm⁻³ iodine, I2.
FA 5 is 0.100 mol dm⁻³ sodium thiosulfate, Na2S2O3.
FA 6 is starch indicator.

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