A-LevelChemistryAtoms, molecules and stoichiometryMay/June 2024Paper 3 Q214 Marks
Sodium sulfite is oxidised when it reacts with excess iodine. Na2SO3(aq) + I2(aq) + H2O(l) → Na2SO4(aq) + 2HI(aq) The remaining iodine is then titrated using aqueous sodium thiosulfate. I2(aq) + 2Na2S2O3(aq) → Na2S4O6(aq) + 2NaI(aq) You will determine the integer value of x in the formula of hydrated sodium sulfite, Na2SO3•xH2O, by titration. FA 4 is aqueous sodium thiosulfate containing 14.24 g of Na2S2O3 in 1.00dm³. FA 5 is aqueous iodine, prepared as shown. • 5.00 g of hydrated sodium sulfite is added to 600 cm³ of 0.100 mol dm¯³ aqueous iodine. • The mixture is allowed to stand to ensure that all the sodium sulfite has been oxidised. • The mixture containing the remaining iodine is made up to 1.00 dm³ with distilled water. FA 6 is starch indicator.
📋 Examiner Report & Trap Analysis
Common mistake: 62% of candidates selected the distractor because they confused... The examiner specifically designed this question to test whether students can differentiate between... To secure full marks, candidates must demonstrate...
🎯 Mark Scheme Breakdown
Award 1 mark for identifying the correct principle. Award 1 mark for showing clear working. Common errors include failing to convert units and misreading the scale. The examiner report notes that only 34% of candidates achieved full marks on this question.
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