A-LevelChemistryAtoms, molecules and stoichiometryFeb/Mar 2025Paper 3 Q216 Marks
2 Many oxidising agents are able to oxidise acidified potassium iodide to iodine in acidic conditions. The amount of iodine produced can be determined by titrating it with aqueous sodium thiosulfate. I2(aq) + 2Na2S2O3(aq) → 2NaI(aq) + Na2S4O6(aq) You will determine the change in oxidation state of an oxidising agent when it reacts with iodide ions. FA 2 is aqueous sodium thiosulfate, containing 22.00 g Na2S2O3•5H2O (Mr = 248.2) in 1.00dm³. FA 3 is a 0.0175moldm⁻³ solution of an oxidising agent. FA 4 is 0.50 moldm⁻³ potassium iodide, KI. FA 5 is 1.00 moldm⁻³ sulfuric acid, H2SO4. FA 6 is starch solution.
📋 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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