A-LevelChemistryAtoms, molecules and stoichiometryOct/Nov 2017Paper 3 Q116 Marks
In this experiment you will determine the value of x in the formula for hydrated copper(II) sulfate, CuSO4.xH2O. You will first react a solution of Cu2+ ions with excess iodide ions, I-. This reaction produces iodine. 2Cu2+(aq) + 4I-(aq) → 2CuI(s) + I2(aq) The amount of iodine produced can be determined by titrating with thiosulfate ions, S2O32-. I2(aq) + 2S2O32-(aq) → 2I-(aq) + S4O62-(aq) FA 1 is 0.150 moldm-³ sodium thiosulfate, Na2S2O3. FA 2 is dilute sulfuric acid. FA 3 is 1.00 mol dm⁻³ potassium iodide, KI. FA 4 is a solution made by dissolving 32.5g of CuSO4.xH2O in 1.00 dm³ of solution. 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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