A-LevelChemistryAnalytical techniquesOct/Nov 2020Paper 3 Q119 Marks
In this experiment you will determine the value of x in the formula of hydrated sodium thiosulfate, Na2S2O3•xH2O, where x is an integer. You will first prepare a solution of the salt and then use this solution in a titration with aqueous iodine. The thiosulfate ions react with iodine as shown. 2S2O32-(aq) + I2(aq) → S₄O‰²¯(aq) + 21-(aq) FA 1 is hydrated sodium thiosulfate, Na2S2O3•xH2O. FA 3 is 0.0500 mol dm⁻³ iodine, I2. 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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