A-LevelChemistryAnalytical techniquesMay/June 2024Paper 5 Q116 Marks
Titration can be used to determine the concentration of dissolved oxygen in samples of river water. The procedure for the experiment is given. step 1 Use five 50 cm³ graduated syringes, A, B, C, D and E, to collect five separate 30.0cm³ samples of river water. step 2 In the laboratory, carefully add 5.0 cm³ of 0.220 mol dm⁻³ manganese(II) sulfate, MnSO₄(aq), into syringe A and mix well. step 3 Add 5.0 cm³ of alkaline aqueous potassium iodide into syringe A and mix well. step 4 Add 10.0cm³ of dilute sulfuric acid into syringe A and mix well. step 5 Transfer the contents of syringe A into a 150cm³ conical flask. Rinse syringe A using 10 cm³ of distilled water and add washings to the conical flask. step 6 Carry out one accurate titration of all the contents in the conical flask with 0.00200 mol dm⁻³ aqueous sodium thiosulfate, Na₂S₂O₃(aq), using starch indicator. Repeat steps 2–6 for the samples in syringes B–E.
📋 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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