A-LevelChemistryAnalytical techniquesOct/Nov 2025Paper 5 Q116 Marks
The concentration of aqueous chloride ions can be found by titration with aqueous silver nitrate, AgNO3(aq). Ag+(aq) + Cl-(aq) → AgCl(s) The indicator used is aqueous potassium chromate(VI), K2CrO4(aq). As AgNO3(aq) is added to aqueous chloride ions, a white precipitate of AgCl(s) is formed. When all the chloride ions have reacted, further addition of AgNO3(aq) leads to the formation of a red precipitate of silver chromate(VI), Ag2CrO4(s). The first appearance of the red precipitate shows the end-point of the titration. A student carries out an experiment to determine the number of molecules of water of crystallisation, x, in hydrated barium chloride, BaCl2•xH2O(s).
📋 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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