A-LevelChemistryEquilibriaOct/Nov 2018Paper 5 Q112 Marks
A sample of waste water contains calcium ions, copper(II) ions and lead(II) ions. You are to plan an experimental procedure to separate the metal ions in the waste water sample by forming precipitates of the metal ions. The table shows the numerical values of the solubility products, Ksp, of some compounds. For example, the solubility product of calcium chloride, Ksp = [Ca²⁺][Cl⁻]² = 2 × 10¹ mol³ dm⁻⁹. The smaller the solubility product, the easier it is for a precipitate to form. Ksp values | | Cl⁻ | CO₃²⁻ | S²⁻ | |:---|:---|:---|:---| | Ca²⁺ | 2 × 10¹ | 5 × 10⁻⁹ | 3 × 10⁻¹⁰ | | Cu²⁺ | 1 × 10¹ | 3 × 10⁻¹⁰ | 6 × 10⁻³⁶ | | Pb²⁺ | 2 × 10⁻⁵ | 7 × 10⁻¹⁴ | 3 × 10⁻²⁸ | You are provided with * a sample of waste water containing Ca²⁺(aq), Cu²⁺(aq) and Pb²⁺(aq), * HCl(aq), * (NH₄)₂CO₃(aq), * (NH₄)₂S(aq). No other reagents should be used. Standard laboratory equipment is available.
📋 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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