A-LevelChemistryAtoms, molecules and stoichiometryFeb/Mar 2021Paper 5 Q220 Marks
Ethanedioic acid is a white crystalline solid. If excess aqueous potassium hydroxide, KOH(aq), is added to dilute ethanedioic acid, H₂C₂O₄(aq), full neutralisation occurs and potassium ethanedioate, K₂C₂O₄(aq), forms. H₂C₂O₄(aq) + 2KOH(aq) → K₂C₂O₄(aq) + 2H₂O(l) If a small amount of potassium hydroxide is added, partial neutralisation takes place and not all H⁺ ions in the acid are replaced by K⁺ ions. Instead an acid salt forms, which crystallises to form a solid with the formula KₐHᵦ(C₂O₄)c•dH₂O. The letters a, b and c represent a ratio of the numbers of species present in the compound and may not necessarily be whole numbers. The relative number of water molecules associated with one formula of the compound is represented by d. A student attempted to determine the values of a, b, c and d in a sample of an acid salt, KₐHᵦ(C₂O₄)c•dH₂O.
📋 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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