A-LevelChemistryChemical bondingMay/June 2017Paper 4 Q46 Marks
A number of isomers with the formula Cr(H2O)6Cl3 exist. Their general formula is [Cr(H2O)6-nCln]Cl3-n.nH2O. Each isomer contains a six co-ordinated Cr(III) ion in an octahedral complex. Water molecules not directly bonded with the Cr atom are held in the crystal lattice as water of crystallisation. The Cr-Cl bond is not easily broken and so chloride bonded with the Cr(III) ion in the complex does not react. 1.00g samples of three of the isomers, A, B and C, were dissolved in separate samples of water. An excess of AgNO3(aq) was added to each and the mass of AgCl(s) formed was measured. Ag+(aq) + Cl-(aq) → AgCl(s) The number of moles of AgCl(s) formed was calculated. The table shows the results. isomer moles of AgCl formed from 1.00 g of isomer A 3.75 x 10-3 B 7.50 x 10-3 C 1.13 x 10-2
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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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