OCR A Jan 2012 Paper 5 Q8

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8 Haematite is the main ore of iron. The percentage of iron in a sample of haematite can be determined using the method below. 20 Method Stage 1. An excess of concentrated hydrochloric acid is added to a 3.25 g sample of haematite. The iron(III) oxide in the haematite reacts to form a solution containing Fe3+ ions. Stage 2. An excess of aqueous tin(II) chloride is added. Sn2+ reduces the Fe3+ present to Fe2+. Excess Sn2+ is removed. Stage 3. The solution is diluted and made up to 250.0 cm3 in a volumetric flask. Stage 4. A 25.0 cm3 sample of this solution is pipetted into a conical flask. Stage 5. The solution in the conical flask is titrated with 0.0200 mol dm3 aqueous potassium dichromate(VI), K2Cr2O7. The Fe2+ ions are oxidised to Fe3+ ions. Stage 6. Stages 4 and 5 are repeated to obtain an average titre of 26.5 cm3. You are provided with the following electrode potentials. You may need to use this information throughout this question. Sn4+(aq) + 2e Fe3+(aq) + e 2(aq) + 14H+(aq) + 6e Cl2(aq) + 2e (aq) + 8H+(aq) + 5e Cr2O7 MnO4 E o = +0.15 V Sn2+(aq) E o = +0.77 V Fe2+(aq) 2Cr3+(aq) + 7H2O(l) E o = +1.33 V E o = +1.36 V 2Cl (aq) Mn2+(aq) + 4H2O(l) E o = +1.51 V (a) Write an equation for the reaction between iron(III) oxide and concentrated hydrochloric acid, occurring in Stage 1.[1] (b) Write equations for the reactions involving iron ions in Stages 2 and 5. Stage 2 .Stage 5 .[2]OCR 2012<br />
 (c) Calculate the percentage by mass of iron in the haematite ore. 21 percentage iron =% [5] (d) Aqueous potassium manganate(VII), KMnO4(aq), is not suitable for titrating the solution in this method. Aqueous potassium dichromate(VI), K2Cr2O7(aq), is used instead. Suggest and explain why potassium dichromate(VI), K2Cr2O7, is suitable for this titration whereas potassium manganate(VII), KMnO4, is not suitable.[2] [Total: 10] END OF QUESTION PAPEROCR 2012<br />

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