OCR A Jun 2012 Paper 5 Q7

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7 Potassium manganate(VII) can be prepared in the laboratory by a two-step synthesis starting from manganese(IV) oxide. 16 Step 1 In this step, manganese(IV) oxide is heated strongly with potassium hydroxide and potassium chlorate(V), a powerful oxidising agent. Manganese(IV) oxide, MnO2, is oxidised to manganate(VI) ions. Step 2 Potassium manganate(VI) is separated from the alkaline mixture from step 1 as a green solid. In this step, potassium manganate(VI) is heated in water. Manganate(VI) ions disproportionate forming manganate(VII) ions and a precipitate of manganese(IV) oxide. (a) In step 1, a redox reaction takes place. Add the correct number of electrons to the correct sides of the incomplete oxidation and reduction half-equations shown below. MnO2 + 4OH3H2O + Cl O3 2 + 2H2O MnO4 6OH + Cl[2] (b) In step 2, an equilibrium is set up. 3MnO4 2(aq) + 2H2O(l) 2MnO4 (aq) + MnO2(s) + 4OH(aq) The equilibrium position can be shifted by bubbling carbon dioxide gas through the mixture. Suggest, with the aid of an equation, how the equilibrium position shifts.[3]OCR 2012<br /> (c) Aqueous potassium manganate(VII), KMnO4, in acidic conditions can be used in analysis. A student analyses a sample of sodium sulfite, Na2SO3, using the following method. 17The student dissolves 0.720 g of impure sodium sulfite in water. The solution is made up to 100.0 cm3. The student titrates 25.0 cm3 of this solution with 0.0200 mol dm3 KMnO4 under acidic conditions. The volume of KMnO4(aq) required to reach the end-point is 26.2 cm3. The equation for the reaction is shown below. 2MnO4+ 6H+ + 5SO3 2 2Mn2+ + 5SO4 2 + 3H2O Determine the percentage purity of the sample of sodium sulfite. percentage purity =% [5] [Total: 10] END OF QUESTION PAPEROCR 2012<br />

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