OCR A Jun 2011 Paper 5 Q8

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8 Brass is an alloy which contains copper. The percentage of copper in brass can be determined using the steps below. 22 Step 1 2.80 g of brass is reacted with an excess of concentrated nitric acid, HNO3. The half-equations taking place are shown below. Step 2 Step 3 Step 4 Step 5 Cu2+(aq) + 2e Cu(s) 2HNO3(l) + e NO3 (aq) + NO2(g) + H2O(l) Excess aqueous sodium carbonate is added to neutralise any acid. The mixture effervesces and a precipitate forms. The precipitate is reacted with ethanoic acid to form a solution which is made up to 250 cm3 with water. A 25.0 cm3 sample of the solution is pipetted into a conical flask and an excess of aqueous potassium iodide is added. A precipitate of copper(I) iodide and a solution of iodine, I2(aq), forms. The resulting mixture is titrated with 0.100 mol dm3 sodium thiosulfate to estimate the iodine present: I2(aq) + 2S2O3 2(aq) 2I(aq) + S4O6 2(aq) Step 6 Steps 4 and 5 are repeated to obtain an average titre of 29.8 cm3. For steps 1, 2 and 4, write ionic equations, including state symbols, for the reactions taking place.Determine the percentage, by mass, of copper in the brass. Give your answer to one decimal place.OCR 2011<br /> 23[9] [Total: 9] END OF QUESTION PAPEROCR 2011<br />

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