CIE Nov 2016 v2 Paper 4 Q4

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PMT (a) Explain why compounds of transition elements are usually coloured.[3] (b) Copper is used to make alloys such as brass. The percentage of copper in a sample of brass can be determined by dissolving the sample in concentrated nitric acid and reacting the mixture with potassium iodide. The resulting solution is then titrated. A 1.75 g sample of the brass was dissolved in excess concentrated nitric acid. The reaction of the copper metal in the brass with the concentrated nitric acid released a brown gas and formed a green-blue solution. (i) Write an equation for this reaction.[2] The resulting solution was neutralised and made up to 250 cm3 in a volumetricask with distilled water. An excess of aqueous potassium iodide was added to a 25.0 cm3 portion of this solution to liberate iodine. The resulting solution required 22.40 cm3 of 0.100 mol dm3 aqueous sodium thiosulfate solution to react with the iodine produced. The reactions taking place in this titration are shown. 2Cu2+ + 4I2CuI + I2 I2 + 2S2O3 22I + S4O6 2 (ii) Calculate the percentage of copper, by mass, in the sample of brass to three signi cantgures.UCLES 2016 9701/42/O/N/16 % of copper =[4] [Total: 9]<br />

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