1 9729 / YJC / 2017 / JC2 Preliminary Examination / Paper 4 Answer all the questions in the spaces provided. 1 Determination of the concentration of sodium thiosulfate in solution Z Sodium thiosulfate is commonly used in an alternative method for the extraction of gold from its ores, through the formation of a stable and soluble complex of gold(I) ions, [Au(S2O3)2]3−. One of your tasks is to determine the concentration of sodium thiosulfate in a commercially produced solution Zused in the extraction process. Solution Zcontains sodium thiosulfate in deionised water. You are not provided with solution Z, as it is too concentrated for use in this experiment. A diluted solution of Zhas been prepared for you. This diluted solution is FA 5. FA 1 is 0.0230 mol dm−3of aqueous potassium iodate(V), KIO3FA 2is 0.50 mol dm−3potassium iodide, KIFA 3 is 0.75 mol dm−3sulfuric acid, H2SO4FA 4is a solution of sodium thiosulfate, Na2S2O3of an unknown concentration FA 5a diluted solution of Z, in which 31.00 cm3of solution Zwas made up to 250 cm3with deionised water in a volumetric flask You are also provided with starch solution. In acidic medium, iodate(V) ions oxidise iodide ions to iodine IO3−(aq) + 5I−(aq) + 6H+(aq) 3I2(aq) + 3H2O(l) reaction 1If an excess of sodium thiosulfate is present in the reaction mixture, the iodine produced in reaction 1is immediately reduced back to iodide ions, as shown in reaction 2, and the dark brown colour of iodine does not form. I2(aq) + 2S2O32−(aq) 2I−(aq) + S4O62−(aq) reaction 2In the experiments that you will perform, insufficient volumes of the first sodium thiosulfate solution will be added initially, and some iodine will remain in the solution. The amount of iodine left-over can be determined by titration with the second solution of sodium thiosulfate, as shown in reaction 2. You will prepare a number of reaction mixtures, each of which will contain: a fixed volume of FA 1, excess amounts of FA 2and FA 3, a different volume of FA 4. All reaction mixtures will contain some iodine, produced in reaction 1, that will be titrated against the FA 5solution. You are required to use a graphical method to determine the concentrations of sodium thiosulfate in FA 4, FA 5and solution Z.
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