Edexcel Jun 2014 (R) Paper 2 Q17

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SECTION B Answer ALL the questions. Write your answers in the spaces provided. 17 Potassium dichromate(VI), K2Cr2O7, can be used to accurately determine the concentration of other chemicals, such as sodium thiosulfate, Na2S2O3. (a) The dichromate(VI) ion has two chromium atoms sharing one oxygen to give two tetrahedral units. Each chromium atom uses six electrons in bonding and expands its outer shell to accommodate a total of 12 electrons. Complete the dot and cross diagram for this ion below. Only show outer shell electrons. Use x for chromium electrons and(cid:135) for oxygen electrons. Use the symbol * to represent the extra electrons which give the ion its charge. (3) Cr O Cr O 2 10 *P42991A01024*<br /> (b) Four chemistry students were given a solution of sodium thiosulfate with a concentration of approximately 0.1 mol dm3 and asked to determine its exact concentration using potassium dichromate(VI) solution. They were each given separate tasks to carry out as described below. (i) The first student was given the task of making up a potassium dichromate(VI) solution. A mass of 14.71 g of K2Cr2O7 was weighed out, dissolved in deionized water, the volume made up to 250 cm3 in a volumetric flask and the mixture shaken. Calculate the concentration of this potassium dichromate(VI) solution, in mol dm3. Use the Periodic Table as a source of data. (2) (ii) The second student was asked to calculate the mass of potassium iodide that would be required to add to 0.00250 mol of potassium dichromate(VI) to ensure complete reaction. The equation for the reaction is Cr2O7 2(cid:236)(aq) + 14H+(aq) + 6I(cid:236)(aq) (cid:314) 2Cr3+(aq) + 3I2(aq) + 7H2O(l) Calculate the minimum mass of potassium iodide, KI, required and hence suggest a suitable mass to use if the potassium iodide is to be in excess. You must show your working and your mass should be reasonable. (2) Minimum mass required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . g Suitable mass to use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . g *P42991A01124* 11 Turn over<br /> (iii) The third student was given the following equation. I2(aq) + 2S2O3 2-(aq) (cid:314) 2I(cid:236)(aq) + S4O6 2(cid:236)(aq) This student was asked to estimate the titration reading at the end-point if a solution that contained 0.00260 mol of iodine was in the conical flask and the concentration of sodium thiosulfate was about 0.16 mol dm-3. Calculate the volume of sodium thiosulfate solution, in cm3, that would have been added at the end-point of the titration. (2) 12 *P42991A01224*<br /> *(iv) The fourth student was given the following experimental information. 0.1 g of potassium dichromate(VI) was dissolved in a total volume of 25.00 cm3. An excess of potassium iodide and acid was added and then used in a titration with the sodium thiosulfate solution of concentration approximately 0.1 mol dm3. The titre was 25.15 cm3. The student suggested that the greatest uncertainty in the result arose from the mass that was measured and that the procedure was unreliable. Explain why these views were justified. No calculation is required. (2)(c) (i) Acidified sodium dichromate(VI) solution is commonly used for the oxidation of alcohols. It is important not to use hydrochloric acid in this reagent mixture because the chloride ions are oxidized to chlorine. Write the ionic half-equation for the oxidation of chloride ions. State symbols are not required. (ii) Fumes of hydrogen chloride gas can be identified by bringing the fumes into contact with another gas, X. Identify gas X and state the observation you would make. (1) (2) Gas XObservation*P42991A01324* 13 Turn over<br /> (d) Potassium bromide can be distinguished from potassium chloride by its reaction with silver nitrate solution, followed by the addition of aqueous ammonia solution. State what you would see on the addition of silver nitrate solution to potassium bromide solution. How could both dilute and concentrated ammonia be used to confirm that silver bromide is formed, rather than silver chloride? (3)(Total for Question 17 = 17 marks) 14 *P42991A01424*<br />

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