Edexcel Jun 2014 (IAL) Paper 2 Q19

Answers available below

19 Potassium bromate(V), KBrO3, is a primary standard, meaning that it can be obtained as a pure substance and used to accurately determine the concentrations of solutions of other chemicals, such as sodium thiosulfate, Na2S2O3. (a) (i) Complete the dot and cross diagram for the bromate(V) ion. Show only the outer shell electrons. In this ion, the bromine expands its outer shell to accommodate 12 electrons. Use x for bromine electrons and (cid:120) for oxygen electrons. The symbol * on the diagram represents the extra electron which gives the ion its charge. Br O* (ii) Suggest how elements in Period 3 and higher can accommodate more than eight electrons in their outer shell. (2) (1)12 *P42977A01224*<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. 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 bromate(V) solution. A mass of 8.35 g of KBrO3 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 bromate(V) solution, in mol dm3. (ii) The second student was asked to determine a suitable mass of potassium iodide to add to 0.0025 mol of potassium bromate(V) to ensure complete reaction. The equation for the reaction is BrO3 (aq) + 6H+(aq) + 6I(aq) (cid:314) Br(aq) + 3I2(aq) + 3H2O(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) (3) Minimum mass requiredg Suitable mass to useg *P42977A01324* 13 Turn over<br /> (iii) The third student was given the following equation. I2(aq) + 2S2O3 2(aq) (cid:314) 2I(aq) + S4O6 2(aq) This student was asked to estimate the titration reading. Calculate the volume of 0.1 mol dm3 of sodium thiosulfate solution, in cm3, that would be needed to react with 0.00100 mol of iodine present in the conical flask. (iv) The fourth student carried out an alternative method for determining the concentration of the sodium thiosulfate solution. A known mass of solid potassium bromate(V) was dissolved in water in a conical flask. An excess of potassium iodide and acid were added and the mixture titrated with the sodium thiosulfate solution. The following measurements were obtained. Mass of KBrO3 Volume of water Volume of Na2S2O3(aq) 0.07 g 25 cm3 26.85 cm3 The student calculated the concentration of the sodium thiosulfate, Na2S2O3, to be 0.0937 mol dm3. There is uncertainty in the value of the calculated concentration of the sodium thiosulfate. Which measurement, given in the table, has the greatest effect on the uncertainty of this value? Justify your answer. No calculation is required for this answer. (2) (2)(Total for Question 19 = 12 marks) 14 *P42977A01424*<br />

Show answer