VJC_H2_CHEM_P2_Answers Prelim

 VJC 2014 9647/02/PRELIM/14 [Turn over 2014 Preliminary Examination H2 Chemistry Paper 2 1 Planning (P) Aqueous chlorine, Cl2, displaces iodine, I2, from aqueous potassium iodide. If a solution of chlorine is mixed with an excess of aqueous potassium iodide, iodine is displaced in a 1:1 molar ratio with chlorine. The concentration of chlorine in the original solution can therefore be calculated from the concentration of the displaced iodine. I2(aq) + 2Na2S2O3(aq)  2NaI(aq) + Na2S4O6(aq) (a) Write an equation for the reaction between aqueous ch lorine, and an excess of aqueous potassium iodide. Cl2(aq) + 2KI(aq)  I2(aq) + 2KCl(aq) [1] [Accept ionic equation, state symbols are optional.] (b) You are to plan an experiment to determine as accurately as possible the concentration o f a saturated aqueous solution of chlorine by titration. The approximate solubility of chlorine is 5 g dm–3 at 25 oC. You are provided with the following materials:  saturated aqueous chlorine;  solid sodium thiosulfate Na2S2O3.5H2O;  concentrated aqueous potassium iodide. This will be used in excess.  the apparatus and chemicals normally found in a school or college laboratory. Your plan should include details of:  a calculation of the approximate concentration of saturated aqueous chlorine in mol dm–3 at 25 oC; [Ar: Cl, 35.5]  a detailed description of the method for preparing a solution of aqueous sodium thiosulfate that can be used in the titration. In a titration, it is usual for the titre volume to be approximately equal to that of the volume of solut ion pipetted. Calculate the mass of sodium thiosulfate, Na 2S2O3.5H2O, which will produce a solution suitable for use in this titration. The relevant calculations and reasoning must be shown in full; [Ar: H, 1.0; O, 16.0; Na, 23.0; S, 32.1]  a step–by–step description of how you would carry out sufficient titrations using a suitable indicator to allow an accurate end–point to be obtained;  an outline calculation to show how the results are to be used to determine the accurate concentration of the aqueous chlorine. Assume that the titre volume used is w cm3. Approximate concentration of saturated Cl2(aq) = = 0.0704 mol dm–3 Assume 10.0 cm3 of saturated Cl2(aq) is pipetted in each titration. No. of moles of saturated Cl2(aq) used = 0.0704 x = 7.04 x 10–4 mol = no. of moles of I2 formed

 VJC 2014 9647/02/PRELIM/14 2 No. of moles of Na2S2O3 needed for titration = 2 x 7.04 x 10–4 = 1.41 x 10–3 mol Assume 10.0 cm 3 of Na 2S2O3 is needed for each titration and 100 cm 3 volumetric flask is used to prepare Na2S2O3 solution. Concentration of Na2S2O3 needed = 1.41 x 10–3  = 0.1