Edexcel Jun 2014 Paper 5 Q21

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21 (a) The following method was used to estimate the concentration of ethanol in an aqueous solution, Q, prepared by the fermentation of sucrose. 25 cm3 of Q was measured using a pipette and transferred to a 250 cm3 volumetric flask; the flask was made up to the mark with distilled water and mixed thoroughly, forming a diluted solution, R. 25 cm3 samples of R were mixed with an equal volume of a 0.200 mol dm3 potassium dichromate(VI) solution and excess dilute sulfuric acid. The mixture was allowed to stand for several hours and then the amount of unreacted potassium dichromate(VI) was determined by titration against a 0.255 mol dm3 iron(II) ammonium sulfate solution. The mean titre was 23.85 cm3. (i) Use the ionic half-equations below to write the full ionic equation for the reaction between potassium dichromate(VI) and iron(II) ammonium sulfate. State symbols are not required. Fe3+ + e (cid:111) Fe2+ Cr2O7 2 + 14H+ + 6e (cid:111) 2Cr3+ + 7H2O (ii) Calculate the number of moles of potassium dichromate(VI) that remained unreacted after standing for several hours with solution R. (1) (2) 20 *P42973A02028*<br /> (iii) Calculate the number of moles of potassium dichromate(VI) that reacted with the ethanol while standing for several hours with solution R. (iv) Write an ionic half-equation for the oxidation of ethanol to ethanoic acid. Use your equation, and the half-equation for the reduction of dichromate(VI) ions, to show that 3 mol of ethanol are oxidized by 2 mol of potassium dichromate(VI). (2) (2)(v) Calculate the concentration of ethanol (in mol dm3) in solution Q. (3) *P42973A02128* 21 Turn over<br /> *(b) The indicator used in this titration was barium diphenylamine sulfonate, which turns from deep purple to colourless at the end-point. Identify the ion responsible for turning the indicator from deep purple to colourless at the end-point. By considering the type of reaction involved when this ion reacts with barium diphenylamine sulfonate, suggest how barium diphenylamine sulfonate acts as an indicator in this titration. Note that complex formation does not occur. The detailed reactions of this particular indicator are not required. (3)*(c) This method of determining ethanol concentration does not give very reliable results, although the titration is very accurate. Suggest one reason why this might be the case, explaining how the measured concentration would differ from the actual concentration of the ethanol. (3)(Total for Question 21 = 16 marks) TOTAL FOR SECTION B = 50 MARKS 22 *P42973A02228*<br />

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