Edexcel Jan 2014 (IAL) Paper 5 Q21

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SECTION B Answer ALL the questions. Write your answers in the spaces provided. 21 Potassium manganate(VII) can be used in redox titrations to determine the concentration of iron(II) ions and ethanedioate ions in aqueous solution. Aqueous solutions of potassium manganate(VII) are unstable, so it is often standardized using solutions of iron(II) ammonium sulfate, freshly prepared from Mohrs salt, Fe(NH4)2(SO4)2.6H2O. (a) Use the relevant ionic half-equations, and standard reduction potentials on page 17 of the Data Booklet, to answer the following questions. State symbols are not required in the equations. (i) Write the ionic half-equation for the reduction of manganate(VII) ions in acid solution. (ii) Write the ionic half-equation for the oxidation of water to form oxygen and hydrogen ions. (iii) Combine your answers to (a)(i) and (a)(ii) to derive an equation to show the production of oxygen in acidified manganate(VII) solution. (iv) Calculate E (cid:57) cell for the reaction in (a)(iii) and hence explain why aqueous solutions of potassium manganate(VII) are unstable. (1) (1) (1) (2)10 *P42993A01028*<br /> (b) 250 cm3 of a solution containing 10.00 g of Mohrs salt was prepared. Separate 25.0 cm3 samples of this solution were pipetted into conical flasks, excess sulfuric acid added and then each mixture was titrated against potassium manganate(VII) solution. The mean titre was 25.85 cm3. *(i) Describe in outline how you would prepare the 250 cm3 of Mohrs salt solution, given 10.00 g of the solid. (4)(ii) State what you would see at the end-point of the titration. (1)(iii) Write the ionic equation showing that 1 mol of manganate(VII) ions reacts with 5 mol of iron(II) ions in acid conditions. State symbols are not required. (1) *P42993A01128* 11 Turn over<br /> (iv) The concentration of the potassium manganate(VII) solution was stated to be 0.0200 mol dm3. Calculate the percentage of the potassium manganate(VII) that had reacted between its preparation and the titration. The molar mass of Mohrs salt is 392 g mol1. (4) (Total for Question 21 = 15 marks) 12 *P42993A01228*<br />

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