2 Many oxidising agents are able to oxidise acidified potassium iodide to iodine in acidic conditions. The amount of iodine produced can be determined by titrating it with aqueous sodium thiosulfate. I2(aq) + 2Na2S2O3(aq) → 2NaI(aq) + Na2S4O6(aq) You will determine the change in oxidation state of an oxidising agent when it reacts with iodide ions. FA 2 is aqueous sodium thiosulfate, containing 22.00 g Na2S2O3•5H2O (Mr = 248.2) in 1.00dm³. FA 3 is a 0.0175moldm⁻³ solution of an oxidising agent. FA 4 is 0.50 moldm⁻³ potassium iodide, KI. FA 5 is 1.00 moldm⁻³ sulfuric acid, H2SO4. FA 6 is starch solution.

1. (a)Method • Fill the burette with FA 2. • Pipette 25.0 cm³ of FA 3 into a conical flask. • Use the 10.0 cm³ measuring cylinder to add 10cm³ of FA 4, an excess, to the conical flask. • Use the 25.0 cm³ measuring cylinder to add 20 cm³ of FA 5, an excess, to the conical flask. • Add FA 2 from the burette until the solution becomes yellow. • Add about 10 drops of FA 6 to the conical flask. • Perform a rough titration and record your burette readings in the space below. The rough titre is = .............................................. cm³. Carry out as many accurate titrations as you think necessary to obtain consistent results. Make sure any recorded results show the precision of your practical work. Record, in a suitable form in the space below, all your burette readings and the volume of FA 2 added in each accurate titration. Keep FA 3, FA 4, FA 5 and FA 6 for use in Question 3.[7]
2. (b)From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtain the mean value. 25.0 cm³ of FA 3 required .............................................. cm³ of FA 2 [1][1]
3. (c)(i)Give your answers to (c)(ii), (c)(iii) and (c)(iv) to an appropriate number of significant figures.[1]
4. (c)(ii)Calculate the amount, in mol, of sodium thiosulfate in the volume of FA 2 in (b). amount of Na2S2O3 = .............................................. mol [1][1]
5. (c)(iii)Calculate the amount, in mol, of iodine that reacts with the amount of sodium thiosulfate in (c)(ii). amount of I2 = .............................................. mol [1][1]
6. (c)(iv)Calculate the amount, in mol, of FA 3 used to produce the amount of iodine in (c)(iii). amount of FA 3 = .............................................. mol [1][1]
7. (c)(v)Calculate the amount, in mol, of iodine produced by the reaction of 1 mol of FA 3 with potassium iodide. Give your answer to one decimal place. amount of I2 = .............................................. mol [1][1]
8. (c)(vi)The oxidising agent in FA 3 is a compound of a transition metal, M. The redox reaction of FA 3 with iodide ions produces M2+ ions. Use your answer to (c)(v) to calculate the change in the oxidation state of M during this reaction. Show your working. The oxidation state of M changes from .............................................. to .............................................. [2][2]
9. (d)A student suggests that the experiment in (a) would be more accurate if the FA 5, sulfuric acid, is measured using a pipette. State whether the student is correct. Explain your answer.[1]