In this experiment you will determine the oxidation number of iodine in one of its compounds by titration. FA 1 is a 0.0197 mol dm-³ solution of the iodine-containing compound. FA 2 is dilute sulfuric acid, H₂SO₄. FA 3 is aqueous potassium iodide, KI. FA 4 is 0.105 mol dm-³ sodium thiosulfate, Na₂S₂O₃. starch indicator FA 1 reacts with excess acidified potassium iodide to produce iodine, I₂. This iodine is then titrated with aqueous sodium thiosulfate using starch indicator.

1. (a)Method • Fill the burette with FA 4. • Pipette 25.0 cm³ of FA 1 into a conical flask. • Using the measuring cylinder, add 10 cm³ of FA 2 to the same conical flask. • Using the same measuring cylinder, add 20 cm³ of FA 3 to the mixture in the conical flask. • The mixture will now be a red-brown colour, due to iodine produced. • Carry out a rough titration by adding FA 4 from the burette until the mixture becomes light brown. • Then add 10 drops of starch indicator. The mixture will change to a dark blue colour. • Continue titrating until the mixture becomes colourless. This is the end-point. 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 below all of your burette readings and the volume of FA 4 added in each accurate titration.[7]
2. (b)From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value. The iodine produced required .............................................................. cm³ of FA 4.[1]
3. (c)(i)Calculate the number of moles of sodium thiosulfate in the volume of FA 4 calculated in (b). moles of Na₂S₂O₃ = .............................................. mol[1]
4. (c)(ii)The equation for the reaction of iodine with sodium thiosulfate is shown. I₂(aq) + 2Na₂S₂O₃(aq) → Na₂S₄O₆(aq) + 2NaI(aq) Calculate the number of moles of iodine that reacted with the sodium thiosulfate calculated in (i). moles of I₂ = .............................................. mol[1]
5. (c)(iii)Use the information on page 2 to calculate the number of moles of iodine-containing compound in the 25 cm³ of FA 1 used in each titration. moles of iodine-containing compound in 25 cm³ FA 1 = .............................................. mol
6. (c)(iv)Use your answers to (ii) and (iii) to calculate the number of moles of iodine produced when 1 mole of the iodine-containing compound in FA 1 reacts with excess FA 3. Give your answer as an integer. moles of I₂ = .............................................. mol[1]
7. (c)(v)The anion in FA 1 is IOₓ⁻ where x is the number of oxygen atoms present in the formula. Use your answer to (iv) to balance the ionic equation for the reaction between FA 1 and FA 3 under acidic conditions. Hence deduce the value of x in the formula IOₓ⁻. IOₓ⁻ + .........I⁻ + .........H⁺ → .........I₂ + .........H₂O x = ......................................[2]
8. (c)(vi)Calculate the oxidation state of iodine in FA 1. (If you were unable to calculate x in part (v), assume that x = 4.) oxidation state of iodine = ......................................[1]