2 Iodate ions contain iodine and oxygen. They have the formula IOx⁻ where x is an integer. In this experiment you will determine the value of x in an iodate. You will first react IOx⁻ with an excess of iodide ions, I⁻, to form iodine, I2. The amount of iodine produced is then determined by titration with thiosulfate ions, S2O3²⁻. I2(aq) + 2S2O3²⁻(aq) → 2I⁻(aq) + S4O6²⁻(aq) FB 2 is 0.100 mol dm⁻³ sodium thiosulfate, Na2S2O3. FB 3 is a solution containing 0.0140 mol dm⁻³ IOx⁻ ions. FB 4 is dilute sulfuric acid, H2SO4. FB 5 is 0.500 mol dm⁻³ potassium iodide, KI. FB 6 is starch indicator.

1. (a)Method • Fill the burette with FB 2. • Pipette 25.0 cm³ of FB 3 into a conical flask. • Use the 25 cm³ measuring cylinder to add 10 cm³ of FB 4 to the conical flask. • Use the same measuring cylinder to add 10 cm³ of FB 5 to the conical flask. • Add FB 2 from the burette until the solution turns yellow. • Add 10-15 drops of FB 6 to the solution in the conical flask. • Continue to add more FB 2 from the burette until the blue-black colour just disappears. • 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 FB 2 added in each accurate titration. Keep the remaining FB 2 for use in Question 3. I II III IV V VI VII[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 FB 3 required ................................. cm³ of FB 2.[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)Use your answer to (b) and the information given to calculate the amount, in mol, of iodine formed when 25.0 cm³ of FB 3 reacts with 10 cm³ of FB 5. amount of I2 = ................................. mol[1]
5. (c)(iii)Calculate the amount, in mol, of IOx⁻ ions in 25.0 cm³ of FB 3. amount of IOx⁻ ions = ................................. mol[1]
6. (c)(iv)An unbalanced equation for the reaction of IOx⁻ ions with iodide ions, I⁻, and hydrogen ions, H⁺, is shown. ΙOx⁻ + .... Ι⁻ + ........ H⁺ → .... I2 + ........ H2O Use the ratio of your answers to (c)(ii) and (c)(iii) to balance this equation and determine the value of x. Show your working. ratio IOx⁻ : I2 = 1 : ................................. x = .................................[2]
7. (d)A student carries out the same experiment as in (a) but uses 0.0140 mol dm⁻³ IO2⁻ ions in place of FB 3. Tick the correct box in Table 2.1. Explain your answer. (If you were unable to determine the value of x in (c)(iv) or you determined the value of x to be 2, assume x = 4. Note that this may not be the correct value). Table 2.1 Volume of FB 2 will be smaller. Volume of FB 2 will be unchanged. Volume of FB 2 will be larger.[2]