In this experiment you will determine the relative formula mass of a copper salt by titration. A solution of the copper salt reacts with excess acidified potassium iodide, producing iodine. This iodine is then titrated with aqueous sodium thiosulfate, using starch indicator. FA 1 is an aqueous solution of the copper salt prepared by dissolving 26.0g of the salt to make 1.00 dm³ of solution. FA 2 is dilute sulfuric acid, H2SO4. FA 3 is aqueous potassium iodide, KI. FA 4 is 0.110 moldm⁻³ sodium thiosulfate, Na2S2O3. starch indicator

1. (a)Method • Fill the burette with FA 4. • Pipette 25.0 cm³ of FA 1 into a conical flask. • Use the measuring cylinder to add approximately 10 cm³ of FA 2 to the same conical flask. • Use the measuring cylinder to add approximately 20 cm³ of FA 3 to the mixture in the conical flask. The mixture will now be a brown colour, due to iodine produced in the reaction. • Begin your rough titration by adding FA 4 from the burette until the mixture becomes light brown. • Add 10 drops of starch indicator. The mixture will become darker. • Continue titrating until the mixture becomes an off-white colour. This is the end-point. • Add one drop of starch indicator to check that no traces of dark colour are produced. If the mixture stays off-white, the titration is finished. If some dark colour is produced, because iodine is still present, continue the titration. • Record your burette readings and the rough titre 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 below all of your burette readings and the volume of FA 4 added in each accurate titration. I II III IV V VI VII[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, Na2S2O3, in the volume of FA 4 calculated in (b). moles of Na2S2O3 = ................................... mol[1]
4. (c)(ii)Balance the equation for the reaction of iodine with sodium thiosulfate. State symbols are not required. ...I₂ + .Na2S2O3 → .Na2S4O6 + .......NaI
5. (c)(iii)Using your answer to (ii), calculate the number of moles of iodine that reacted with the number of moles of Na2S2O3 calculated in (i). moles of I₂ = ................................... mol[1]
6. (c)(iv)Iodine, I2, is produced in the reaction between FA 1 and FA 3. FA 3 is in excess. 2Cu²+(aq) + 4I-(aq) → 2CuI(s) + I2(aq) Using your answer to (iii), calculate the number of moles of copper(II) ions in 25.0cm³ of FA 1. moles of Cu2+ ions = ................................... mol[1]
7. (c)(v)Using your answer to (iv) and the information on page 2, calculate the relative formula mass of the copper compound in FA 1. M of copper compound = ...................................[1]