CIE Jun 2017 v1 Paper 3 Q1

Answers available below

In this experiment you will determine the relative formula mass of a copper salt by titration. PMT II III IV VI VII 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 26.0 g 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 mol dm3 sodium thiosulfate, Na2S2O3. starch indicator (a) MethodFill 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 iscm3. 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. Keep FA 3 and starch indicator for use in Question 3. (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. [7]UCLES 2017 9701/31/M/J/17 The iodine produced requiredcm3 of FA 4. [1]<br /> PMT (c) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate the number of moles of sodium thiosulfate, Na2S2O3, in the volume of FA 4 calculated in (b). moles of Na2S2O3 =mol (ii) Balance the equation for the reaction of iodine with sodium thiosulfate. State symbols are not required. I2 + Na2S2O3 Na2S4O6 + NaI (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 (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 25.0 cm3 of FA 1. (v) Using your answer to (iv) and the information on page 2, calculate the relative formula mass of the copper compound in FA 1. moles of Cu2+ ions =molUCLES 2017 9701/31/M/J/17 Mr of copper compound =[4] [Total: 12] [Turn over<br />

Show answer