Introduction Sulphate ions and iodine can be formed from reactions between peroxodisulphate ions and iodine ions. S2O82-aq + 2I-aq -----------> 2S042-aq + I2aq The reactants and the sulphate ions are colourless so the achievements of the experiment must be determined by the iodine produced. If starch is added to the reaction iodine is more clearly visible as a strong complex of blue-black colour is formed. For this experiment I will be measuring the time taken for the iodine to appear in the solution. To make this more clear and accurate thiosulphate ions are added...
mixtures. The reactant that is not in excess in the reaction will be used up and it is this which will determine the total amount of iodine produced.

D Moles = mol dm-3 x dm3

= 0.04 x 2/1000

= 8x10-5 mols of S2O8 2-

E i 2cm3 of Na2S2O3 0.0100 mol dm-3

Moles of Na2S2O3 = 0.01 x 2/1000

= 2x10-5 moles of S2O3

ii Moles of = 1.0 X10-5

I2

iii Percentage % of reaction = 1.0 X10-5 x100

8.0 X10-5

= 12.5 %