In this experiment, you assumed that only a small amount of S2O82– was used during the time trial so that the concentration of this reactant did not change appreciably during the course of the reaction. Calculate how much S2O82– was used in Trial 3 and the percent remaining at the end of the reaction. Was this assumption valid?   

for the third trial 6.25 mL of 0.2M ammonium persulfate was used.

The reaction was done with 25mL of 0.2M potassium iodide and 5mL of 0.2% starch in beaker 1, and 6.25 mL of 0.2M ammonium persulfate, 18.75mL of 0.2M  ammonium sulfate and 10mL of 0.01M sodium thiosulfate in beaker 2.  The second beaker was then dumped into the first and the time for the color change was recorded, which took 87 seconds for the change.  

In the reaction table the concentraion of 6.25 mL of ammonium persulfate used was 0.00125 M.