[syd]

Hi-

I think I'm completely lost on this question.  It has been a while since I had to do chemistry problems.   I'm attempting to calculate the reaction rate in M s^-1 in a reaction between:

2I^- + S₂O₈^2- I₂ + 2 SO₄^2-

In my experiment I used one mL of .200 M KI solution, 2 mL of .100 M (NH₄)₂S₂O₈ and 1 mL of .005 M Na₂S₂O₃ solution.

I know I'm supposed to divide (the negative of) my change in I- concentration by the change in time.. but the answer I get seems way too small I must have messed up my calculation somewhere.  The reaction was finished in 253 seconds.  From what I calculated, my initial I- concentration in M was 2.00 * 10^-4, and initial S2O8 ion was also 2.00 * 10 ^-4.  Based on how much S2O3 ion I had in there, (which was 5.00*10^-6 moles right)  So from that information I know that 2.50 * 10^-6 moles of my S2O8 ion reacted.  Based on the stoichiometric ration between my I- and S208, which I found to be 2:1, I should have also used up 5.00 * 10^-6 moles of the I- right?  But this makes my change in I- concentation tiny, the change is actually just the 5.00 * 10^-6 moles and dividing that by the change in time, 253, gives a very small answer in the 10^-9 range.  Even multiplying that by 2 (which I think I should right?) since 2 I-'s react with every 1 mole of my S2O8.. the answer is tiny.  I think I'm just way off on my method any clarification or nudge in the right direction would be most appreciated.

Thanks in advance.

edit- wrong information