Question

How do you calculate delta[S2O8 2-] in iodine clock reaction?

Answer 1

Here is a site that explains the iodine clock reaction and all of its various forms.
http://en.wikipedia.org/wiki/Iodine_clock_reaction
The one you want to read about (read them all--all are interesting and contain interesting chemistry) is the persulfate one. The change in persulfate can be calculated knowing about you started with. It is just moles initially -0 = moles used. The clock runs cosn when all of the persulfate is used.