chemistry
posted by Aria .
Balance each of the following redox equations using the halfreaction method.
HI(s) + H2SO4 (aq) > H2S(g) + I2 (s)
My only question about this equation is...how do I know how many electrons I need to put on both sides? Other than that I know how to do it.

I in HI has an oxidation state of 1. On the other side I2 is zero. So in going from 2I^ to I2, I lost 2electrons.
S in H2SO4 is +6. S in H2S is 2. In going from +6 to 2, there is a gain of 8 electrons. I do this intuitively but you can do it mathematically.
+6(2) = +8 so gain of 8.
2(0) = 2 so loss of 2. 
You carefully measure 20.00 of powder and add it to 81.00 of solution in a reaction flask of known mass. You notice bubbles as a reaction takes place. You then determine that the contents of the flask have a mass of 92.80 . The relevant equation is
Assuming no other reactions take place, what mass of was produced in this reaction?
Express the mass numerically in