Question

SO3^2- + Fe^3+ -> SO4^2- + Fe^2+

Answer 1

this is a redox reaction. if we assign a -2 to oxygen in SO3^2-, then the oxidation number of S is 4+, because the sum of the oxidation numbers must be equal to the net charge of the anion. using this rule, the oxidation number of S in SO4^2- is +6.

this tells us that this SO3 to SO4 is a two-electron oxidation. on the other hand, Fe(III) to Fe(II) is a one-electron oxidation. so we need to add the half-equations together like this:

SO3 --> SO4 + 2e
2Fe(III) + 2e --> 2 Fe(II)

because the two electrons taken from one SO3 must all go to Fe(III), we need 2 mol of Fe(III) to react with 1 mol of SO3.