Soo I'm sure I am being too picky here, but I'm also very confused. The question is just asking for the dS given the dS products ands reactants per mole. So you multiply by the number of moles of each, and then do dSrxn = dSproducts - dSreactants. But doesn't that now mean that the units are J/K and not J/(K mol)? Since you've already multiplied by the number of moles? I just don't understand why it would still be per mole, or why you wouldn't then have the answer divided by the number of moles of product to put it back into "/mol". And when it is /mol, for a reaction like 2O3 --> 3O2, if the Delta S is 137J/K mol, is it Moles of O2 or moles of O3 ?