You've gone a little far with your answer for part A... to find out which is the limiting reactant, you'll have to stop once you get to mols of Zn and mols of S. You did the right thing in finding out the mols of Zn and S, so let's go from there.

Your calculations: (FW = Formula Weight)

25.0g Zn / 63.39g/mol Zn FW = 0.39mol Zn

30.0g S / 32.07g/mol S FW = 0.93mol S

Now, take a look at your equation:

Zn + S = ZnS

The stoichiometry dictates that for every mol of Zn you use, you use one mol of S. Remember, mols = mols, but grams don't equal grams.

So, if you have 0.39mol Zn and 0.93mol of S, how many mols of ZnS can you make, total? Only 0.39... the extra 0.54mol of S just sits there and does nothing because it can't react. (More correctly, it may melt, because this reaction is very exothermic, but melting isn't reacting). You now know which is the limiting reactant.

Knowing that only 0.39mol of ZnS can be formed, assuming there are no problems while performing the reaction, you can calculate the number of grams of ZnS formed by using the formula weight. If you have questions about this, ask.