How many grams of the excess reactant remain assuming the reaction goes to completion and that you start with 15.5g of na2s and 12.1g cuso4? reaction: na2s cuso4 ? na2so4 cus a 0.05 b 15.45 c 9.58 d 5.92 e not enough information?

The reaction between Na2S and CuSO4 will give us the balanced chemical reaction of,
                                     Na2S + CUSO4 --> Na2SO4 + CuS
This means that for every 78g of Na2S, there needs to be 159.6 g of CuSO4. The ratio is equal to 0.4887 of Na2S: 1 of CuSO4. Thus, for every 12.1g of CuSO4, we need only 5.91 g of Na2S. Thus, there is an excess of 9.58 g of Na2S. The answer is letter C. 

True, the standard unit for volume is liters which can be represented by either a lower case "l" or a capital "l"