The limiting reagent is the reactant which completely reacts before the other reactant(s) is used up. When 9.0 moles Na₂S and 8.0 moles CuSO₄ react, it appears that CuSO₄ is the limiting reagent. You can tell because it results in the production of less product.
You can determine the mass of CuS by multiplying the moles by the molar mass. It is important to arrange the ratio in a way that allows for the cancellation of units.
Molar Mass (CuS): 95.62 g/mol
8.0 moles CuS 95.62 g ------------------------- x ----------------------- = 765.0 grams CuS 1 mole
