Iron(ii) sulfide reacts with hydrochloric acid according to the reaction:
fes(s) + 2 hcl(aq) right arrow fecl2(s) + h2s(g)
a reaction mixture initially contains 0.175 mol fes and 0.481 mol hcl. once the reaction has occurred as completely as possible, what amount (in moles) of the excess reactant is left?

We must first determine which of FeS and HCl is the limiting reactant. Since the balanced reaction shows that 1 mole of FeS will react with 2 moles of HCl, this means that 0.175 mol FeS will react with 0.350 mol HCl. However, the initial amount of HCl is greater than 0.350, so it is not the limiting reactant.

All the FeS is used up (making it the limiting reactant), but there will be 0.481 - 0.350 = 0.131 mol HCl remaining.