Iron (II) sulfide reacts with hydrochloric acid according to the reaction:

FeS (s) + 2HCl (aq) →FeCl2 (s) + H2S (g)

A reaction mixture initially contains 0.240 mol FeS and 0.646 mol HCl.

what amount (in moles) of the excess reactant is left?

The balanced chemical reaction would be:

FeS (s) + 2HCl (aq) →FeCl2 (s) + H2S (g)

We are given the amount of the reactants to be used for the reaction. We use these amounts. First, we determine the limiting reactant of the reaction. From the data, we can say that FeS is the limiting ad HCl is the excess reactant. We calculate as follows:

Amount of HCl used: 0.240 mol FeS x 2 mol HCl / 1 mol FeS = 0.48 mol HCl

0.646 - 0.48 = 0.166 mol HCl left