First you need to balance any equation given. then you need to figure the mole ratios.say 2 moles of iron (II) sulfide form 1 mole of iron (II) chloride, then
it is a 2 to 1 ratio, then you say you have~ 1 mole of iron sulfide X 2
moles of iron chloride/1mole of iron sulfide X of iron sulfide/1 mole of iron sulfide. Just make sure to start with a balanced equation.
there is a formula which is importent : (n) moles = (m) mass / (M)molar mass