Answer: The amount (in moles) of the excess reactant left is, 0.166 mol
Explanation : Given,
Moles of FeS = 0.240 m ol
Moles of HCl = 0.646 mol
Now we have to calculate the limiting and excess reagent.
The balanced chemical equation is:

From the balanced reaction we conclude that
As, 1 mole of
react with 2 mole of 
So, 0.240 moles of
react with
moles of 
From this we conclude that,
is an excess reagent because the given moles are greater than the required moles and
is a limiting reagent and it limits the formation of product.
Now we have to calculate the excess moles of 
Remaining moles of HCl = 0.646 - 0.48 = 0.166 mol
Thus, the amount (in moles) of the excess reactant left is, 0.166 mol