Question

If 3.0 moles of x and 4.0 moles of y react according to the hypothetical reaction below, how many moles of the excess reactant will be left over at the end of the reaction? x + 2 y yields xy2

Answer 1

Firstly the limiting reactant should be identified. Limiting reactant is the reactant that is in limited supply, the amount of product formed depends on the moles present of the limiting reactant.

the stoichiometry of x to y = 1:2
1 mole of x reacts with 2 moles of y
if x is the limiting reactant, there are 3 moles of x, then 6 moles of y should react, however there are only 4 moles of y. Therefore y is the limiting reactant and x is in excess.
4 moles of y reacts with 2 moles of x 
since there are 3 moles of x initially and only 2 moles are used up, excess amount of x is 1 mol thats in excess.