How do you find the excess reagent?

Question

Consider the reaction: 2 HCl + Ca(OH)2 → 2 H2O + CaCl2

Calculate the amount of the excess reagent, in moles, when 2.6 moles of HCl and 1.4 moles of Ca(OH)2 are mixed.

Answer 1

2 HCl + Ca(OH)₂ ==> 2 H₂O + CaCl₂

One of the easiest ways to find the limiting reactant is to divided the moles of each reactant by the coefficient in the balanced equation, and whichever is smaller, that is limiting.  In this problem we have the following:

2.6 moles HCl.  Divide that by 2 (coeff. of HCl) and you get 1.3

1.4 moles Ca(OH)₂.  Divide that by 1 (coeff.) and you get 1.4.

1.3 is less than 1.4 so HCl is limiting.  Now forget these calculated values of 1.3 and 1.4 and go back and use the original number of moles given in the problem.

We know Ca(OH)₂ is in excess.  To find out how much excess there is, use mole ratios of the balanced equation:

2.6 moles HCl x 1 mole Ca(OH)₂/2 moles HCl = 1.3 moles Ca(OH)₂ reacted or used up.

moles Ca(OH)₂ remaining = 1.4 moles - 1.3 moles = 0.1 moles remaining