How many molecules/atoms of excess reactant will remain after the reaction is complete?

Once you have identified the limiting reactant, you calculate how much of the other reactant it must have reacted with and subtract from the original amount.

Explanation:

 1.  Write the chemical equation.

   2.Calculate the moles of product from the first reactant.

   3.Calculate the moles of product from the second reactant.

   4.Identify the limiting reactant and the excess reactant.

   5.Calculate the mass of excess reactant used up.

   6.Calculate the mass of unused excess reactant.

   7.Calculate the mass of limiting reactant needed to react with the unused excess reactant.