Limiting Reactant and Excess Reactant Problems
1. If amounts of 2 different reactants are given, then you are dealing with a limiting
reactant/excess reactant problem.
2. You must convert amount of each reactant to moles and STOP!
3. Divide each of these mole amounts by each reactant’s own coefficient to
calculate what fraction of the “recipe” you have. Use these numbers to
determine the identity of the limiting and excess reactants.
4. To determine the amount of desired product formed begin your stoichiometry
calculation with the original amount of limiting reactant and do the necessary
conversions to calculate the amount of desired product formed.
Be sure to start this calculation with the actual mole amount of limiting reactant
you calculated, and not with the “fraction of the recipe you have.”
(This is a common mistake)
5. Some of the excess reactant will react and some will be left over – unreacted.
a) Calculate how much excess reactant actually reacted by doing a
stoichiometry problem from the limiting reactant to the excess reactant.
(remember to calculate the amount of excess reactant which reacted in the
appropriate units – the same as given at the beginning of the problem.)
b) Calculate how much excess reactant is left over unreacted by using
subtraction.
6. You might be asked to determine the concentration of the left over reactant.
a) Calculate how many moles of excess reactant remain after the reaction.
(This means you will either have to convert g to mol using the molar mass or
mL to mol using the concentration.)
b) Divide the remaining number of moles by the number of Liters of solution
in the container. This is the concentration of remaining excess reactant.