limiting reactant
Now you say that stoichiometry is great, but what if you don't have such pretty numbers that work out evenly? That is where limiting reactant comes into play. In most situations one reactant will run out before the other. The reactant that is totally consumed is called the limiting reactant because it stops the reaction. Any other reactant that does not run out is called the excess reactant. Therefore the amount of product that can be produced is directly related to the limiting reactant.
The first step in a limiting reactant question is to determine the limiting reactant. To do this, for all reactants calculate the amount of product that would be produced if all of the reactant was used and there was excess of all the other reactants. The reactant that has the least product is the limiting reactant.
The amount of products is due to the amount of the limiting reactant.
An example:
Using the following reaction
Zn(s) + 2HCl(aq) --> ZnCl2(aq) + H2(g)
Which is the limiting reactant and how much H2 is produced if there is 0.30 mol of Zn and 0.52 moles of HCl?
Step one:
Since HCl is the limiting reactant, the number of moles of H2 produced is .26 moles.