In order to solve any stoichiometry problem, you must first write a balanced stoichiometric equation:
xA + yB –► wC + zD
This equation is read as:
x motes of A react with y moles of B to yield w moles of C and z moles of D.
If the amounts of both reactants are specified, one of the two reactants is likely to be used up before the other reactant. The reactant that gets used up first is called the limiting reactant. The other is the excess reactant. You have to do two calculations to determine which is the limiting reactant and how much product can be produced. For example, if you are given 3 moles of A and 5 moles of B, the calculations below show how to calculate the mass of C that can be produced: