Example Limiting Reactant Calculation:
A 2.00 g sample of ammonia is mixed with 4.00 g of oxygen. Which is the limiting reactant and how much excess reactant remains after the reaction has stopped?
First, we need to create a balanced equation for the reaction:
4 NH3(g) + 5 O2(g)4 NO(g) + 6 H2O(g)
Next we can use stoichiometry to calculate how much product is produced by each reactant. NOTE: It does not matter which product is chosen, but the same product must be used for both reactants so that the amounts can be compared.
The reactant that produces the lesser amount of product: in this case the oxygen.
Next, to find the amount of excess reactant, we must calculate how much of the non-limiting reactant (oxygen) actually did react with the limiting reactant (ammonia).
We're not finished yet though. 1.70 g is the amount of ammonia that reacted, not what is left over. To find the amount of excess reactant remaining, subtract the amount that reacted from the amount in the original sample.