A problem which is more fun involves specifying the amounts of both reactants without indicating which is in excess. It becomes necessary to determine which of the reactants is in excess.

Lets begin with a simple example to show the logic. In the reaction,

2C₂H_6(g) + 7O_2(g) ---> 4CO_2(g) + 6H₂O _(g)

Exactly 2 mols of C₂H₆ are required to react completely with 7 mols of O₂.

Sample Limiting Reagent Problem #1

Sample Limiting Reagent Problem #2

Sample Limiting Reagent Problem #3

Here are two animations showing the second problem using two different approaches. One as we solved the problem in class the other a slightly different way.

• Stoichiometry animation #1

• Stoichiometry animation #2