Excess and Limiting Quantities

The chemical equation for a reaction describes what is supposed to occur. However sometimes there is not enough of one reactant for the full reaction to occur. The reactant is called the limiting reactant because it limits how far the reaction can go. The limiting reactant is always fully used up. The other reactant(s) are called the excess reactant because there are more than enough moles for the reaction to occur.





Now how do we find out which reactant is which?
There are two ways: the first way is to convert both reactant to the same product and see which one produces the least, the second way is to convert one reactant to the other and see how much is needed to react with each other.

If you have 67.0g of Cl₂ and 35.0g of O₂ in the reaction 2Cl₂ + O₂ à2OCl₂ which reactant is the limiting quantity?

67.0g Cl₂ x 1 Mol Cl₂ x 2 Mol OCl₂ x       87.0g        = 82.1 g of OCl₂

                     71.0g         2 Mol CL₂        1 Mol OCl₂

44.0g O₂ x 1 Mol O₂ x 2 Mol OCl₂ x       87.0g        = 239 g of OCl₂

                     32.0g         1 Mol O₂        1 Mol OCl₂

 This means the Cl2 is the limiting reactant because it can only make 82.1g of OCl2

Now the second way using the same question

44.0g O₂ x 1 Mol O₂ x 2 Mol Cl₂ x       71.0g        = 195 g of Cl₂

                     32.0g         1 Mol O₂        1 Mol Cl₂

Now we can see that we would need 195g of Cl2 in order for all of the O2 to be used up. But we only have 67.0g and therefore we know that Cl2 is the limiting reactant.

Using the second way we can also determine how much excess we have or how much is missing for the whole reaction to occur. 

195- 67= 128g

This means that 128g of Cl is missing.

Here is another example question and tutorial

http://www.khanacademy.org/video/stoichiometry--limiting-reagent?playlist=Chemistry