How to Use Limiting Reagents...

How to Use Limiting Reagents...
In order to use limiting reagents you must first be comfortable with Finding The Limiting Reagent.
Once you have found the limiting reagent, there is only one more step left. This is done, like most chemistry problems, by using a formula. The first formula will leave you with an answer in moles, the second formula applied after the first will leave you with an answer in grams (what we want.)

Theoretical yield in moles = (moles of the limiting reagent) * (coefficient of the product/coefficient of the limiting reagent)

Theoretical yield in grams = (theoretical yield in moles) * (the molar mass of the product)

Example:

For the balanced equation shown below, if 7.42 grams of P4 were reacted with 5.77 grams of O2, how many grams of P4O10 would be produced?

P4+5O2=>P4O10

Step 1: Find the Limiting Reagent (if you need to review how to do this step click here...Find the Limiting Reagent.

P4 = 31 * 4 = 124             O2 = 16 * 2 = 32
      
7.42                                        5.77
124                                           32

.059                                           .180
                           
                  1      =    .059
                  5            .180

                .180   =   .295

O2 = the limiting reagent

Step 2: Plug the appropriate #'s into the formula.

Theoretical yield in moles = (moles of the limiting reagent) * (coefficient of the product/coefficient of the limiting reagent)
Theoretical yield in moles = (.180) * (1/5) = .036

now plug in this answer to the 2nd formula to convert it from moles to grams.

Theoretical yield in grams = (theoretical yield in moles) * (the molar mass of the product) ...in this case the "molar mass of the product" is the molar mass of P4O10 which = 284
Theoretical yield in grams = (.036) * (284)  = 10.244 grams


If you are comfortable with this additional step required to use the limiting reagent, then visit the practice problems section here and put yourself to the test! Using Limiting Reagents Practice Problems