Up until now, all our reactions have taken place under ideal conditions. However, ideal conditions don't exist in real life. Reactants may be impure, reactions may not go to completion, or given reactions may have to compete with several smaller side reactions. In fact, in the laboratory, if you get 60% of the expected amount of product, that is considered very good.

Now, let's talk terminology. The calculated or expected amount of product is called the theoretical yield. The amount of product actually produced is called the actual yield. When you divide actual yield by theoretical yield you get a decimal percentage known as the percent yield of a reaction.

Percent Yield =    

Once again it's time for an example problem:

Problem: What is the percent yield of the following reaction if 60 grams of CaCO3 is heated to give 15 grams of CaO?

CaCO3→CaO + CO2    

Solution: Ideally, how many grams of CaO should be produced? First verify the equation is balanced; it is. Now convert to moles, based on the amount of CaCO3 present.

× = 0.6 mole CaO    

How many grams is this?

= 33.6 grams CaO    

So, ideally, 33.6 grams of CaO should have been produced in this reaction. This is the theoretical yield. However, the problem tells us that only 15 grams were produced. 15 grams is the actual yield. It is now a simple matter to find percent yield.

= 0.446 = 44.6%