Yields
Dr. MJ Patterson

When we carry out a reaction in the lab, we typically measure how much of each starting material we are using.  From these amounts, we can calculate how much of the product should be made.  This calculated amount is referred to as the theoretical yield, and it is based on the type of calculation we did in the last module.

In the real world, when you isolate your product and weigh it, the mass is almost never the same as the theoretical yield.  The real, experimentally measured amount is called the actual yield.  Sometimes the actual yield is less than the theoretical yield, and sometimes it is greater.  Very frequently, a percent yield is reported:
 
 

% yield =

     actual yield     

 x 100

theoretical yield

 

 

Example 1:
In a lab experiment, 0.80 g of copper metal should be produced.  If a student actually made 0.77 g of copper, what is the percent yield?

Solution 1:
The theoretical yield is 0.80 g.  The actual yield is 0.77 g.  We have all of the information we need to calculate the % yield.
 

% yield =

     actual yield 

 x 100 = 

(0.77 g)

x 100 = 96.25 % = 96 %

 

theoretical yield

(0.80 g)

 

 

 

 

Factors Making Percent Yield Less Than 100%

Any time the % yield is less than 100%, it simply means that less product was made than would be expected.  Another way of stating it is that the actual yield is less than the theoretical yield.

What would make the actual yield smaller than expected?  There are many possibilities, but the following items are some of the most commonly encountered reasons.

  • Dropping the sample, knocking over a beaker, etc.
  • Leaving some sample behind when transferring from one container to another.
  • Using impure starting materials.
  • Reaction equilibrium.  Some reactions simply do not go to completion.  No matter how long you wait, there will always be unreactted starting material.
  • Formation of side products.  Frequently, more than one reaction can occur under a given set of conditions.  If other reactions take place, less of the desired product will be made.
  • Loss of product upon purification.  Most purification processes sacrifice some of the desired product as a trade-off for achieving higher purity.

Factors Making Percent Yield Exceed 100%

The opposite case is when more product is found than was expected.  Does this mean that we have found a way of creating matter from empty space?  No, it typically means that the product contains impurities.  The following are some of the most common reasons.

  • The product is still wet with solvent.
  • Unreacted starting material is still present.
  • More than one product was made, and they were not thoroughly separated from one another.
  • Garbage is in the sample - broken glass, boiling stones, pieces of filter paper...