Percent Yield

When we do the math for a stoichiometry problem we can predict how much product the reaction should produce based on the amount of reactant used. Of course, nothing ever works perfectly.

When a chemist does a lab, perhaps one that produces a solid precipitate, they must collect the product (perhaps by filtering) and then wash away any other ions of substances present. All of these processes may cause a loss of product. Then the chemist must dry the product to remove any remaining water. If this doesn’t work properly, or if the product was not fully cleaned and purified there may be extraneous material in the product. 

More complex, multi-step processes add to this list of problems and some processes, especially organic reactions simply don’t work all that well. At times only a small fraction of the material actually reacts the way it should.

All of this means that the amount of product actually obtained in a laboratory process is never exactly what the math predicts. We measure the success of our experiment by calculating the percent yield with the following formula

Actual yield, in the equation, is the amount of product that the lab produces. This is the value you measure at the end of the experiment. Theoretical yield is the answer to the stoichiometry problem - the amount that the math says should be produced. It is important to note that the units of actual yield and theoretical yield MUST be the same. Here is an example:

A student measures some silver nitrate and dissolves it in deionized water. She then drops in a piece of copper wire. After the reaction occurs, she collects the silver that was produced on some filter paper and weighs it. Using her data, in the table below, determine the percent yield.

Since we are looking for the percent yield, we need to know the actual yield and the theoretical yield.

First, we’ll determine the actual yield, that is, the amount of silver that was collected. This is a simple subtraction problem.

Next, we’ll need to find the theoretical yield. This requires a stoichiometry problem, which in turn, requires us to start with the balanced reaction. As before, I’ll put the relevant information under the respective compounds.

The value on the right (the g of Ag) is the actual yield, so we’re going to leave that alone. We need to calculate the theoretical yield. In other words we need to determine how many grams of silver should have been produced by the reaction. So, we’ll start our stoichiometry problem with the only other thing we know - the grams of silver nitrate used, and we’ll solve for the mass of silver produced.

Note that this answer does NOT match our actual yield. This should not be surprising. As discussed above, labs are never perfect.

Now we’re ready to determine the percent yield

Percent yield problems on a test or quiz

When you are doing an experiment, the “actual yield” is easy to determine, as above. When you are solving a problem on paper (where you don’t have an “actual” yield) the “actual yield” is always given as the amount of product produced.

Here is the exact same problem written as a test question:

A student places a piece of copper into a solution of silver nitrate and allows them to react. If she used 1.032 g of silver nitrate to make the solution and recovered 0.602 g of silver, what is the percent yield?

Notice that the actual yield is given (always an amount of product) and that the other amount given is the beginning of the stoichiometry problem that must be done.