What is the formula for calculating
the percentage yield from the actual yield and the theoretical yield?
The theoretical yield is the
maximum amount of product that can be formed from the given amounts of
reactants. For example, if we react 24.3 grams
of magnesium with 32 grams of oxygen, we should be able to produce 56.3 grams of
magnesium oxide. 56.3 grams is the theoretical yield
of this experiment. But when we perform an experiment
such as this, it is not possible to produce the theoretical yield.
There are a variety of reasons as
to why producing the theoretical yield is not possible. Material may have been lost during
experimental procedure, such as transferring a substance from one container to
another or during filtration. Other reasons as to why it’s not
possible to obtain the theoretical yield include that the reaction may not have
completed, unwanted side reactions or competing reactions may have occurred, the
reaction may be reversible or in equilibrium, or there may have been impurities in
the reactants. Regardless of the reason, the
amount of product we produce during an experiment will be less than the theoretical
yield. The amount of product actually
obtained from carrying out a chemical reaction is the actual yield.
A useful way to express the output
or yield of a chemical reaction is as a percentage. We may recall that we can calculate
a percentage by dividing the part in question by the total and multiplying by 100
percent. For example, if we wanted to know
what percentage of the dots were pink, we would divide four, the number of pink
dots, by 10, the total number of dots, then multiply by 100 percent.
We can think of percentage yield in
a similar fashion. When thinking about a chemical
reaction, the total is represented by the theoretical yield, the maximum amount of
product that can be formed from the given amounts of reactants. And the part is represented by the
actual yield as when we perform a chemical reaction, we only produce a part of the
total amount that we should have been able to make.
So to calculate a percentage yield,
we need to take the actual yield and divide it by the theoretical yield, then
multiply by 100 percent. So the formula for calculating the
percentage yield from the actual yield and the theoretical yield is percentage yield
equals actual yield divided by theoretical yield times 100 percent.