Percent Yield and Stoichiometry
Stoichiometry
• In the reaction between 5.5g magnesium and
an excess amount of silver nitrate how many
grams of silver should I produce?
• When I actually performed the experiment
45.3g of silver was isolated.
• What percent of the amount I should have
had did I actually end up with?
Why didn’t I recover 100% of the
product?
• Reactions do not always work perfectly. In any
given experiment there will be sources of
experimental error. Because of this, the
amount of product made will not usually be
the same as the calculated amount.
Definitions
• Theoretical Yield: The maximum amount of
product that can be formed in a reaction –
determined using stoichiometry.
– 48.8 g of Ag in previous example.
• Actual Yield: The amount of product that is
actually formed/isolated from a chemical
reaction – this is a measured value.
– 45.3 g of Ag in previous example.
Percent Yield
• Percent Yield: The amount of product that was
recovered relative to the theoretical
(calculated) amount.
Percent Yield =
Actual Yield x 100
Theoretical Yield
Example
• When I was in second year at university, we
had a lab where we were supposed to isolate
caffeine from tea leaves. Most of my caffeine
was accidentally washed down the drain.
Although I should have had 5.0 g of caffeine, I
only ended up with 0.040 g of caffeine and a
bad grade. What was my percent yield?
Example
• Johnny was sloppy and rushing so he did not
wait for his PbI2 to dry before measuring its
mass. As a result, his product weighed 2.25g
when it should have been 1.75g. What was his
percent yield?
Example
• What is the theoretical yield if 5.50g of
hydrogen react with nitrogen to form
ammonia?
• If 51.6214g of ammonia is produced, what is
my percent yield?
A Big Example
• 5.3 mol of calcium and 3.8 mol of aluminum
chloride are added to the reaction flask.
a. Are these quantities in stoichiometric amounts?
If not determine the limiting reagent.
b. What is the theoretical yield of aluminum in
grams?
c. 90.521g of aluminum was isolated. What is the
percent yield?
Homework
Read:
Section 7.5
Questions:
p.338 # 1-2
p. 399 # 6, 7, 8,
13.