Chemistry Reference
In-Depth Information
10.5
Theoretical Yield and Percent Yield
When a quantity of product is calculated from a quantity or quantities of reac-
tants, as was done in Sections 10.1 through 10.4 of this chapter, that quantity
of product is called the
theoretical yield.
When a reaction is run, however, less
product than the calculated amount is often obtained: Some of the product may
stay in the solution in which the reaction was run; some side reaction may use
up some of the reactants; or the reaction may be stopped before it is completed.
No matter why, the fact is that many reactions produce less product than the
calculated quantity; that is, the actual yield is less than the theoretical yield. No
reaction can produce more than the theoretical yield. The
percent yield
is
defined as 100% times the ratio of the actual yield to the theoretical yield:
Actual yield
Theoretical yield
a
b
Percent yield
100%
Example 10.26
Calculate the percent yield of a reaction if calculations indicated that 7.44 g of
product could be obtained, but only 7.02 g of product was actually obtained.
Solution
Actual yield
Theoretical yield
a
7.02 g
7.44 g
Percent yield
a
b
100%
b
100%
94.4%
Example 10.27
Calculate the percent yield if 14.1 g of solid
PCl
5
is obtained in a certain exper-
iment in which 10.0 g of liquid
PCl
3
is treated with excess gaseous
Cl
2
.
Solution
The theoretical yield in grams is calculated as discussed in Section 10.2:
PCl
3
(
/
)
Cl
2
(g) £ PCl
5
(s)
Balanced
chemical
equation
Grams
of PCl
3
Molar
mass
Moles
of PCl
3
Moles
of PCl
5
Molar
mass
Grams
of PCl
5
a
1 mol PCl
3
138 g PCl
3
a
1 mol PCl
5
1 mol PCl
3
a
209 g PCl
5
1 mol PCl
5
10.0 g PCl
3
b
b
b
15.1
4
g PCl
5
The percent yield is
Actual yield
Theoretical yield
a
14.1 g
15.1
4
g
a
b
100%
b
100%
93.1%
