The Foundations of Chemistry

simultaneously, forming undesired products as well as desired products. Reactions other

than the desired one are called “side reactions.” (3) In some cases, separation of the de-

sired product from the reaction mixture is so difficult that not all of the product formed

is successfully isolated. The actual yieldis the amount of a specified pure product actu-

ally obtained from a given reaction.

The term percent yieldis used to indicate how much of a desired product is obtained

from a reaction.

percent yield100%

Consider the preparation of nitrobenzene, C 6 H 5 NO 2 , by the reaction of a limited

amount of benzene, C 6 H 6 , with excess nitric acid, HNO 3. The balanced equation for the

reaction may be written as

C 6 H 6 
HNO 3 88nC 6 H 5 NO 2 
H 2 O

1 mol 1 mol 1 mol 1 mol

78.1 g 63.0 g 123.1 g 18.0 g

EXAMPLE 3-10 Percent Yield

A 15.6-gram sample of C 6 H 6 is mixed with excess HNO 3. We isolate 18.0 grams of C 6 H 5 NO 2.

What is the percent yield of C 6 H 5 NO 2 in this reaction?

Plan

First we interpret the balanced chemical equation to calculate the theoretical yield of C 6 H 5 NO 2.

Then we use the actual (isolated) yield and the previous definition to calculate the percent

yield.

Solution

We calculate the theoretical yield of C 6 H 5 NO 2.

__?g C 6 H 5 NO 2 15.6 g C 6 H 6 

24.6 g C 6 H 5 NO 2 mtheoretical yield

This tells us that if allthe C 6 H 6 were converted to C 6 H 5 NO 2 and isolated, we should obtain

24.6 grams of C 6 H 5 NO 2 (100% yield). We isolate only 18.0 grams of C 6 H 5 NO 2 , however.

percent yield100%100%

 73.2 percent yield

You should now work Exercise 38.

The amount of nitrobenzene obtained in this experimentis 73.2% of the amount that

would be expected ifthe reaction had gone to completion, ifthere were no side reactions,

and ifwe could have recovered all of the product as pure substance.

18.0 g



24.6 g

actual yield of product



theoretical yield of product

123.1 g C 6 H 5 NO 2



1 mol C 6 H 5 NO 2

1 mol C 6 H 5 NO 2



1 mol C 6 H 6

1 mol C 6 H 6



78.1 g C 6 H 6

actual yield of product



theoretical yield of product

It is not necessary to know the mass of
one mole of HNO 3 to solve this
problem.

100 CHAPTER 3: Chemical Equations and Reaction Stoichiometry

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 5.6, Percent Yield.