The Foundations of Chemistry

EXAMPLE 3-12 Sequential Reactions

Phosphoric acid, H 3 PO 4 , is a very important compound used to make fertilizers. It is also pres-

ent in cola drinks.

H 3 PO 4 can be prepared in a two-step process.

Rxn 1: P 4 
5O 2 88nP 4 O 10

Rxn 2: P 4 O 10 
6H 2 O88n4H 3 PO 4

We allow 272 grams of phosphorus to react with excess oxygen, which forms tetraphosphorus

decoxide, P 4 O 10 , in 89.5% yield. In the second step reaction, a 96.8% yield of H 3 PO 4 is ob-

tained. What mass of H 3 PO 4 is obtained?

Plan

1. We interpret the first equation as usual and calculate the amount of P 4 O 10 obtained.

P 4 
 5O 2 88nP 4 O 10

1 mol 5 mol 1 mol

124 g 5(32.0 g) 284 g

g P 4 88n mol P 4 88n mol P 4 O 10 88n g P 4 O 10

2. Then we interpret the second equation and calculate the amount of H 3 PO 4 obtainedfrom
   the P 4 O 10 from the first step.

P 4 O 10 
 6H 2 O 88n4H 3 PO 4

1 mol 6 mol 4 mol

284 g 6(18.0 g) 4(98.0 g)

g P 4 O 10 88n mol P 4 O 10 88n mol H 3 PO 4 88n g H 3 PO 4

Solution

1. __?g P 4 O 10 272 g P 4 


 558 g P 4 O 10

2.__?g H 3 PO 4 558 g P 4 O 10 


 746 g H 3 PO 4

You should now work Exercises 48 and 50.

Chemists have determined the structures of many naturally occurring compounds. One

way of proving the structure of such a compound is by synthesizing it from available start-

ing materials. Professor Grieco, now at Indiana University, was assisted by Majetich and

Ohfune in the synthesis of helenalin, a powerful anticancer drug, in a 40-step process.

This 40-step synthesis gave a remarkable average yield of about 90% for each step, which

still resulted in an overall yield of only about 1.5%.

96.8 g H 3 PO 4 actual



100 g H 3 PO 4 theoretical

98.0 g H 3 PO 4 theoretical



1 mol H 3 PO 4 theoretical

4 mol H 3 PO 4



1 mol P 4 O 10

1 mol P 4 O 10



284 g P 4 O 10

89.5 g P 4 O 10 actual



100 g P 4 O 10 theoretical

284 g P 4 O 10 theoretical



1 mol P 4 O 10 theoretical

1 mol P 4 O 10



1 mol P 4

1 mol P 4



124 g P 4

102 CHAPTER 3: Chemical Equations and Reaction Stoichiometry

Two large uses for H 3 PO 4 are
in fertilizers and cola drinks.
Approximately 100 lb of
H 3 PO 4 – based fertilizer are used
per year per person in America.

The unit factors that account for less
than 100% reaction and less than
100% recovery are included in
parentheses.