Chemistry, Third edition

LIMITING REAGENTS 131

Example 8.9

Methane is converted to carbon dioxide and water when burned

in a plentiful supply of oxygen (complete combustion):

CH 4 (g)2O 2 (g)CO 2 (g)2H 2 O(I)

If 10 g of CO 2 were obtained when 16 g of CH 4 were burned in a

limited supply of oxygen gas, what would be the percentage

yield of carbon dioxide?

Answer

If 16 g of methane is burned in oxygen and 10 g of carbon dioxide are formed, then

the theoretical yield of carbon dioxide formed can be found from the equation:

CH 4 (g)2O 2 (g)CO 2 (g)2H 2 O(I)

1 mol CH 4 (g) produces 1 mol CO 2 (g)

16 g CH 4 (g) produces 44 g CO 2 (g)

percentage yield of CO 2 

experimental yield

 100

theoretical yield



10

 100

44

23%

Comment

In a limited supply of oxygen, not all of the methane would be converted to carbon

dioxide – carbon monoxide and soot (carbon) would also be formed. In this case,

incomplete combustion occurs.

Percentage yields

A student prepared solid magnesium oxide by burning 4.8 g

of magnesium metal in air. Unfortunately, much of the white

magnesium oxide produced in this way formed a white

smoke which dispersed into the air of the laboratory. The

student recovered 4.5 g of magnesium oxide. What was the

percentage yield of magnesium oxide?

Exercise 8K

Limiting reagents

We have very often specified that a particular reagent is in excess; this means that

there are more than enough moles of it to react with the other reactant. Under these

circumstances it is the reagent that is notin excess that controls the amount of

product. This reagent is called the limiting reagent.

8.8