The Foundations of Chemistry

known mass of a compound is burned in a furnace in a stream of oxygen. The carbon and
hydrogen in the sample are converted to carbon dioxide and water vapor, respectively.
The resulting increases in masses of the CO 2 and H 2 O absorbers can then be related to
the masses and percentages of carbon and hydrogen in the original sample.

EXAMPLE 2-14 Percent Composition

Hydrocarbons are organic compounds composed entirely of hydrogen and carbon. A 0.1647-
gram sample of a pure hydrocarbon was burned in a C-H combustion train to produce 0.4931
gram of CO 2 and 0.2691 gram of H 2 O. Determine the masses of C and H in the sample and
the percentages of these elements in this hydrocarbon.

Plan

Step 1: We use the observed mass of CO 2 , 0.4931 grams, to determine the mass of carbon in
the original sample. There is one mole of carbon atoms, 12.01 grams, in each mole of CO 2 ,
44.01 grams; we use this information to construct the unit factor

Step 2: Likewise, we can use the observed mass of H 2 O, 0.2691 grams, to calculate the amount
of hydrogen in the original sample. We use the fact that there are two moles of hydrogen
atoms, 2.016 grams, in each mole of H 2 O, 18.02 grams, to construct the unit factor

Step 3: Then we calculate the percentages by mass of each element in turn, using the rela-
tionship

% element
100%

Solution

Step 1: ? g C0.4931 g CO 2
0.1346 g C

Step 2: ? g H0.2691 g H 2 O
0.03010 g H

Step 3: % C
100% 81.72% C

%H
100% 18.28% H

Total100.00%

You should now work Exercise 58.

When the compound to be analyzed contains oxygen, the calculation of the amount
or percentage of oxygen in the sample is somewhat different. Part of the oxygen that goes
to form CO 2 and H 2 O comes from the sample and part comes from the oxygen stream
supplied. For that reason, we cannot directly determine the amount of oxygen already in

0.03010 g H



0.1647 g sample

0.1346 g C



0.1647 g sample

2.016 g H



18.02 g H 2 O

12.01 g C



44.01 g CO 2

g element



g sample

2.016 g H



18.02 g H 2 O

12.01 g C



44.01 g CO 2

Hydrocarbons are obtained from coal

and coal tar and from oil and gas wells.

The main use of hydrocarbons is as

fuels. The simplest hydrocarbons are

methane CH 4

ethane C 2 H 6

propane C 3 H 8

butane C 4 H 10

2-9 Derivation of Formulas from Elemental Composition 73

We could calculate the mass of H

by subtracting mass of C from mass

of sample. It is good experimental

practice, however, when possible,

to base both on experimental

measurements, as we have done here.

This would help to check for errors in

the analysis or calculation.

Can you show that the hydrocarbon

in Example 2-14 is propane, C 3 H 8?