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Law of Definite Proportions
An important characteristic of compounds is that the elements comprising them always
combine in definite proportions by mass. This observation is so fundamental that it is
summarized as the law of definite proportions. The
law of definite proportions
states
that a compound is always composed of the same elements in the same proportion by
mass, no matter how large or small the sample. The mass of the compound is equal to
the sum of the masses of the elements that make up the compound.
The relative amounts of the elements in a compound can be expressed as percent by
mass. The
percent by mass
is the ratio of the mass of each element to the total mass of
the compound expressed as a percentage.
For example, consider the compound granulated sugar (sucrose). This compound is
composed of three elements—carbon, hydrogen, and oxygen. The analysis of 20.00 g of
sucrose from a bag of granulated sugar is given in
Table 3
. Note that the sum of the
individual masses of the elements found in the sugar equals 20.00 g, which is the
amount of the granulated sugar sample that was analyzed. This demonstrates the law of
conservation of mass as applied to compounds: the mass of a compound is equal to the
sum of the masses of the elements that make up the compound.
Get It?
State
the law of definite proportions.
Percent by Mass
percent by mass (%)
=
mass of element ______________
mass of compound
×
100
Percent by mass is obtained by dividing the mass of the element by the mass of the
compound and then by multiplying this ratio by 100 to express it as a percentage.
Table 3
Sucrose Analysis
20.00 g of Granulated Sugar
500.0 g of Sugarcane
Element Analysis
by Mass
(g)
Percent by Mass (%)
Analysis
by Mass
(g)
Percent by Mass (%)
Carbon
8.44
8.44 g C
___________ 20.00 g sucrose
×
100
=
42.20% 211.0
211.0 g C
___________ 500.0 g sucrose
×
100
=
42.20%
Hydrogen
1.30
1.30 g H
___________ 20.00 g sucrose
×
100
=
6.50% 32.5
32.50 g H
___________ 500.0 g sucrose
×
100
=
6.500%
Oxygen
10.26
10.26 g O
___________ 20.00 g sucrose
×
100
=
51.30% 256.5
256.5 g O
___________ 500.0 g sucrose
×
100
=
51.30%
Total
20.00
100%
500.0
100%
64
Module 2 • Matter—Properties and Changes