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10.3 Percent Composition
and Chemical Formulas >
10.3 Percent Composition
and Chemical Formulas >
CHEMISTRY
& YOU
What does the percent composition of a
compound tell you?
Chapter 10
Chemical Quantities
A tag sewn into the
seam of a shirt usually
tells you what fibers
were used to make
the cloth and the
percent of each.
10.1 The Mole: A Measurement of
Matter
10.2 Mole-Mass and Mole-Volume
Relationships
10.3 Percent Composition and
Chemical Formulas
1
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Percent Composition of a
10.3 Percent Composition
and Chemical Formulas > Compound
2
Percent Composition of a
10.3 Percent Composition
and Chemical Formulas > Compound
Percent Composition of a Compound
The relative amounts of the elements in a
compound are expressed as the percent
composition or the percent by mass of
each element in the compound.
How do you calculate the percent
composition of a compound?
3
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Percent Composition of a
10.3 Percent Composition
and Chemical Formulas > Compound
Percent Composition of a
10.3 Percent Composition
and Chemical Formulas > Compound
These percents must total 100%.
The percent composition of potassium
chromate, K2CrO4, is:
K = 40.3%
Cr = 26.8%
O = 32.9%
5
K = 40.3%
Cr = 26.8%
+ O = 32.9%
100%
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Percent Composition of a
10.3 Percent Composition
and Chemical Formulas > Compound
6
Percent Composition of a
10.3 Percent Composition
and Chemical Formulas > Compound
Percent Composition from Mass Data
These percents must total 100%.
If you know the relative masses of each
element in a compound, you can calculate
the percent composition of the compound.
•  The percent composition of a compound is
always the same.
7
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Percent Composition of a
10.3 Percent Composition
and Chemical Formulas > Compound
10.3 Percent Composition
and Chemical Formulas >
Calculating Percent
Composition from Mass Data
Percent Composition from Mass Data
The percent by mass of an element
in a compound is the number of
grams of the element divided by the
mass in grams of the compound,
multiplied by 100%.
% by mass of element =
9
mass of element
mass of compound
10.3 Percent Composition
and Chemical Formulas >
When a 13.60-g sample of a
compound containing only
magnesium and oxygen is
decomposed, 5.40 g of oxygen is
obtained. What is the percent
composition of this compound?
× 100%
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Sample Problem 10.9
10
Sample Problem 10.9
2 Calculate Solve for the unknowns.
Determine the percent by mass of
Mg in the compound.
The percent by mass of an element in a
compound is the mass of that element divided by
the mass of the compound multiplied by 100%.
KNOWNS
mass of compound = 13.60 g
mass of oxygen = 5.40 g O
mass of magnesium = 13.60 g – 5.40 g O = 8.20 g Mg
% Mg =
=
UNKNOWNS
percent by mass of Mg = ?% Mg
percent by mass of O = ?% O
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10.3 Percent Composition
and Chemical Formulas >
1 Analyze List the knowns and the unknowns.
11
Sample Problem 10.9
mass of Mg
× 100%
mass of compound
8.20 g
× 100%
13.60 g
= 60.3% Mg
12
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10.3 Percent Composition
and Chemical Formulas >
Sample Problem 10.9
10.3 Percent Composition
and Chemical Formulas >
2 Calculate Solve for the unknowns.
3 Evaluate Does the result make sense?
Determine the percent by mass of
O in the compound.
%O=
=
mass of O
mass of compound
5.40 g
13.60 g
Sample Problem 10.9
The percents of the elements add up
to 100%.
× 100%
60.3% + 39.7% = 100%
× 100%
= 39.7% O
13
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Percent Composition of a
10.3 Percent Composition
and Chemical Formulas > Compound
14
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Percent Composition of a
10.3 Percent Composition
and Chemical Formulas > Compound
Percent Composition from the
Chemical Formula
You can also calculate the percent
composition of a compound using its
chemical formula.
Percent Composition from the
Chemical Formula
You can also calculate the percent
composition of a compound using its
chemical formula.
•  The subscripts in the formula are used to
calculate the mass of each element in a mole of
that compound.
% by mass mass of element in 1 mol compound
=
× 100%
of element
molar mass of compound
•  Using the individual masses of the elements and
the molar mass, you can calculate the percent
by mass of each element.
15
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10.3 Percent Composition
and Chemical Formulas >
Sample Problem 10.10
10.3 Percent Composition
and Chemical Formulas >
Calculating Percent Composition
from a Formula
Sample Problem 10.10
1 Analyze List the knowns and the unknowns.
Calculate the percent by mass of each element
by dividing the mass of that element in one mole
of the compound by the molar mass of the
compound and multiplying by 100%.
Propane (C3H8), the fuel
commonly used in gas grills, is
one of the compounds
obtained from petroleum.
Calculate the percent
composition of propane.
KNOWNS
mass of C in 1 mol C3H8 = 3 mol × 12.0 g/mol = 36.0 g
mass of H in 1 mol C3H8 = 8 mol × 1.0 g/mol = 8.0 g
molar mass of C3H8 = 36.0 g/mol + 8.0 g/mol = 44.0 g/mol
UNKNOWNS
percent by mass of C = ?% C
percent by mass of H = ?% H
17
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10.3 Percent Composition
and Chemical Formulas >
Sample Problem 10.10
18
10.3 Percent Composition
and Chemical Formulas >
2 Calculate Solve for the unknowns.
=
mass of C in 1 mol C3H8
molar mass of C3H8
36.0 g
44.0 g
Determine the percent by mass of H
in C3H8.
× 100%
%H=
× 100%
=
= 81.8% C
19
Sample Problem 10.10
2 Calculate Solve for the unknowns.
Determine the percent by mass of C
in C3H8.
%C=
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mass of H in 1 mol C3H8
molar mass of C3H8
8.0 g
44.0 g
× 100%
× 100%
= 18% H
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10.3 Percent Composition
and Chemical Formulas >
Sample Problem 10.10
Percent Composition of a
10.3 Percent Composition
and Chemical Formulas > Compound
Percent Composition as a Conversion
Factor
3 Evaluate Does the result make sense?
You can use percent composition to
calculate the number of grams of any
element in a specific mass of a compound.
The percents of the elements add up to
100% when the answers are expressed to
two significant figures (82% + 18% =
100%).
21
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Percent Composition of a
10.3 Percent Composition
and Chemical Formulas > Compound
•  To do this, multiply the mass of the
compound by a conversion factor based on
the percent composition of the element in
the compound.
22
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10.3 Percent Composition
and Chemical Formulas >
Percent Composition as a Conversion
Factor
CHEMISTRY
& YOU
What information can you get from the
percent composition of a compound?
Propane is 81.8% carbon and 18%
hydrogen.
•  You can use the following conversion factors
to solve for the mass of carbon or hydrogen
contained in a specific amount of propane.
81.8 g C
100 g C3H8
23
and
18 g H
100 g C3H8
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10.3 Percent Composition
and Chemical Formulas >
25
CHEMISTRY
& YOU
10.3 Percent Composition
and Chemical Formulas >
What information can you get from the
percent composition of a compound?
Calculating the Mass of an
Element in a Compound Using
Percent Composition
You can use percent
composition to determine
the mass of an element in
a sample of a compound
of a given size. You can
also determine the
empirical formula of the
compound.
Calculate the mass of carbon and
the mass of hydrogen in 82.0 g of
propane (C3H8).
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10.3 Percent Composition
and Chemical Formulas >
Sample Problem 10.11
26
•  To calculate the mass of C, first write the
conversion factor to convert from mass of C3H8
to mass of C.
From Sample
81.8 g C
100 g C3H8
UNKNOWNS
mass of carbon = ? g C
mass of hydrogen = ? g H
Problem 10.10, the
percent by mass of C
in C3H8 is 81.8%.
•  Multiply the mass of C3H8 by the
conversion factor.
82.0 g C3H8 ×
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Sample Problem 10.11
2 Calculate Solve for the unknowns.
Use the conversion factors based on the percent
composition of propane to make the following
conversions: grams C3H8 → grams C and grams
C3H8 → grams H.
KNOWN
mass of C3H8 = 82.0 g
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10.3 Percent Composition
and Chemical Formulas >
1 Analyze List the known and the unknowns.
27
Sample Problem 10.11
28
81.8 g C
= 67.1 g C
100 g C3H8
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10.3 Percent Composition
and Chemical Formulas >
Sample Problem 10.11
10.3 Percent Composition
and Chemical Formulas >
2 Calculate Solve for the unknowns.
3 Evaluate Does the result make sense?
•  To calculate the mass of H, first write the
conversion factor to convert from mass of C3H8
to mass of H.
From Sample
18 g H
100 g C3H8
Sample Problem 10.11
The sum of the two masses equals 82 g,
the sample size, to two significant figures
(67 g C + 15 g H = 82 g C3H8).
Problem 10.10, the
percent by mass of H
in C3H8 is 18%.
•  Multiply the mass of C3H8 by the
conversion factor.
82.0 g C3H8 ×
29
18 g H
100 g C3H8
= 15 g H
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