AP 1.3 draft

1.3 Composition of pure

substances

John D. Bookstaver

St. Charles Community

College

Cottleville, MO
© 2009, Prentice-Hall, Inc.

Chemistry, The Central Science, 11th edition

Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E.

Bursten

Stoichiometry

Formula Weight (FW)

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●A formula weight is the sum of the atomic weights
for the atoms in a chemical formula.

●So, the formula weight of calcium chloride, CaCl2,
would be

Ca: 1(40.1 amu)
+ Cl: 2(35.5 amu)

111.1 amu

●Formula weights are generally reported for ionic
compounds.

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Stoichiometry

Molecular Weight (MW)

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●A molecular weight is the sum of the atomic
weights of the atoms in a molecule.

●For the molecule ethane, C2H6, the molecular
weight would be

C: 2(12.0 amu)

30.0 amu

+ H: 6(1.0 amu)

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Stoichiometry

Percent Composition

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One can find the percentage of the mass of a

compound that comes from each of the elements in
the compound by using this equation:

% element =

(number of atoms)(atomic weight)

(FW of the compound)

x 100

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Stoichiometry

Percent Composition

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So the percentage of carbon in ethane (C2H6) is…

%C =

(2)(12.0 amu)

(30.0 amu)

24.0 amu

30.0 amu

=

x 100

= 80.0%

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Stoichiometry

Calculating Empirical Formulas

One can calculate the empirical formula from

the percent composition.

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Prentice-Hall, Inc.

The compound para-aminobenzoic acid (PABA used to be included in
sunscreen) is composed of carbon (61.31%), hydrogen (5.14%), nitrogen
(10.21%), and oxygen (23.33%). Find the empirical formula of PABA.

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Prentice-Hall,
Inc.

Stoichiometry

Calculating Empirical Formulas

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Assuming 100.00 g of para-aminobenzoic acid,

C:

61.31 g x

= 5.105 mol C

H:

5.14 g x

= 5.09 mol H

N:

10.21 g x

= 0.7288 mol N

O:

23.33 g x

= 1.456 mol O

1 mol
12.01 g

1 mol
14.01 g

1 mol
1.01 g

1 mol
16.00 g

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Stoichiometry

Calculating Empirical Formulas

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Calculate the mole ratio by dividing by the smallest number of moles:

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C:

= 7.005 ≈ 7

H:

= 6.984 ≈ 7

N:

= 1.000

O:

= 2.001 ≈ 2

5.105 mol
0.7288 mol

5.09 mol
0.7288 mol

0.7288 mol
0.7288 mol

1.458 mol
0.7288 mol

Stoichiometry

Calculating Empirical Formulas

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These are the subscripts for the empirical formula:

C7H7NO2

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Stoichiometry

Copyright © Houghton
Mifflin Company. All rights
reserved.

3a–*

Figure 3.6: Examples of substances whose
empirical and molecular formulas differ. Notice that
molecular formula = (empirical formula)n, where n
is an integer.

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Stoichiometry

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Stoichiometry

© 2009, Prentice-Hall, Inc.