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Chapter 7 Chemical Quantities

7.1

Atomic Mass and Formula Mass

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Atomic Mass

Atomic mass is the• Mass of a single atom in

atomic mass units (amu)• Mass of an atom

compared to a 12C atom• Number below the

symbol of an element

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Periodic Table and Atomic Mass

Ag has atomic mass = 107.9 amu

C has atomic mass = 12.01 amu

S has atomic mass = 32.07 amu

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Atomic Mass Factors

The atomic mass, • Can be written as an equality

Example: 1 P atom = 30.97 amu

• Can be written as two conversion factors

Example:

1 P atom and 30.97 amu

30.97 amu 1 P atom

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Uses of Atomic Mass Factors

The atomic mass is used to convert: • A specific number of atoms to mass (amu)• An amount in amu to number of atoms

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Using Atomic Mass Factors

1. What is the mass in amu of 75 P atoms?

75 P atoms x 30.97 amu = 2323 amu (2.323 x103 amu)

1 P atom

2. How many Cu atoms have a mass of 4.500 x 105 amu?

4.500 x 105 amu x 1 Cu atom = 7081 Cu atoms

63.55 amu

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Learning Check

What is the mass in amu of 75

silver atoms?

1) 107.9 amu

2) 8093 amu

3) 1.439 amu

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Solution

What is the mass in amu of 75 silver atoms?

2) 8093 amu

75 Ag atoms x 107.9 amu = 8093 amu

1 Ag atom

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Learning Check

How many gold atoms have a

mass of 1.85 x 105 amu?

1) 939 Au atoms

2) 3.64 x 107 Au atoms

3) 106 Au atoms

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Solution

How many gold atoms have a mass of 1.85 x 105 amu?

1) 939 Au atoms

1.85 x 105 amu x 1 Au atom = 939 Au atoms

197.0 amu

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Formula Mass

The formula mass is• The mass in amu

of a compound• The sum of the

atomic masses of the elements in a formula

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Calculating Formula Mass

To calculate formula mass of Na2SO4,

• Multiply the atomic mass of each element by its subscript

• Total the masses

2 Na x 22.99 amu = 45.98 amu Formula mass

1 Na Na2SO4

1 S x 32.07 amu = 32.07 amu 142.05 amu1 S

4 O x 16.00 amu = 64.00 amu1 O

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Learning Check

Using the periodic table, calculate the formula mass of aluminum sulfide Al2S3.

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Solution

Using the periodic table, calculate the formula mass of aluminum sulfide Al2S3.

2 Al x 26.98 amu = 53.96 amu

1 Al

3 S x 32.07 amu = 96.21 amu

1 S 150.17 amu