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1 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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2 Atomic Mass Atomic mass is the Mass of a single atom in atomic mass units (amu) Mass of an atom compared to a 12 C atom Number below the symbol of an element

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3 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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4 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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5 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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6 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 x10 3 amu) 1 P atom 2. How many Cu atoms have a mass of 4.500 x 10 5 amu? 4.500 x 10 5 amu x 1 Cu atom = 7081 Cu atoms 63.55 amu

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7 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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8 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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9 Learning Check How many gold atoms have a mass of 1.85 x 10 5 amu? 1) 939 Au atoms 2) 3.64 x 10 7 Au atoms 3) 106 Au atoms Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

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10 Solution How many gold atoms have a mass of 1.85 x 10 5 amu? 1) 939 Au atoms 1.85 x 10 5 amu x 1 Au atom = 939 Au atoms 197.0 amu

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11 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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12 Calculating Formula Mass To calculate formula mass of Na 2 SO 4, 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 Na 2 SO 4 1 S x 32.07 amu = 32.07 amu 142.05 amu 1 S 4 O x 16.00 amu = 64.00 amu 1 O

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13 Learning Check Using the periodic table, calculate the formula mass of aluminum sulfide Al 2 S 3.

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14 Solution Using the periodic table, calculate the formula mass of aluminum sulfide Al 2 S 3. 2 Al x 26.98 amu = 53.96 amu 1 Al 3 S x 32.07 amu = 96.21 amu 1 S150.17 amu

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