1 Chapter 7 Chemical Quantities 7.1 Atomic Mass and Formula Mass Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

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

3 Periodic Table and Atomic Mass Ag has atomic mass = amu C has atomic mass = amu S has atomic mass = amu

4 Atomic Mass Factors The atomic mass, Can be written as an equality Example: 1 P atom = amu Can be written as two conversion factors Example: 1 P atom and amu amu 1 P atom

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

6 Using Atomic Mass Factors 1. What is the mass in amu of 75 P atoms? 75 P atoms x amu = 2323 amu (2.323 x10 3 amu) 1 P atom 2. How many Cu atoms have a mass of x 10 5 amu? x 10 5 amu x 1 Cu atom = 7081 Cu atoms amu

7 Learning Check What is the mass in amu of 75 silver atoms? 1) amu 2) 8093 amu 3) amu Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

8 Solution What is the mass in amu of 75 silver atoms? 2) 8093 amu 75 Ag atoms x amu = 8093 amu 1 Ag atom

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

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 amu

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

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 amu = amu Formula mass 1 Na Na 2 SO 4 1 S x amu = amu amu 1 S 4 O x amu = amu 1 O

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

14 Solution Using the periodic table, calculate the formula mass of aluminum sulfide Al 2 S 3. 2 Al x amu = amu 1 Al 3 S x amu = amu 1 S amu