Example
What is average atomic mass of Lithium if 7.42% exists as 6Li (6.015
amu) and 92.58% exists as 7Li (7.016 amu)?
Isotope
Atomic Mass*
Lithium – 6
6.015 amu
Percent
Abundance
7.42%
Lithium – 7
7.016 amu
92.58%
Subtotal
How to Solve
1.) Set up chart like one above. *If you are given the atomic mass
use that. If you are not given the atomic mass just put the
mass number there.
2.) Convert the the percent abundance to a decimal by dividing by
100.
a. 7.42%  7.42/100 = 0.0742
b. 92.58  92.58/100 = 0.9258
3.) Multiple your atomic mass by the percent abundance
a. 6.015 x 0.0742 = .446
b. 7.016 x 0.9258 = 6.495
4.) Add subtotals
0.446 amu
+
6.495 amu
6.941 amu
1. Neon has two major isotopes, Neon-20 and Neon-22. Out of every
250 neon atoms, 225 will be Neon-20 (19.992 amu), and 25 will be
Neon-22 (21.991 amu). What is the average atomic mass of Neon?
2. Rubidium has two common isotopes, 85Rb and 87Rb. If the
abundance of 85Rb is 72.2% and the abundance of 87Rb is 27.8%,
what is the average atomic mass of rubidium?
Intermediate
3. What is the atomic mass of Hafnium if out of every 200 atoms, 10
have mass 176.00 amu, 38 have mass 177.00 amu, 54 have mass
178.00 amu, 28 have mass 179.00 amu, and 70 have mass 180.00
amu?
4. Without doing any math, are there more Bromine-79 atoms or
more Bromine-80 atoms on earth? (Hint: look at the periodic table.)
Advanced
5. In a sample of 200 Chlorine atoms, it is found that 151 are 35Cl
(34.969 amu), and 49 are 37Cl (36.966 amu). What is the average
atomic mass of Chlorine?