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Average Atomic Mass Practice
Average atomic mass is the weighted average of the masses of all the isotopes of that element. A weighted
average reflects both the mass and the abundance of the isotopes as they occur in nature.
To watch a YouTube video on this topic, search YouTube for: atomic mass introduction dewitt
Formula
Average atomic mass = (Atomic mass of isotope #1) x (Abundance of Isotope #1) + (Atomic mass of
isotope #2) x (Abundance of Isotope #2)… and so on for each isotope
Example: In nature, chlorine consists of two isotopes:
Isotope
Cl-35
Cl-37
Atomic mass
34.97 amu
36.97 amu
Abundance (%)
75.8 %
24.2 %
Abundance (as decimal)
0.758
0.242
To calculate the average atomic mass, multiply each atomic mass by the percent abundance (as decimal) and
add them together.
(34.97 amu) x (0.758) + (36.97 amu) x (0.242) = 26.51 + 8.95 = 35.46 amu
Practice Problems
Fill in the missing information and calculate the average atomic mass of each element (show your work).
1. Carbon
Isotope
Atomic Mass
% Abundance
C-12
12.00 amu
98.93%
C-13
13.00 amu
1.07%
Abundance as
decimal
Show your work :
Average Atomic Mass of Carbon=__________
2. Potassium
Isotope
K-39
Atomic Mass
38.96 amu
% Abundance
93.26%
K-40
39.96 amu
0.01%
K-41
40.96 amu
6.73%
Abundance as decimal
Show your work :
Average Atomic Mass of Potassium=___________
For the next two problems, construct the table yourself and solve the problem.
3. Three isotopes of argon occur in nature, Ar-36, Ar-38, Ar-40. Calculate the average atomic mass of
argon given the following:
Ar-36 (35.97 amu; abundance = 0.337 %), Ar-38 (37.96 amu; abundance 0.063 %), and Ar-40 (39.96 amu;
abundance 99.6 %).
4. Rubidium has two common isotopes, Rb-85 and Rb-87. What is the average atomic mass of rubidium
given the following: Rb-85 (84.911 amu, abundance = 72.2%) and Rb-87 (86.91 amu, abundance = 27.83%)
5. Without doing any math, are there more Bromine-79 atoms or more Bromine-80 atoms on earth? (Hint:
look at the periodic table.) How do you know?