How to Find
Average Atomic Mass
Copy when you see…
Do you know what
a weighted average is?
• Your grades are calculated by weighted average
– 40% minor
– 60% major
• When you want to calculate your class average
this is how you do it!
For example:
• Quiz #1 = 67%
• Quiz #2 = 75%
• Quiz #3 = 83%
Quiz Average = 75%
Exam Average = 79%
• Exam #1 = 78%
• Exam #2 = 80%
(% weight x grade)
Then, add them up!
0.4 x 75 = 30
0.6 x 79 = 47.4
77.4 = 77%
Your class average in chemistry!
Since isotopes are atoms of the same
element with different masses:
• We use a weighted average to calculate the
average atomic mass because some isotopes
are more common than others.
• We use this formula:
% abundance x mass (for each isotope)
THEN add them all together!
REMEMBER, change the % to a decimal!!
Example:
• Chlorine-35 (35Cl) makes up 76% of chlorine
atoms and chlorine-37 (37Cl) makes up 24%.
What is the average atomic mass for chlorine?
(% abundance x mass)
0.76 x 35 = 26.6
Then, add them up!
0.24 x 37 = 8.88
← This is the average
35.48 amu
atomic mass for
chlorine!