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1. Average Atomic Mass
What is average atomic mass?
Average atomic mass is the
weighted average of the atomic
masses of the naturally occurring
isotopes of an element

2. Calculating a Weighted Average
Example
A box contains two size of marbles. If
25.0% have masses of 2.00 g and 75.0%
have masses of 3.00 g what is the weighted
average?
(.250) (2.00) + (.750) (3.00) = .500 + 2.25 =
2.75g

3. Calculating Average Atomic Mass
EXAMPLE
Boron has two isotopes:
B-10 (mass 10.013 amu) 19.8% abundance
B-11 (mass 11.009 amu) 80.2% abundance
Calculate the average atomic mass.
(.198) (10.013) + (.802) ( 11.009) =
1.98 amu + 8.83 amu = 10.81 amu

4. Calculating Average Atomic Mass
Calculate the average atomic mass of Mg.
Isotope 1 - 23.985 amu (78.99%)
Isotope 2 - 24.986 amu (10.00%)
Isotope 3 – 25.982 amu (11.01%)
(23.985)(.7899)+(24.986)(.1000)+(25.982)(.1101)
18.95 amu + 2.499 amu + 2.861 amu = 24.31 amu

5. Average Atomic Mass
Helium has two naturally occurring isotopes,
He-3 and He-4. The atomic mass of
helium is 4.003 amu. Which isotope is
more abundant in nature?
He-4 is more abundant in nature because
the atomic mass is closer to the mass of
He-4 than to the mass of He-3.

6. Isotopic Pennies – number of pre and post 1982
a. Let X be the number of pre-1982 pennies
b. Let 10-X be the number of post-1982 pennies
c. (X)(3.1g) + (10-X)(2.5g) = mass of 10 pennies
pre-82 post-82
EXAMPLE (Mass of a sample of pennies is 31.0g)
(X)(3.1g) + [(10-X)(2.5g)] = 31.0 g
3.1X + 25 - 2.5X = 31.0g
.6X + 25 = 31.0g
.6X = 6.0g X = 6.0g/.6
X = 10 pre-82 pennies
10-X = 0 pre-82 pennies

7. Isotopic Penny Lab- Average Atomic Mass
Calculate percent of pre-82 and post-82 pennies
# of pre-82 pennies x 100% # post-82 pennies x 100%
10 10
Calculate the average atomic mass of coinium
(% pre-82)(3.1g) + (% post-82)(2.5) = average atomic mass