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38 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 4
First, calculate the contribution of each isotope to the average
atomic mass, being sure to convert each percent to a fractional
abundance.
Mass contribution ϭ mass of isotope ϫ abundance of isotope
For Cu-63:
Mass contribution ϭ 62.930 amu ϫ 0.6917 ϭ 43.529 amu
For Cu-65:
Mass contribution = 64.928 amu ϫ 0.3083 ϭ 20.017 amu
The average atomic mass of the element is the sum of the mass contributions of each isotope.
Atomic mass Cu ϭ mass contribution Cu-63
ϩ mass contribution Cu-65
Atomic mass Cu ϭ 43.529 amu ϩ 20.017 amu ϭ 63.546 amu
You can easily confirm this result by checking the periodic table.
Practice Problems
11. Gallium occurs in nature as a mixture of two isotopes. They are
Ga-69 with a 60.108% abundance and a mass of 68.926 amu
and Ga-71 with a 39.892% abundance and an atomic mass of
70.925. Calculate the atomic mass of gallium.
12. The following table shows the five isotopes of germanium
found in nature, the abundance of each isotope, and the atomic
mass of each isotope. Calculate the atomic mass of germanium.
Isotope
Abundance (%)
Atomic Mass (amu)
Germanium-70
21.23
69.924
Germanium-72
27.66
71.922
Germanium-73
7.73
72.923
Germanium-74
35.94
73.921
Germanium-76
7.44
75.921