Average Atomic Mass Practice Problems
1. What is the atomic mass of hafnium if, out of every 100 atoms, 5 have a mass of
176, 19 have a mass of 177, 27 have a mass of 178, 14 have a mass of 179, and 35
have a mass of 180.0?
2. Calculate the average atomic mass of lithium, which occurs as two isotopes that
have the following atomic masses and abundances in nature: 6.017 u, 7.30% and
7.018 u, 92.70%.
3. Hydrogen is 99% 1H, 0.8% 2H, and 0.2% 3H. Calculate its average atomic mass.
4. Calculate the average atomic mass of magnesium using the following data for three
magnesium isotopes.
Isotope mass (u) relative abundance
Mg-24 23.985 0.7870
Mg-25 24.986 0.1013
Mg-26 25.983 0.1117
5. Lithium has two naturally occurring isotopes: lithium-6 and lithium-7. If the average
atomic mass of lithium is6.941 amu, which isotope is the most abundant? How do
you know?
6. Calculate the average atomic mass of chromium. (not in percents)
Isotope
Mass (amu)
Relative Abundance
Chromium – 50
49.946
0.043500
Chromium – 52
51.941
0.83800
Chromium – 53
52.941
0.095000
Chromium – 54
53.939
0.023500
7. The average atomic mass of copper is 63.55 amu. If the only two isotopes of copper
have masses of 62.94 amu and 64.93 amu, what are the percentages of each?
(Think algebra)
8. Rubidium is a soft, silvery-white metal that 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?
9. Uranium is used in nuclear reactors and is a rare element on earth. Uranium has
three common isotopes. If the abundance of 234U is 0.01%, the abundance of 235U is
0.71%, and the abundance of 238U is 99.28%, what is the average atomic mass of
uranium?
10. Titanium has five common isotopes: 46Ti (8.0%), 47Ti (7.8%), 48Ti (73.4%),
49Ti (5.5%), 50Ti (5.3%). What is the average atomic mass of titanium?
11. Calculate the percentages of each isotope present in a mixture of:
a) 113In and 115In has an average mass of 114.8 u.
b) Naturally occurring silicon consists of three isotopes, 28Si, 29Si, and 30Si, whose
atomic masses are 27.9769, 28.9865, and 29.9838, respectively. The most
abundant isotope is 28Si, which accounts for 92.23% of naturally occurring silicon.
Given that the observed atomic mass of silicon is 28.0855, calculate the
percentages of 29Si and 30Si in nature.
