Name _____________________
Isotopes and Average Atomic Mass #2
1. Neon has two isotopes, Ne-20 has a mass of 19.992 u and Ne-22 which has a mass of 21.991 u. 90% of neon
atoms are isotope Ne-20, while only 10% of neon atoms are isotope Ne-22. Find the average atomic mass for
neon.
2. Find the atomic mass of hafnium, element # 72. 5% of the atoms have mass 176, 19% have mass 177, 27%
of the atoms have mass 178, 14% have mass 179, and 35% have mass 180.0.
3. What is the atomic mass of silicon if 92.21% of its atoms have mass 27.977 u, 4.70% have mass 28.976 u,
and 3.09% have mass 29.974 u?
4. Find the average atomic mass of Silver if 51.83% of the silver atoms occurring in nature have mass 106.905
u and 48.17% of the atoms have mass 108.905 u.
5. Find the average atomic mass of krypton if the relative amounts are as follows:
isotopic mass
percentage
77.920 u
0.350
79.916 u
2.27
81.913 u
11.56
82.914 u
11.55
83.912 u
56.90
85.911 u
17.37
6. The element boron has two naturally occurring isotopes. One has atomic mass 10.0129 u, while the other
has atomic mass 11.0093 u. The average atomic mass of boron is 10.811 u. Determine the percentage
occurrence of each isotope.
7. Copper in the Earth’s crust is present in two isotopes – copper-63 and copper-65. Copper-63 represents
69.1% of the copper and has a mass of 62.93 g/mol. Copper-65 is 30.9% of the copper available and has a mass
of 64.93 g/mol.
a) How many protons, neutrons, and electrons are in each isotope?
Cu-63 – protons ______
neutrons _____
electrons _____
Cu-65 – protons ______
neutrons _____
electrons _____
b) Determine the average molar mass of copper and compare this to the value on the periodic chart.
8. Naturally occurring uranium is a mixture of 3 isotopes – 99.28% is U-238 with a molar mass of 238.08
g/mol, 0.7110% is U-235 (used in power plants and weapons) with a molar mass of 235.04 g/mol, and 0.0054%
is U-234 with a molar mass of 234.04 g/mol.
a) How many protons, neutrons, and electrons are in each isotope?
U-238 – protons ______
neutrons _____
electrons _____
U-235 – protons ______
neutrons _____
electrons _____
U-234 – protons ______
neutrons _____
electrons _____
b) Before doing the calculation, look at the data and predict if the average molar mass of naturally
occurring Uranium closer to 238, 235, or 234. ____________
c) Using the data, calculate the average molar mass of uranium.
9. Lithium exists as a combination of 2 isotopes: Li-6, molar mass of 6.0151 g/mol and Li-7, molar mass of
7.0160 g/mol. Using the average molar mass of lithium from the periodic chart, calculate the percentage
abundance of each isotope.
10. Hydrogen is composed of 3 naturally occurring isotopes: hydrogen-1 (known as protium) – 1.0078 g/mol,
hydrogen-2 (known as deuterium) – 2.0414 g/mol, and hydrogen-3 (known as tritium) – 3.016 g/mol.
Deuterium, though rare, is extracted from seawater as heavy water (D2O) for use in some nuclear reactors.
Tritium is synthetically produced in nuclear reactors and is used in weapons and tracer studies. The percent of
tritium is negligible, so discount it altogether and then calculate the percentage abundance of each of the other
two naturally occurring isotopes of hydrogen, given that the molar mass of hydrogen is 1.0079 g/mol.