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Atomic Mass Notes & Practice Problems
Average Atomic Mass
Average atomic mass is the ____________________ of the atomic masses of the
_______________ of an element.
Measured in ________________________________ (
).
Atomic number is typically found ___________ an element’s symbol in the periodic
table and is generally not a ___________ number.
Q: What is the average atomic mass of lithium?
A: ________________
Q: What element has an average atomic mass of 32.07 amu?
A: ________________
Calculating Average Atomic Mass
SAMPLE PROBLEM 1
Two isotopes of chlorine occur in nature: chlorine-35 and chlorine-37. Calculate the average
atomic mass if 75% of the naturally occurring chlorine is chlorine-35 and 25% is chlorine-37.
1. Multiply each mass number by its percentage abundance over 100.
2. Add answers from step 1.
3. Interpret answer.
Atomic Mass Notes & Practice Problems
SAMPLE PROBLEM 2
Two isotopes of boron occur in nature: boron-11, and boron-10. Calculate the average atomic
mass if 80% of the naturally occurring boron is boron-11 and 20% is boron-10.
1. Multiply each mass number by its percentage abundance over 100.
2. Add answers from step 1.
3. Interpret answer.
Practice Problems
1. Rubidium has two common isotopes, Rubidium-85 and Rubidium-87. If the abundance
of Rb-85 is 72.2% and the abundance of Rb-87 is 27.8%, what is the average atomic
mass of rubidium?
2. Find the average atomic mass for lithium if 7.5% of lithium atoms are Li-6 92.5% are Li-7.
Atomic Mass Notes & Practice Problems
3. Uranium has three common isotopes. If the abundance of Uranium-234 is 0.01%, the
abundance of Uranium-235 is 0.71%, and the abundance of Uranium-238 is 99.28%,
what is the average atomic mass of uranium?
4. Titanium has five common isotopes: Ti-46 (8.0%), Ti-47 (7.8%), Ti-48 (73.4%), Ti-49
(5.5%), Ti-50 (5.3%). What is the average atomic mass of titanium?
5. Thallium has two stable isotopes, thallium-203 and thallium-205. Thallium-203 has a
relative abundance of 29.52%. Thallium-205 has a relative abundance of 70.48%. What
is the average atomic mass of thallium?
6. Find the average atomic mass for magnesium if 78.99% of Mg atoms are Mg-24, 10.00%
are Mg-25, and 11.01% are Mg-26.
Atomic Mass Notes & Practice Problems
7. There are 2 isotopes of gallium that occur naturally: Ga-69 and Ga-71. The Ga-69 atoms
represent 60% of the naturally occurring isotopes. The remaining atomas are Ga-71.
What is the average atomic mass of gallium?
8. There are 2 isotopes of an unknown element X that occur naturally: X-63 and X-65. The
X-63 isotope has an abundance of 69% and the X-65 isotope has an abundance of 31%.
a. What is the average atomic mass of element X?
b. What is the identity of element X?
9. There are 3 isotopes of an unknown element Y that occur naturally: Y-40, Y-42, and Y44. The Y-40 isotope has an abundance of 96.9%, the Y-42 isotope has an abundance of
1.0% and the Y-44 isotope has an abundance of 2.1%
a. What is the average atomic mass of element Y?
b. What is the identity of element Y?