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Average Atomic Mass (Grade 10)

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Average Atomic Mass

Instructions: The following questions require the use of a calculator.

1. 
Which statement correctly describes the average atomic mass of an element?
  1. The average of the two most abundant isotopes of an element
  2. The average of all of the naturally-occurring atomic mass values of isotopes
  3. The atomic mass of the most abundant isotope of an element
  4. The atomic mass of the least abundant isotope of an element
2. 
The isotopes of nitrogen have masses of 14 amu and 15 amu. The average atomic mass of nitrogen is 14.01 amu. Based on this information, the most abundant isotope of nitrogen has a mass of
  1. 15 amu
  2. 16 amu
  3. 14 amu
  4. 13 amu
3. 
Imagine that a new element has been discovered, which has the chemical symbol Rx. The isotope Rx-298 has a mass of 298 amu and an abundance of 75%. The isotope Rx-299 has a mass of 299 amu and an abundance of 25%. Which of the following values is closest to the average atomic mass of this element, Rx?
  1. 298 amu
  2. 299 amu
  3. 300 amu
  4. 301 amu
4. 
Which combination of abundances and atomic masses would lead to sulfur's average atomic mass of 32.065 amu?
  1. 90% S-32 and 10% S-33
  2. 93% S-32 and 7% S-33
  3. 75% S-32 and 25% S-33
  4. 85% S-32 and 15% S-33
5. 
Chlorine has two common isotopes, Cl-35 and Cl-37, and an average atomic mass of 35.5 amu. Which of the following isotopes of chlorine is most abundant, based on the average atomic mass?
  1. Cl-35
  2. Cl-37
  3. Both have equal abundances
  4. Neither of the isotopes has a greater abundance
6. 
An element has an atomic number of 26 and has isotopes with mass numbers of 56 and 54. Its average atomic mass is 55.845 amu. Based on the average atomic mass, which of the following isotopes is most abundant?
  1. The isotope with a mass number of 54
  2. The isotope with a mass number of 56
  3. Both isotopes have equal abundances
  4. An isotope with a mass number other than 56 or 54
7. 
An element has an atomic mass of 355.5 amu. Its isotopes have mass numbers of 355 and 356. Which statement is true about the isotopes of the element?
  1. The isotope with a mass number of 355 is more abundant
  2. The isotope with a mass number of 356 is more abundant
  3. The isotopes have an equivalent abundance
  4. An isotope with a mass number of 357 is more abundant
8. 
Mg has three different isotopes. Mg-24 has an abundance of 78.70%, Mg-25 has an abundance of 10.13%, and Mg-26 has an abundance of 11.17%. Which setup is correct for calculating the atomic mass of magnesium?
  1. (78.70)(24 u)+(10.13)(25 u)+(11.17)(26 u)
  2. (78.70)(24 u)+(11.17)(25 u)+(10.13)(26 u)
  3. (0.7870)(24 u)+(0.1013)(25 u)+(0.1117)(26 u)
  4. (0.7870)(24 u)+(0.1117)(25 u)+(0.1013)(26 u)
9. 
There are three isotopes of silicon. Si-28 has an atomic mass of 27.98 amu and an abundance of 92.21%, Si-29 has an atomic mass of 28.98 amu and an abundance of 4.70%, and Si-30 has an atomic mass of 29.97 amu and an abundance of 3.09%. Calculate the average atomic mass of silicon.



10. 
When calculating the average atomic mass of an element, the average atomic mass value falls closest to that of the least abundant isotope.
  1. True
  2. False

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