Atomic Mass Concepts and Problems

1. Silicon has three major isotopes: Si-28 (27.98 amu), Si-29 (4.70% abundance), and Si-30 (3.09% abundance). What is the missing percentage abundance of Si-28?

90.00% 95.00% 88.00% 92.21%

2. Silver has two isotopes, Ag-1 (106.90509 amu, 51.84% abundance) and Ag-2. The average atomic mass is 107.90 amu. Calculate the atomic mass of Ag-2.

108.9 amu 109.0 amu 106.0 amu 107.0 amu

3. Calculate the average atomic mass of an element with isotopes A (mass 23.985 amu, 78.99%), B (mass 24.985 amu, 10.00%), and C (mass 25.983 amu, 11.01%).

25.00 amu 24.25 amu 24.00 amu 23.50 amu

4. Which isotope is used as the reference point for calculating atomic masses of all other elements?

Nitrogen-14 Hydrogen-1 Oxygen-16 Carbon-12

5. What does the average atomic mass represent?

Sum of protons and neutrons Mass of the heaviest isotope Mass of the lightest isotope Weighted average mass of all naturally occurring isotopes of an element

6. What is the atomic mass (atomic weight)?

The total number of electrons in an atom The number of neutrons in an atom The number of protons in an atom The mass of an individual atom measured in atomic mass units (amu)

7. Atomic mass is measured in which units?

Grams (g) Kilograms (kg) Moles (mol) Atomic mass units (amu)

8. Given the isotopes of Silicon: Si-28 (27.98 amu, 92.21%), Si-29 (unknown mass, 4.70%), and Si-30 (29.97 amu, 3.09%), calculate the atomic mass of Si-29.

28.09 amu 29.00 amu 27.00 amu 30.00 amu

9. Given isotopes C-12 (12.00 amu, 98.9%) and C-13 (13.0034 amu, 1.11%), calculate the average atomic mass of carbon.

13.00 amu 11.99 amu 12.50 amu 12.01 amu

10. Nitrogen has two isotopes, N-14 (14.0031 amu) and N-15 (15.0001 amu). The average atomic mass is 14.0067 amu. Calculate the percentage abundance of the lighter isotope.

100% 0.36% 99.64% 50%

11. The isotopes of lithium, Li-6 and Li-7, have atomic masses 6.0151 amu and 7.0610 amu. The average atomic mass is 6.941 amu. Calculate the natural abundances of these isotopes.

11.4% Li-6 and 88.6% Li-7 50% each 88.6% Li-6 and 11.4% Li-7 100% Li-7