Atomic Mass Concepts and Problems

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

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

2. 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?

95.00% 88.00% 90.00% 92.21%

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%).

24.25 amu 24.00 amu 23.50 amu 25.00 amu

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

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

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

12.50 amu 13.00 amu 12.01 amu 11.99 amu

6. Atomic mass is measured in which units?

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

7. 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 100% Li-7 88.6% Li-6 and 11.4% Li-7

8. What does the average atomic mass represent?

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

9. 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.

107.0 amu 108.9 amu 109.0 amu 106.0 amu

10. 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.

29.00 amu 28.09 amu 30.00 amu 27.00 amu

11. 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.

50% 99.64% 100% 0.36%