Atomic Mass Concepts and Problems

1. 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 30.00 amu 27.00 amu 28.09 amu

2. 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 12.50 amu 11.99 amu 12.01 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 23.50 amu 24.00 amu 24.25 amu

4. 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.

106.0 amu 108.9 amu 107.0 amu 109.0 amu

5. 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.

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

6. What does the average atomic mass represent?

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

7. Atomic mass is measured in which units?

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

8. 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% 100% 0.36% 99.64%

9. 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% 92.21% 88.00% 95.00%

10. 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

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

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