Question

Chlorine has two naturally occurring isotopes, 35Cl with an atomic mass of 34.969 amu and 37Cl with an atomic mass of 36.966 amu. The weighted average atomic mass of chlorine is 35.45 amu. What is the percent abundance of each isotope? Be sure your work supports your answer.

Copper has two naturally occurring isotopes, 63Cu with an atomic mass of 62.9296 amu and 65Cu with an atomic mass of 64.9278 amu. The weighted average atomic mass of copper is 63.546 amu. What is the percent abundance of each isotope? Be sure your work supports your answer.

Carbon has two naturally occurring isotopes, 12C with an atomic mass of 12.00000 amu and 13C with an atomic mass of 13.00335 amu. The weighted average atomic mass of carbon is 12.01 amu. What is the percent abundance of each isotope? Be sure your work supports your answer.

Answer 1

1) Let the percent abundance of 35Cl be x

then the percent abundance of 37Cl be (100-x)

34.969x + (36.966)(100-x) = 35.45(100)

34.969x + 3696.6 - 36.966x = 3545

-1.997x = 151.6

x = 75.91

Percent abundance of 35Cl = 75.91%, abundance of 37Cl = 24.09%

2) Let the percent abundance of 63Cu be x

then the percent abundance of 65Cu be (100-x)

62.9296x + (64.9278)(100-x) = 63.546(100)

62.9296x + 6492.78 - 64.9278x = 6354.6

-1.9982x = 138.18

x = 69.15

Percent abundance of 63Cu = 69.15%, abundance of 65Cu = 30.85%

3) Let the percent abundance of 12C be x

then the percent abundance of 13C be (100-x)

12.0000x + (13.00335)(100-x) = 12.01(100)

12.00000x + 1300.35 - 13.00335x = 1201

-1.00335x = 99.35

x = 99.01

Percent abundance of 12C = 99.01%, abundance of 13C = 0.99%