Chemistry help please...?

Question

Lithium has two naturally occuring isotopes. Lithium-6 has an atomic mass of 6.015 amu, and lithium-7 has an atomic mass of 7.016 amu. The average atomic mass of lithium is 6.941 amu. What is the percentage of naturally occuring lithium-7?

How do I work this problem to find the answer.

Answer 1

This essentially turns into an algebra problem. We'll call the percent (in decimal form) of lithium-7 "x". Since there are only 2 isotopes, that means that the percent of lithium-6 has to be "1-x". The problem becomes:

7.016x + 6.015(1-x) = 6.941

7.016x + 6.015 - 6.015x = 6.941

1.001x = .926

x = .9251

92.51% of naturally occurring lithium is lithium-7