A sample of gallium has an atomic weight of approximately 69.8

Question

If the sample consists of two isotopes of masses 69.0 and 71.0 respectively, what is the approximate percentage of the lighter isotope in the sample?

A) 80%

B 60%

C 40%

D 20%

Answer 1

Let X = the percent of the lighter isotope.

Because the two isotopes together must add to 100%, we can write the equation:

69.0 * X% + 71.0 * (100% - X) = 69.8 * 100%

X = 60%

As a reality check, the average mass (69.8) should be closer to the isotope which there is more of.