Lithium has 2 isotopes. The relative atomic masses are 6.02 and 7.02. The average relative atomic mass of lithium is 6.94. What is the calculated relative abundance of the lighter isotope?

Answer 1

Say you have 1 gram of Lithium. If x grams have mass 6.02, then

6.02x + 7.02(1-x) = 6.94
x = .08

so, 8% of Li is the lighter isotope. Makes sense, since 6.94 is so much closer to the heavier value.