Lithium has only two naturally occurring isotopes. the mass of li-6 is 6.015 amu and the mass of li-7 is 7.016 amu. the average atomic mass of lithium is 6.941. (a) calculate the relative abundances of the two isotopes. (b) what fraction of a sample of lithium atoms actually has a mass of 6.941 amu

The answer to your question is :

a) Abundance of Li-6 = 7.49

b) Abundance of Li-7 = 100-7.49 = 92.5

Explanation:

Data           Isotopes                       Mass               Abundance

                     Li-6                           6.015 amu                 y

                     Li-7                            7.016 amu            100 -y

      Lithium atomic mass = 6.941

6.941 = (6.015y) + (7.016)(100-y) /100

6.941 = (6.015y  + 701.6 - 7.016y) / 100

6.941 x 100 = 6.015y - 7.016y + 701.6

694.1 -701.6 = - 1.001y

-7.5 = -1.001 y

y = 7.49

Abundance of Li-6 = 7.49

Abundance of Li-7 = 100-7.49 = 92.5