How would you find the percent abundance of 2 isotopes of an element, given the atomic weight and mass number of each isotope?

See explanation.

The average of is calculated by:

##Mass_(avrg.)=sum("isotope mass")*("percent abundance")##

For example, suppose we want to find the percent abundance of chlorine ##""^35Cl## and ##""^37Cl## given that the average atomic mass of chlorine is ##35.45 "a.m.u.##.

Assume that the percent abundance of ##""^35Cl## is ##x##, therefore, the percent abundance of ##""^37Cl## is ##1-x##.

Thus, ##35.45=(35xxx)+(37xx(1-x)) => 35.45 = 37 -2x##.

Solve for ##x##, ##x=0.775##

Therefore, the percent abundance of ##""^35Cl## is ##77.5%## and the percent abundance of ##""^37Cl## is ##22.5%##