Boron occurs in nature in the form of two isotopes having mass 10 and 11. Calculate the percentage abundances of B11 isotopes in a sample of boron having average atomic mass 10.8.

Isotopes of boron =10 B and  11 B. Let, total number of moles of =10 B and  11B=1Let fractional abundance of  10B=α;Fractional abundance of  11B=1−αBut, average atomic mass = [Fractional abundance of 10B× mass of 10B+[Fractional abundance of  11B× mass of  11B.10.8=(α×10)+(1−α)×11=10α+11−11αor, α=11−10.8;α=0.2 Hence 1−α=1−0.2=0.8∴ % age abundance of  10B                                  = 0.2 X 100 = 20%    % age abundance of  11B                                  = 0.8 X 100 = 80%.