Question

The relative atomic mass of bromine which consists of Br 79 35 and Br 85 35 isotopes is 79.9 amu.

Calculate the percent abundance of Br 79 35 and Br 85 35 in the mixture of isotopes?

Answer 1

Br-79        78.92?

Br-81         80.92?

average atomic mass=(mass% +mass% +mass%)

Let y be % abundance of Br-79

Let 1-y be % abundance of Br-81

Br is 79. 904g/mol on the periodic table 

79.904% =y+(1−y)

79.904% = (78.92×y) + (80.92×(1−y))

79.904% =(78.92y) + (80.92−80.92y))

79.904% =78.92y + 80.92−80.92y

79.904%=−2y+80.92

79.904%−80.92=78.92y

−1.016=−2y

y=0.508

Use our y to find % abundance of Br-79

y×100%

0.508⋅100%

=50.8 % abundance

Use our y to find that % abundance of Br-79

(1−x)⋅100%

0.492⋅100%

=49.2 %  abundance