Bromine consists of two isotopes with masses of 78.92 and 80.92 amu. What is the abundances of these two isotopes?

1 Answer
Mar 24, 2018

Isotopescolor(white)(XX)mass(amu)color(white)(X)Isotopic abundance

Br-79color(white)(XXXX)78.92color(white)(XXXXXXX)?

Br-81color(white)(XXXX)80.92color(white)(XXXXXXX)?

"average atomic mass"=("mass*%"+"mass*%"+"mass*%"...)

Let x be % abundance of Br-79

Let 1-x be % abundance of Br-81

Br is 79. 904g/mol on the periodic table color(orange)"(average atomic mass)"

79.904%=x+(1-x)

79.904%=(78.92*x)+(80.92*(1-x))

79.904%=(78.92x)+(80.92-80.92x))

79.904%=78.92x+80.92-80.92x

79.904%=-2x+80.92

79.904%-80.92=78.92x

-1.016=-2x

x=0.508

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

x*100%

0.508*100%

=50.8 "% abundance"

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

(1-x)*100%

0.492*100%

=49.2 "% abundance"

:. The % abundance of the isotope with a mass of 78.92 amu has a % abundance of 50.8% and the isotope with a mass 80.92 amu has a % abundance of 49.2%