Isotopescolor(white)(XX)XXmass(amu)color(white)(X)XIsotopic abundance
Br-79color(white)(XXXX)XXXX78.92color(white)(XXXXXXX)XXXXXXX?
Br-81color(white)(XXXX)XXXX80.92color(white)(XXXXXXX)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%