Isotopes#color(white)(XX)#mass(amu)#color(white)(X)#Isotopic abundance
Br-79#color(white)(XXXX)#78.92#color(white)(XXXXXXX)#?
Br-81#color(white)(XXXX)#80.92#color(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%
