Question

If the natural abundance of Br−79 is 50.69 %, what is the natural abundance of Br−81?

Answer 1

the average mass of both isotopes of Br is found on the periodic table, 79.9 amu. That weighted average is the sum of the contributions of both Br-79 and Br-81. Set up an equation that will use the percentages and isotopes to get the weighted average.\[\%^{79}Br *(79amu) + \%^{81}Br*(81amu) = 79.9amu\]all that's missing is the percent of Br-81