Br consists of Br-79 and Br-81. The atomic mass of Br is 79.9. Find the % of Br-81 in bromine.
Do you know a formula for calculating the atomic mass from percentages of different isotopes?
Atomic mass is equal, in this case, to:
[(Br-79 percentage abundance/100) x 79] + [(Br-81 percentage abundance/100) x 81]
Because you know the atomic mass and not the abundances, let percentage abundance of Br-81 be X. Then you can write an expression for the percentage abundance of Br-79, and so can calculate the value you need.
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