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Homework Help: Percent Abundance

  1. Mar 1, 2005 #1
    Br consists of Br-79 and Br-81. The atomic mass of Br is 79.9. Find the % of Br-81 in bromine.
  2. jcsd
  3. Mar 1, 2005 #2
    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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