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Homework Help: Percent Abundance

  1. Mar 1, 2012 #1
    1. The problem statement, all variables and given/known data
    Naturally occurring bromine contains two isotopes; Br-79, which has a weight of 78.9183 and Br-81 with a weight of 80.9163. If the atomic weight of Br is 79.904, determine the % abundance of Br-79 and Br-81.

    2. Relevant equations

    To solve this equation I try to put the equation as

    (x)(78.9183) + (x)(80.9163) = 79.904

    The value of x = .499946789

    3. The attempt at a solution

    The answers were given because this is a practical exam I'm studying for. Can someone point me in the right direction. Something seems to be missing from the equation.

    Br-81 49.33%
    Br-79 50.66%
  2. jcsd
  3. Mar 1, 2012 #2
    Nevermind I found the answer.. Thank you
  4. Mar 2, 2012 #3


    User Avatar

    Staff: Mentor

    For the record:

    is wrong, as abundances of both isotopes are different, not identical - so you need two variables. The correct equation is

    (x1)(78.9183) + (x2)(80.9163) = 79.904

    and you need another equation:

    x1 + x2=1
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