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Homework Help: Bromine isotopes

  1. Feb 10, 2013 #1
    There are two different isotopes of bromine atoms. Under normal conditions, elemental bromine consists of molecules, and the mass of a molecule is the sum of the masses of the two atoms in the molecule. The mass spectrum of consists of three peaks:

    Mass Relative Size
    157.836 0.2569
    159.834 0.4999
    161.832 0.2431

    a)What is the isotopic composition of each peak?
    b)What is the atomic mass of each isotope?
    c)Determine the average mass of Br2 molecule.
    d)Determine the average atomic mass of a bromine atom.
    e)What is the percent abundance of the two isotopes?

    here is some of my work, im not sure if this correct, can someone help me get through the questions with explantons on how to do them.
    (157.836 x 0.2569 + 159.834 x 0.4999 + 161.832 x 0.2431)/2 <- i think this gives me a mass
  2. jcsd
  3. Feb 11, 2013 #2


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    Staff: Mentor

    Yes, that would be a molar mass of the Br2.

    Each molecule is made of two atoms. There are two kinds of atoms - a lighter one, and a heavier one. What combinations of these atoms are possible? Can you assign these combinations to the peaks?
  4. Feb 11, 2013 #3


    User Avatar
    Gold Member

    That's a standard procedure of finding the weighted average molecular mass depending on the abundance of each isotope. Its just like finding the centre of mass for a system.
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