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Antimony has 2 naturally occurring isotopes. The mass of antimony-121 is 120.904 amu and the mass of antimony-123 is 122.904 amu. Using the average atomic mass from the periodic table, find the abundance of each isotope. (Remember that the sum of the two abundances must be 100. Show your work.)

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Alex19id
Wow, now this one is a hard one, took me quite a while to figure it out but here it is:
[tex]120.904(x)+122.904(1-x)=121.76 \\ 120.904x-122.904x+122.904=121.76 \\ -2x+122.904=121.76 \\ -2x=-1.144 \\ x=.572 \\ 1-.572=.428[/tex]
The abundances of the two isotopes is:
Antimony-121 = 57.2%
Antimony-123 = 42.8%