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This exercise will lead you to verify that the average atomic mass of magnesium is 24.31 amu, based on the following information:
isotope mass (amu) percent abundance
24Mg 23.985042 78.99%
25Mg 24.985837 10.00%
26Mg 25.982593 11.01%
The average atomic mass is the weighted average of the atomic masses of all isotopes.
What is the fractional abundance of 24Mg?

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rspill6

Answer: 24.305052 (8 sig figs)  [Mg]

Explanation:  The AMU of each isotope is multiplied by it's percent abundance to yield a weighted average for each isotope.  The sum of these weighted averages will be the wighted average of the element's isotopes:

Isotope    Mass (AMU)     %            Weighted AMU

24Mg 23.985042 78.99% 18.94578468

25Mg 24.985837 10.00%   2.4985837

26Mg 25.982593 11.01%   2.860683489

                                   24.305052 (8 sig figs)

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