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Thread: Isotope question

  1. #1 Isotope question 
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    7) Magnesium consists of three naturally occurring isotopes. The percent abundance of these isotopes is as follows: 24Mg (78.70%), 25Mg (10.13%), and 26Mg (11.7%). The average atomic mass of the three isotopes is 24.3050 amu. If the atomic mass of 25Mg is 24.98584 amu, and 26Mg is 25.98259 amu, calculate the actual atomic mass of 24Mg.
    Correct answer is 23.98504, I keep getting 23.8041. Can anyone see where I am going wrong?

    Isotope. AMU. %abun. AMUx%abun. Adv AMU of Mg 24.3050.

    24mg ? .7870. 25Mg. 24.98584. .1013. 2.531065592. 26Mg. 25.98259. .1170. 3.03996303 ----------------- 5.571028622Adv AMU. 24.3050 -AMUx%. 5.5710 ----------- 18.7339/.7870= 23.8041


    Last edited by Mark h; February 26th, 2013 at 06:08 PM. Reason: Grr cant get table to line up right
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  3. #2  
    Forum Freshman Crimson Sunbird's Avatar
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    Have a look at Wikipedia. I think the abundance of as 10.13% is a little too high.


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  4. #3  
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    Those three percentages add up to >100%.
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