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Help with relative atomic mass of rubidium.

I was doing some summer work given by my chem teacher to help prepare us for chemistry class starting in september. I need help with this question:


"Rubidium has only two isotopes in nature: 85Rb and 87Rb. Calculate the percentage abundance of the isotopes of rubidium given that the relative atomic mass (weighted average) is 85.47."


How would you calculate this? 


Thank you. 

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Ok, so basically you use the formula:

85.47 = (isotope mass x %) + (isotope mass x %)  / 100


since we do not know the % value of the isotopes you sub x as the percentage for the first isotope (Rb-85), which means that the second isotope (Rb-87) would have a percentage of 100 - x

therefore: 85.47 = 85 (x) + 87 (100-x)  /100

multiple each side by 100 and simplify.

8547 = 85x + 8700 - 87x

8547 = -2x + 8700

-153 = -2x

x = 76.5

Once you work out what x is equal to you, you've basically found the percentage of the first isotope (Rb-85), and because there are only 2 isotopes, to work out the percentage of Rb-87 you do 100 - x.

so Rb-85 = 76.5%
    Rb-87 = 100-76.5  = 23.5%


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The above procedure is correct, but the percent abundance numbers are incorrect. I was about to post the solution I had written out but they beat me to it. :P


Instead of assuming 85 and 87 as the mass numbers, use the actual masses, which is 84.9117 for 85Rb and 86.9092 for 87Rb. You will get 72.06% for 85Rb and 27.94% for the other isotope, which is correct according to several internet sources.

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