How to find percent abundance of 2 isotopes of an element?

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3rd February 2010

How to find percent abundance of 2 isotopes of an element?

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I am given two isotopes of the same element, I am given the atomic weight and mass number of each isotope. How would I find the percent of abundance for each isotope?

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1 On February 3rd, 2010, Jeannie Physics Teacher said:
If you know the element, look up the atomic mass on the periodic table. It’s the number with places after the decimal. If there are only two isotopes they have to add up to 100% (or in decimal form 1) so you set it up like this:
(atomic weight of one isotope)(x) + (atomic weight of other isotope)(1-x) = the atomic mass you find on the table.
Use algebra to find x. Don’t forget to use the distributive property. Example: Cl-35 weighs 34.969 amu Cl-37 weighs 36.966 amu
(34.969)(x) + (36.966)(1-x) = 35.453 (on the table for Chlorine)
34.969 x + 36.966 – 36.966 x = 35.453
-1.997 x = -1.513
x = 0.75764 so Cl-35 abundance is 75.764% and Cl-37 is 24.236 %