Three Isotopes Of Argon Occur In Nature

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Three Isotopes Of Argon Occur In Nature

- 36/18 Ar, 38/18 Ar, and 40/18 Ar. Calculate the average atomic mass of argon to two decimal places, given the following relative masses and abundances of each of the isotopes: argon-36 (35.97 amu; 0.337%), argon-38 (37.96 amu; 0.063%), and argon-40 (39.96 amu; 99.600%).
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Ar-40 is 99.6% of Ar and it weighs 39.96 amu and so on. So the atomic mass of Ar is 0.99639.96 + 0.0006337.96 + 0.00337*35.97 = 39.957. I divided the percentages by 100 to get fractions and then each mass contributes to the atomic mass according to its fraction -- weighted average. As almost all Ar is Ar-40, the others contribute very little so the atomic mass of Ar is 39.957 -- or to two decimal places 39.96 amu

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