If 75.0 of the isotopes of an element have a mass of 39.0 and 25.0% of the isotopes have a mass of 36.0 amu, what is the atomic mass of the element?

Atomic mass of element = 38.25 amu

Explanation:

Let X-39 abundance = 75% and X-36 abundance = 25%

Atomic Mass of element = ∑ wt average contributions of each isotope ...

Wt Avg Contribution = fractional abundance x isotopic mass.    

Isotope      Isotopic Mass (amu)     Fractional Abundance     Wt Avg Contbn

 X-39                   39.00                           0.75                              29.25 amu

 X-36                   36.00                           0.25                                9.00 amu

∑ Wt Avg Contributions = 29.25 amu + 9.00 amu = 38.25 amu

=> Atomic mass of element = 38.25 amu

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