Afictional element has two isotopes, each making up 50% of the population. isotope 1 has a mass of 60.0 amu, isotope 2 has a mass of 65.0 amu. calculate the atomic mass of the fictional element.

Atomic mass of the element is the weighted average mass of the element with respect to the abundance of all isotopes
atomic mass is the sum of the products of the atomic mass of isotope by the abundance of the isotope 
atomic mass = 60.0 amu x 50% + 65.0 amu x 50%
                     = 30.0 + 32.5 
                     = 62.5 amu
therefore atomic mass of the element is 62.5 amu

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