Acertain element (x) has isotopes with mass numbers of 10 and 11. the x-10 isotope has an atomic mass of 10.013, and it represents 19.9% of all atoms of this element. the x-11 isotope has an atomic mass of 11.009, and it represents 80.1% of all atoms of this element. therefore, the average atomic mass, or atomic weight, of this element is amu.

The average atomic mass of element X is 10.81 u.

The average atomic mass of X is the weighted average of the atomic masses of its isotopes.

We multiply the atomic mass of each isotope by a number representing its relative importance (i.e., its % abundance).

Thus,  

0.199 × 10.013 u =   1.993 u

0.801 × 11.009 u =  8.818 u

             TOTAL =  10.81  u

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