An element has three naturally occuring isotopes: 90.48% is element-20 (mass = 19.992 amu), 0.27% is element-21 (mass - 20.994 amu), and the remainder is element-22 (mass = 21.991 amu). Calculate the average atomic mass of the element, rounded to three decimal places.

Average atomic mass  = 20.180 amu.

Explanation:

Given data:

Abundance of X-20 = 90.48%

Atomic mass of X-20 = 19.992 amu

Abundance of X-21 = 0.27%

Atomic mass of X-21 = 20.994 amu

Abundance of X-22 =  9.25%

Atomic mass of X-22 = 21.991 amu

Average atomic mass = ?

Solution:

Average atomic mass  = (abundance of 1st isotope × its atomic mass) +(abundance of 2nd isotope × its atomic mass) +(abundance of 3rd isotope × its atomic mass)   / 100

Average atomic mass = ( 90.48×19.992)+(0.27×20.994)+(9.25+ 21.991 ) /100

Average atomic mass = 1808.88  + 5.668+203.42 / 100

Average atomic mass  = 2017.968 / 100

Average atomic mass  = 20.180 amu.

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