Question

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.

Answer 1

Average atomic mass = 20.180 amu.

Step-by-step 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.