Rubidium is a soft silvery-white metal that has two common isotopes, 85rb and 87rb. if the abundance of 85rb is 72.2% and the abundance of 87rb is 27.8%, what is the average atomic mass of ribidium?

Average atomic mass of Rb = 85.6 amu.

Explanation:

Abundance of Rb⁸⁵ = 72.2%

Abundance of Rb⁸⁷ = 27.8%

Average atomic mass = ?

Solution:

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

Average atomic mass of Rb = (85×72.2)+(87×27.8) /100

Average atomic mass of Rb =  6137 + 2418.6 / 100

Average atomic mass of Rb = 8555.6 / 100

Average atomic mass of Rb = 85.6 amu.