Question

Average Atomic Mass Worksheet: show all work.1) Rubidium is a soft, silvery-white metal that has two common isotopes, 85Rb and 87Rb. Ifthe abundance of 85Rb is 72.2% and the abundance of 87Rb is 27.8° o, what is the averageatomic mass of rubidium?

Answer 1

The average atomic mass of Rb = 85.6 amu

Step-by-step explanation

Abundance of ⁸⁵Rb = 72.2% = 0.722

Atomic mass of ⁸⁵Rb = 85

Abundance of ⁸⁷Rb = 27.8% = 0.278

Atomic mass of ⁸⁷Rb = 87

Average atomic mass = ?

First, the exact atomic mass of each isotope must be obtained. Then multiply the percent abundance as a decimal for both isotopes and add them together i.e

Average atomic mass of Rb = (abundance of ⁸⁵Rb × its atomic mass) +(abundance of ⁸⁷Rb × its atomic mass) / 100

`\begin{gathered} \text{Average atomic mass of Rb }=((72.2*85)+27.8*87)/(100) \\ =(6137+2418.6)/(100)=(8555.6)/(100)=85.556\text{ amu} \\ \text{Average atomic mass of Rb }\approx85.6\text{ amu} \end{gathered}`

The average atomic mass of rubidium is 85.6 amu