# Rubidium has two common isotopes, ""^85"Rb" and ""^87"Rb". If the abundance of ""^85"Rb" is 72.2% and the abundance of ""^87"Rb" is 27.8%, what is the average atomic mass of rubidium?

Sep 8, 2016

The average atomic mass of rubidium is 85.467 u.

#### Explanation:

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.

The exact atomic masses of the rubidium-85 and rubidium-86 isotopes.
https://www.ncsu.edu/chemistry/msf/pdf/IsotopicMass_NaturalAbundance.pdf

$\text{Rb-85=84.911789 u}$
$\text{Rb-87=86.909183 u}$

Average Atomic Mass of Rubidium

$\left(0.722 \times 84.911789\right) + \left(0.278 \times 86.909183\right) = \text{85.467 u}$