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?

1 Answer
Sep 8, 2016

Answer:

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

#"Rb-85=84.911789 u"#
#"Rb-87=86.909183 u"#

Average Atomic Mass of Rubidium

#(0.722xx84.911789) + (0.278xx86.909183)="85.467 u"#