Question

Rubidium is a soft, silvery-white metal that has two common isotopes, 85Rb and 87Rb with masses of 84.904 and 87.099 respectively. If the abundance of 85Rb is 72.2% and the abundance of 87Rb is 27.8%, what is the average atomic mass of Rubidium? Submit your answer with three decimal places.

Answer 1

To find the average atomic mass of an element, you use the formula:

AAM = %X1*m1 + %X2*m2 + ... + %Xn*mn

With %X the abundance of an atom/isotope, m the mass of the atom/isotope

(n being the number of isotopes you need to consider)

So this formula gives us:

AAM = 72.2% * 84.904 + 27.8% * 87.099
= 85.514