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?

The average atomic mass of Rubidium : 85.556 u

Isotopes are atoms that have the same atomic number but different mass numbers

Each isotope has an abundance which is usually expressed as a percentage

The average relative mass of atoms is obtained by adding the product of the percentage and mass of each isotope

Can be formulated:

The abundance of 85 Rb is 72.2% and the abundance of 87 Rb is 27.8%, so

mass isotope 1=85, %abundance=72.2

mass isotope 2=87, %abundance=27.8

The averages atomic mass of Rubidium :

the answer is definitely c.