Question

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?

Answer 1

The average atomic mass of Rubidium : 85.556 u

Further explanation

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:

$\tt Average~Mass~atom~X=mass~isotope~ 1\times \%1+mass~isotope~2\times \%2+..$

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 :

$\tt average~atomic~mass=85\times 0.722+87\times 0.278\\\\average~atomic~mass=61.37+24.186\\\\average~atomic~mass=\boxed{\bold{85.556~u}}$