Question

The relative abundance of two rubidium isotopes of atomic weight 85 and 87 are 75% and 25% respectively. The average atomic weight of rubidium is ?
a)75.5 b)85.5
c)87.5 d)86.5

Answer 1

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.
here 85 and 87 and relative abundance 0.75 and 0.25 resectively
so we multiply and add together
(85*0.75)+(87*0.25)=85.5

option B is correct