Question 10
If bromine atom is available in the form of, say, two isotopes ${}_{35}^{79}$Br (49.7%) and ${}_{35}^{81}$Br(50.3%), calculate the average atomic mass of bromine atom.

Isotope of bromine with atomic mass 79 u = 49.7%
Therefore, Contribution of ${}_{35}^{79}$Br to atomic mass $=\frac{79\times 49.7}{100}=39.26u$
Isotope of bromine with atomic mass 81 u = 50.3%
Contribution of ${}_{35}^{81}$Br to the atomic mass of bromine $=\frac{81\times 50.3}{100}=40.74u$
Hence atomic mass of the bromine atom $=39.26+40.74u=80u$