Home/Class 12/Physics/

## QuestionPhysicsClass 12

Boron has two stable isotopes,$$105Br$$ and $$115Br.$$ Their respective masses are $$10.01294u$$  and $$11.00931u$$, and the atomic mass of boron is 10.811 u. Find the abundances of them.

The mass of isotope $$105\text{Br},m=10.01294u$$
The mass of isotope $$115\text{Br},m^\prime =11.00931u$$
Let us suppose first abundance $$=x$$
Abundance of the second isotope $$=100-x$$
It is given in the question that mass of boron $$=10.811u$$
$$10.811=\frac{10.01294x+11.00931\left(100-x\right)}{x+100-x}$$
$$x=19.89\%$$
abundance of second isotope $$=80.11\%$$