Question

The average atomic mass of two isotopes with mass numbers $A$ and $A+2$ is $A+0.25$. Calculate the percentage abundance of the isotopes $A,A+2$.

• A. 85, 15
• B. 87.5, 12.5
• C. 90, 10
• D. 80, 20

Answer 1

The correct option is B 87.5, 12.5

Let the percentage abundance of the isotope with mass number $A+2$ is $x$.
$\therefore \frac{(A+2)x+A(100-x)}{100}=A+0.25$
$\Rightarrow 2x+100A=100A+25\therefore x=12.5$
So, the percentage abundance of the isotopes having mass number $A$ and $A+2$ are $87.5\%$ and $12.5\%$ respectively.