Question

An element is found in nature in two isotopic forms with mass numbers $(A-1)$ and $(A+3)$. If the average atomic mass of the element is found to be A, then the relative abundance of the heavier isotope in the nature will be

(A) $60 \%$

(B) $75 \%$

(C) $25 \%$

(D) $40 \%$

Answer 1

Let relative abundance of heavier isotope be $x \%$.

$$

\begin{array}{l}

A=\frac{(100-x)(A-1)+x(A+3)}{100} \\

100 A=100 A-x A-100+x+3 x+x A \\

4 x=100 \\

\therefore x=25

\end{array}

$$

Therefore, The correct option will be (C) 25 %