Question

Chlorine exists in two isotopic forms, $Cl-37$ and $Cl-35$ but its atomic mass is $35.5$. This indicates the ratio of $Cl-37$ and $Cl-35$ is approximately

• A. $1:2$
• B. $1:4$
• C. $1:3$
• D. $3:1$

Answer 1

Answer: (C)

Let relative abundance of $Cl-37=x\%$
then relative abundance of $Cl-35=(100-x)\%$
Average atomic mass $=\frac{x\times 37+\left(100-x\right)35}{100}$
$=35.5$
$\Rightarrow 37x+3500-35x$
$=3550\Rightarrow x=25$
$\therefore 100-x=75$
Thus, the ratio of $Cl-37$ and $Cl-35$ is $x$ : $(100-x)$
$=25:75$
$1:3$