The average molar mass of chlorine is $35.5\mathrm{g}{\mathrm{mol}}^{-1}$. The ratio of${ }^{35}\mathrm{CI} {\text{to }}^{37}\mathrm{Cl}$  in naturally occurring chlorine is close to 

$\begin{array}{l}{\text{Atomic weight of}}^{\text{35}}\text{Cl=34.9689}\\ {\text{Atomic weight of}}^{\text{37}}\text{Cl=36.9695}\\ \text{Atomic mass of\hspace{0.17em}Cl=35.45}\\ \text{Average atomic mass =}\frac{\text{34.9689(x)+36.9695(100-x)}}{\text{100}}\text{=35.45}\\ \text{x=76\%}\\ \text{Abundance ofCl-35= 76\%}\\ \text{Abundance ofCl-37= 24\%}\\ \text{Hence, the ratio of their natural abundance is approximately equal to3:1.}\end{array}$