Chlorine exists in two isotopic forms \({ }^{35} \mathrm{Cl},{ }^{37} \mathrm{Cl}\). If average atomic mass of chlorine is 35.5 , what is the percentage abundance of these isotopes respectively?

Average atomic mass
atomic mass of \({ }^{35} \mathrm{Cl} \times\) percentage
\(=\frac{+ \text { atomic mass of }{ }^{37} \mathrm{Cl} \times \text { percentage }}{100}\)
Let the \(\%\) abundance of \({ }^{35} \mathrm{Cl}\) isotope \(=x\). \(\%\) abundance of \({ }^{37} \mathrm{Cl}\) isotope \(=100-x\).
Average atomic mass \(=35.5\)
From formula, Average atomic mass
\(\begin{aligned}
& =\frac{35 \times x+37 \times(100-x)}{100}=35.5 \\
& 35 x+3700-37 x=35.5 \times 100 \\
& -2 x=-3700+3550 \\
& 2 x=150 \\
& x=75
\end{aligned}\)
Percentage abundance of
\({ }^{35} \mathrm{Cl}=75 \% \text { and }{ }^{37} \mathrm{Cl}=(100-75)=25 \%\)