Natural chlorine has 2 isotopes ${}_{17}{}^{35}\mathrm{Cl} \mathrm{and} {}_{17}{}^{37}\mathrm{Cl}$ in the ratio of 1:3 calculate the atomic mass of natural chlorine.

Use the following formula to find atomic mass of an isotope of Cl atom.

Average atomic mass of Cl=$\frac{\left(35\times 3\right)+\left(37\times 1\right)}{3+1}=\frac{142}{4}=35.5$