$\underset{1 \mathrm{volume}}{{\mathrm{H}}_{2\left(\mathrm{g}\right)}}+\underset{1\mathrm{volume}}{{\mathrm{Cl}}_{2\left(\mathrm{g}\right)}}\to \underset{2\mathrm{volume}}{2\mathrm{HCl}\left(\mathrm{g}\right)}$ .

The combining ratio of H₂, Cl₂ and HCl is

From Stoichiometry of equation we can say that 1 mole of hydrogen combines with 1 mole of chlorine to form 2 moles of hydrogen chlorine i.e. 1 vol of hydrogen combines with 1 vol of chlorine to form 2 vol of hydrogen chloride.