The composition of the equilibrium mixture $\left ( Cl_{2}\rightleftharpoons 2Cl \right )$ which is attained at $1200^{\circ}C$ is determined by measuring the rate of effusion through a pin hole. It is observed that the given mixture effuses 1.5 times as fast as an unknown gas 'X' effuses under the same conditions. Calculate the fraction of chlorine molecules dissociated into atoms.[Molecular Mass of unknown gas=90]