We can illustrate this problem by relating them to their molecular mass. Carbon monoxide, CO has a molar mass of 28 g/moleChlorine, Cl2 (not Chloride) has a molar mass of 70.9 g/mole. It is harder for Chlorine gas to diffuse than Carbon monoxide because it is heavier. Hence, Carbon monoxide diffuses faster because it is lighter. Hope this helps:)