We can illustrate this problem by relating them to their molecular mass.
Carbon monoxide, CO has a molar mass of 28 g/mole
Chlorine, 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:)