The ratio a/b (the terms used in van der Waals' equation) has the unit

Unit of a = atm ${\mathrm{litre}}^2 {\mathrm{mol}}^{-2}$

$=\mathrm{atm} {\mathrm{dm}}^6 {\mathrm{mol}}^{-2} = \mathrm{dyne} {\mathrm{cm}}^4 {\mathrm{mol}}^{-2}$

$\mathrm{Unit} \mathrm{of} \mathrm{b} = \mathrm{litre} {\mathrm{mol}}^{-1} = {\mathrm{dm}}^3 {\mathrm{mol}}^{-1} = {\mathrm{cm}}^3 {\mathrm{mol}}^{-1}$