The Vander waal's equation of n mole of a real gas is (P + $\frac{a}{V^2}$)(V - b) = nRT. Where P is the pressure, V is the volume, T is absolute temperature, R is molar gas constant and a, b, c are Van der waal constants. The dimensional formula for ab is:

$ [ML^8T^{−2}]$

Since, the dimensions of $P$ must be same as $\frac{a}{V^2}$​,
Hence, $[ML^{-1}T^{-2}]$ = $\frac{[a]}{[L^6]}$ 

[a] = $[ML^5T^{-2}]$

 dimension of b is same as dimension of v , hence [b] = $[L^3]$

dimension of [ab] = $[ML^8T^{-2}]$