(P + $an^2$/$V^2$)(V - nb) = nRT. For 1 mole: (P + a/$V^2$)(V - b) = RT. 'a' = intermolecular attraction correction (units: $L^2$.atm/$mol^2$). Higher a = stronger attraction = easier liquefaction = larger deviation from ideality at moderate pressures. 'b' = excluded volume correction (units: L/mol). b = 4 x actual molecular volume per mole. Higher b = larger molecules. Compressibility factor Z = $\frac{PV}{nRT}$. Ideal gas: Z = 1 at all conditions. Real gas: Z < 1 at moderate P (attraction dominates, gas more compressible). Z > 1 at very high P (volume exclusion dominates, gas less compressible). Z = 1 at Boyle temperature $T_B$ = $\frac{a}{Rb}$.