The **van der Waals equation** (or van der Waals equation of state; named after Johannes Diderik van der Waals) is an equation of state that generalizes the ideal gas law based on plausible reasons that real gases do not act ideally.

\[\left(P+a\dfrac{1}{V_m^2}\right)(V_m-b)=RT\]

where (a) is a constant whose value depends on the gas, (b) is the volume that is occupied by one mole of the molecules, (V_m) is the molar volume of the gas, (R) is the universal gas constant, (T) is temperature, (P) is pressure.

\[a=\dfrac{27R^2T_c^2}{64p_c}\]

\[b=\dfrac{RT_c}{8p_c}\]