<Dd> (P + a n 2 / V 2) (V − n b) = n R T (\ displaystyle (P + an ^ (2) / V ^ (2)) (V - nb) = nRT) </Dd> <P> where V is the molar volume of the gas, R is the universal gas constant, T is temperature, P is pressure, and V is volume . When the molar volume V is large, b becomes negligible in comparison with V, a / V becomes negligible with respect to P, and the van der Waals equation reduces to the ideal gas law, PV = RT . </P> <P> It is available via its traditional derivation (a mechanical equation of state), or via a derivation based in statistical thermodynamics, the latter of which provides the partition function of the system and allows thermodynamic functions to be specified . It successfully approximates the behavior of real fluids above their critical temperatures and is qualitatively reasonable for their liquid and low - pressure gaseous states at low temperatures . However, near the transitions between gas and liquid, in the range of p, V, and T where the liquid phase and the gas phase are in equilibrium, the van der Waals equation fails to accurately model observed experimental behaviour, in particular that p is a constant function of V at given temperatures . As such, the van der Waals model is not useful only for calculations intended to predict real behavior in regions near the critical point . Empirical corrections to address these predictive deficiencies have been inserted into the van der Waals model, e.g., by James Clerk Maxwell in his equal area rule, and related but distinct theoretical models, e.g., based on the principle of corresponding states, have been developed to achieve better fits to real fluid behaviour in equations of more comparable complexity . </P> <Table> <Tr> <Td> </Td> <Td> This section needs expansion with: a proper lay explanation of the equation and the history and context of its discovery . You can help by adding to it . (June 2015) </Td> </Tr> </Table>

Van der waals constants a and b meaning