Abstract: It is well-known that van der Waals equation of state goes over to that for the ideal gas in the limit of large volume, but whether the first derivative of it also works is a problem without explicitly explored hitherto. The exploration shows that the further requirement on the real gas as it in the limit of large volume can reproduce the ideal gas in sense of the first derivative is inappropriate unless asymptotically.

Key words: ideal gas, real gas, van der Waals gas