The behavior of gases is usually described by the ideal gas law pV = NkT. While such a description often gives a sufficiently good account of how gases behave, higher-order approximations are sometimes needed. For example, phase transitions cannot be explained by the ideal gas law. Such phenomena, however, are described by the higher-order approximation Van-der-Waals equation of state. In this paper, we investigate the effect of a weak 1/r potential on gases composed of monatomic molecules that do not interact with each other. 1/r potentials are particularly interesting because the two most dominant interactions in the macroscopic level, gravitational and electrical, are described by such a potential. We evaluate the classical partition function of the system, from which we obtain the equation of state.