2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/23840A new theory on gas-liquid phase transition is given. The new idea is that the total intermolecular potential energy for a classical system in equilibrium is relative with the average distance of molecules. A new space homogeneity assumption is postulated, a new formulation -- the mean distance expansion, instead of Mayer's Cluster Expansion, is introduced, a new explanation on liquid-gas phase transition is given, and the physical quantities in a system of coexistent vapor and liquid -- the densities of the coexistent vapor and liquid, the vapor pressure and the latent heat, are calculated.17 pages, 6 figuresStatistical Mechanicstext