Expressions for the theoretical reversible useful power, as well as the associated theoretical reversible environmental thermal impact power, when processing a steady flow rate of a real pure liquid B at environmental conditions of temperature and pressure, from a real mixture of a gas A and a vapour B at environmental temperature, pressure and vapour molar fraction,, are given. These expressions can be evaluated based only on the knowledge of the gaseous mixture thermal equation of state, pure liquid thermal equation of state, and pure liquid vapour pressure. The obtained results do not depend on any simplifying assumptions regarding the thermodynamic behaviour of substances, and can be generalized for several gases, vapours, liquids, or solids. The general expressions are simplified for the case of an ideal mixture of dry air and water vapour, and incompressible liquid water, with the environment as the only heat source.