One of the most important thermodynamic properties of aqueous solutions is water activity which is an important and key thermodynamic property, because, it is closely related with the other thermodynamic properties such as vapor pressure, osmotic coefficient, activity coefficient, excess enthalpy, excess entropy, excess Gibbs energy and excess volume. It should, therefore, be useful to have information about vapor-liquid equilibria of the aqueous solutions in order to understand better and possibly predict the solution behavior. Vapor-liquid equilibria of electrolyte in water has been studied extensively [1], while the corresponding information on ionic liquids in water and in aqueous electrolyte solutions is more scarce. Therefore, in this study, the improved isopiestic method [2] has been used to obtain activities of water for ternary 1-hexyl-3-methylimidazolium chloride + monomethyl malonate potassium salt + water and the corresponding binary aqueous solutions at 298.15 K.
