Abstract: In this paper a new concept, known as general osmotic coefficient ( Φ_s), is proposed in terms of Bjerrum osmotic coefficient (Φ ), and its definition formula is given.In fact, general osmotic coefficient is an extension of Bjerrum osmotic coefficient.According to the new concept the mutually related formula between general osmotic coefficient and electrolyte activity coefficient, which has just the same form as that in the single solvent electrolyte system, is conducted.Thus the mutual calculation of the thermodynamic properties between solute (electrolyte) and solvent can be easily carried out and the non-ideal degree of the mixed solvent can be measured by it.So general osmotic coefficent has a theoretical and practical signification.

Key words: General osmotic coefficient, General definition formula, Electrolyte activity coefficient.Mutual relation