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Activity coefficients of solutes in a solution containing Fe as solvent and several solutes could be calculated by mathematical relations between the interaction coefficients of the solutes measured in a ternary Fe based solution, but these mathematical relations could be used only for dilute solutes (the solutes with concentration of 1% or less). In this work, a new method has been developed in which the calculation of the activity coefficients of non-dilute solutes as well as dilute solutes in a solution containing Fe as solvent and several solutes could be possible by using the interaction coefficients of the solutes measured in a ternary solution containing Fe as solvent, but only in a concentrate range in which the experimental relation between Ln Ki and Xj is linear (i and j are solutes and K is activity coefficient).