13<> THEORY OF CONCENTRATED SOLUTIONS. 



Mendeleef-Pickering hydrate theory of solution. If, however, 

 one compound largely predominates in solution, then the law of 

 mass action can be applied with some success. 



The application of the law of mass action to the system 

 under discussion gives the relationship at constant temperature : 



[Active mass of .4] [Active mass of B]" = a constant x f Active mass of] 

 in equilibrium J [_ in eciuilibrium J [_ Ah n in equil. J 



It would seem necessary, in discussing matters relating to 

 concentrated solutions, to define what one means by " active 

 mass." This quantity is generally represented by " concentra- 

 tion," i.e., by so many grain-molecules or mols per unit volume. 



Now one's definition of " active mass " must depend some- 

 what on how one defines an ideal solution. 



As is well known, Van 't Iloff has defined his ideal dilute 

 solution as one which obeys the gas laws 



pv= nRT, 



with the substitution of osmotic pressure for gaseous pressure. 

 But even the gas laws in this simple form are quite invalid at 

 moderate pressures, i.e.. at moderate concentrations, and have the 

 necessity of a new general definition of a solution which shall 

 cover both dilute and concentrated solutions. In this paper I 

 propose to take as my ideal solution one which obeys the mixture 

 law throughout the range of its composition. 



If, then, we have a homogeneous mixture of y mols A and 

 (i-y) mols B, and there is no molecular change on mixing (i.e.. 

 we have an ideal binary mixture or solution), then the value of 

 any molecular property for the mixture is given by 



X M = ft*, + ft*, 



where .v, and x., are the values of the given property for pure 



A and pure B respectively, and 



ft and ft are the molar fractions of A and B in the mixture. 



ft thus represents the fraction of any attribute of the 

 mixture which is due to the presence of A, and ft represents 

 the same for the liquid B. 



ft and ft I take as the " active masses " of A and B 

 respectively. 



In the case of mixtures of gases and in dilute solution this 

 conception of active mass coincides with the older one of mols 

 per unit volume, but with liquid mixtures throughout their range 

 of composition this is no longer the case, owing to the fact that 

 the two liquids A and B have different specific and molecular 

 volumes. 



Returning now to the case where A and B form a compound 

 ABn, let us imagine the mixture formed from y mols A and 

 (l-y) mols B. If no combination or other molecular change 

 takes place, the total number of mols present is unity. If, how- 



