DISSOCIATION OF STRONG ELECTROLYTES 



115 



stated in terms of the concentrations of the reacting molecular or 

 ionic species. This is justifiable only insofar as the concentration 

 is proportional to the active mass. In reactions in gaseous systems 

 or in reactions between electroneutral molecules in solution the 

 concept of the active mass may be quite accurately defined. In fact, 

 it is the value which is proportional to the osmotic partial pressure 

 of the molecular species involved. Actually the law of mass action, 

 as it is thermodynamically conceived, makes no mention whatever of 

 any concentration relationships but refers to pressures.^ Since under 

 conditions of ideal gases and of extremely dilute solutions the osmotic 

 pressure of a molecular species is proportional to its concentration, 

 therefore it is permissible, for electroneutral molecules in very dilute 

 solutions, to substitute concentration relations for those of osmotic 

 pressui-e. For higher concentrations, however, this does not hold 



TABLE 20 

 Apparent degree of dissociation of KCl 



true any longer. Only with nonelectric molecular species (e.g., 

 sucrose) concentrations up to 1 M may be considered in this respect, 

 with very httle error, as solutions of infinite dilution. In more con- 

 centrated solutions the osmotic pressure is affected by the forces of 

 attraction operating among the molecules, and because of these the 

 pressure is no longer proportional to the concentration but increases 

 at a lesser rate than the concentration. It has been for a while 

 supposed that with ionized substances the "state of infinite dilution" 

 was to be assumed up to the same concentrations as with electro- 

 neutral molecules. The impossibihty of determining experimentally 

 the concentration of an individual ionic species strengthened this 

 tacit assumption. But this is just where the error is involved. If 

 in the case of electroneutral molecules the effect of the forces of 

 attraction becomes evident in 1 M concentrations, then in the case of 



^ At one time Arrhenius spoke of it on this account as of the "pressure action 

 law" in distinction from the "mass action law." 



