44 DIFFUSION AND OSMOTIC PRESSURE 



equivalent solutions. Most of the tables consider as a 

 standard solutions containing a gram-equivalent per liter. 

 These are easily transformed into gram-molecular solutions 

 by dividing the given concentration by the number repre- 

 senting the valency of the compound. Thus one-tenth 

 gram-equivalent per liter of Na 2 SO 4 is identical with one- 

 twentieth gram-molecular solution of the same salt. 



For very weak solutions of mixed electrolytes the above 

 method may be resorted to. But for solutions of mixed 

 electrolytes and non-electrolytes, and for strong solutions of 

 electrolytes, no method of calculation has yet been discovered. 

 The only practical way open in such a case is to resort to the 

 methods of freezing- and boiling-points. It is often best to 

 make use of both these methods, and to interpolate between 

 them for the normal temperature, inasmuch as ionization often 

 increases rapidly at higher temperatures. Of course, where 

 chemical reaction occurs between the different solutes, the 

 osmotic pressure of the solution will not be constant until 

 chemical equilibrium has been attained. 



