Chapter IV 



— 39 — 



Osmosis 



contained values of the partial specific volume of water S derived from its 

 compressibility instead of the usual mol volume V. The vapor pressures 

 were very carefully measured and the values were corrected for known 

 sources of error. 



Table 15. — Observed and calculated vahies of the osmotic pressure of a-methyl 



glucoside solutions: — 



Calculations by the vapor-pressure formula (equation 2) follow the 

 observed values to a high concentration as shown in Tables 13-16. Where 

 vapor pressure values are available, this basis for calculation is more re- 

 liable because it does not throw the correction upon a postulated mechanism 

 (hydration, association, etc.) but deals with values that integrate these 

 physical properties of the solution in a reliable manner. Freezing point 

 values may be used in a similar way, as pointed out by Lewis. 



The examples given are a sample of the calculations that bear a certain 

 resemblance to the gas laws. Many more such values have been calculated. 

 Over fifty modifications of the van der Waals equation alone have been 

 used to calculate osmotic pressure values. These different laws of solu- 

 tions deduced thermodynamically on various bases are in reality inde- 

 pendent of the phenomenon of osmosis ; they all entail a central concept, 

 namely, that there is a difference between the free energy of the solvent in 

 the solution and in the free state, and that a pressure imposed upon the 

 solution that is sufficient to equalize these free energies will vary with con- 

 centration of the solute in the same general way as does the partial pressure 

 of a gas under analogous conditions. 



There is an obvious and fundamental weakness in this whole concept ; 

 whereas the gas laws apply to a single component, in solutions there are two 

 components to deal with. Because of the philosophy behind van't Hoff's 

 law of osmotic pressure there has been a tendency to treat the solution as 

 a one component system in which the solvent simply provides space for 

 dispersion of the solute molecules. The corrections for space occupied by 

 the molecules of the system and the forces between them as provided in 



