Crafts et al. 



38 



Water in Plants 



These values of Haldane follow the observed values to a concentra- 

 tion about 2 molal and then depart, becoming higher than the observed 

 values. This probably results from use of the ratio of solute to solvent 

 molecules in place of the logarithm of the mol fraction of solvent as pointed 

 out by Lewis (1908) and explained on page 33. 



All of the calculations so far presented have been based upon the con- 

 centration of solute in the solution. By shifting the basis for calculating 

 concentration and by assuming association of the solvent and hydration of 

 the solute fair approximations of the observed values have been obtained. 



Another method of arriving at the osmotic pressures of solutions is to 

 use a different basis for starting calculations ; one that is more reliable than 

 concentration in reflecting the properties of the solution. One such basis is 

 vapor pressure. The vapor pressure of the solvent vapor above a solution 

 is an index of the escaping tendency of its molecules ; such forces as asso- 

 ciation and hydration, in other words, the intermolecular forces determin- 

 ing coordination of solvent and solute molecules are integrated into the 

 vapor pressure value. Though it tells nothing concerning mechanism, it 

 does reflect the true activity of the solvent in the solution and it is the 

 difference between this activity and the activity of pure solvent molecules 

 that determines the true osmotic pressure of the solution (Lewis, 1908). 



Table 13 presents values for the osmotic pressure of sucrose at 30° C. 

 observed by Frazer and Myrick and values calculated from the vapor 

 pressure of solvent molecules above the solutions. 



Table 13 — Observed and calculated osmotic pressure values for sucrose solutions at 



30° C. : — 



Tables 14, 15, and 16 present osmotic pressure values of cane sugar 

 at 30° C. and 0° C, methyl glucoside at 30° C. and 0° C, and calcium 

 ferrocyanide at 0° C. calculated from vapor pressure data compared with 

 directly observed values. These are from Berkeley, Hartley, and Bur- 

 ton (1919) and the formula used for calculating the o.smotic pressures 



Table 14. — Observed and calculated values of the osmotic pressure of sucrose 



solutions: — 



