Chapter IV 



37 



Osmosis 



concentration of the hydrated molecules. Assuming a hydration number 

 of 5, the value of Po for weight normal sucrose at 20° C. becomes 25.96 

 and including the association factor of 1.65 the value becomes 27.50. 



Frazer and Myrick (1916) have used the same formula to calculate 

 osmotic pressure values for cane sugar solutions at 30° C. Table 11 gives 

 their data. 



Table 11. — Osmotic pressure of solutions of cane sugar at 30° C: — 



From these values it is apparent that no one hydration number or asso- 

 ciation factor will account for the deviations at these high concentrations. 

 Up to two molar, the osmotic pressures agree well with values calculated 

 on the assumption of the existence of hexahydrate, but at higher con- 

 centrations the best correspondence occurs where association is assumed 

 and hydration decreases as concentration increases. All of these assump- 

 tions are empirical however and when considered along with the hydration 

 numbers calculated by Porter (Table 10) they suggest a smooth and 

 continuous change of hydration and association with concentration such 

 as would occur if the increase in solute shifted the intermolecular force 

 fields and hence the coordination of the molecules of both solvent and solute. 



Haldane (1918) has recalculated data of Berkeley and Hartley 

 according to the formula 



0.082 NT . (12) 



"' = Nr 



n 



where n, N, and Ni = the gram molecules of solute, pure solvent, and 

 solution per liter. This is identical with the formula of Morse. Haldane, 

 in assuming sucrose to exist as a pentahydrate, corrected for the space 

 occupied by the water of hydration. Table 12 gives Haldane's values. 



Table 12. — Osmotic pressure of sucrose solutions at 0° C. (data of Berkeley and 



Hartley, calculations by Haldane) : — 



