102 OSMOSIS AND OSMOTIC PRESSURE 



This is the same pressure that it would exert were it a gas confined in the 

 same volume under standard conditions. It follows therefore that equal 

 osmotic pressures of solutions of undissociated solutes indicate equal concen- 

 trations of solute molecules and that Avogadro's Hypothesis may be applied 

 to osmotic pressures as well as to gas pressures. In other words the theoretical 

 osmotic pressure of a weight molar solution of any undissociated solute is 

 22.4 atmos. at 0° C. The osmotic pressures of weight molar solutions of 

 several undissociated compounds are given in the left-hand column of Table 15. 

 It should be noted that these experimentally determined values agree only 

 approximately with the theory. In more dilute solutions the values obtained 

 are more nearly concordant with those required by this hypothesis. 



Electrolytes, as first shown by DeVries in 1884, and as indicated by the 

 data in Table 15, exhibit greater osmotic pressures than their molar concen- 

 trations would lead one to expect. This fact remained unexplained until 

 the formulation of the ionization theory by Arrhenius in 1887. In fact 

 DeVries' data were used by Arrhenius as strong evidence for his theory. 

 Apparently each ion produced by the dissociation of an electrolyte is just as 

 effective osmotically as an undissociated molecule. Hence all electrolytes 

 produce higher osmotic pressures than the theoretical values for undissociated 

 compounds; the exact magnitude of the pressures produced will depend upon 

 their degree of dissociation. For example, a molar solution of NaCl is about 

 75 per cent dissociated. This means that 75 out of every 100 molecules of 

 NaCl present have dissociated into ions. There will therefore be 1.75 times 

 as many particles (ions and molecules) present as in a molar solution of a non- 

 electrolyte. The osmotic pressure of a molar solution of NaCl would therefore 

 be theoretically 22.4 X 1.75 = 39-30 atmos. Consultation of Table 15 shows 

 that this value agrees approximately with the experimentally determined value 

 for a molar NaCl solution. 



The hydration of molecules or ions also influences the osmotic pressure 

 of a solution. Water molecules which are bound to solute particles as water 

 of hydration are no longer effective as a part of the solvent. In effect a 

 solution containing hydrated solute particles is therefore more concentrated 

 than its molarity would indicate. This accounts, at least in part, for the 

 fact that the experimentally determined values of osmotic pressures are often 

 in excess of the theoretical ones which would be expected according to the 

 van't Hoff analogy with the gas laws. For example, a molar solution of 

 sucrose should theoretically have an osmotic pressure of 22.4 atmos, at 0° C. 

 Actually its measured pressure is found to be 24.83 atmos. (Table 14). 

 This discrepancy is believed to be due to the hydration of the sucrose mole- 

 cules. One sucrose molecule binds six molecules of water of hydration. 



