42 DIFFUSION AND OSMOTIC PRESSURE 



For mixed solutions of non-electrolytes the total osmotic 

 pressure is the sum of the partial pressures due to the several 

 solutes respectively. 



b) When the pressure is produced by an electrolyte. 

 On account of the phenomena of ionization or dissociation, 

 the calculation of the osmotic pressure of a solution of 

 an electrolyte becomes somewhat complicated. The amount 

 of ionization must be known 'in order to get the relative 

 number of particles per unit volume. For instance, a gram- 

 molecule of NaCl, in aqueous solution, occupying a liter 

 volume, contains more particles than the same volume of a 

 normal solution of sugar ; some of the molecules have sepa- 

 rated into Na and Cl ions. 



The amount of ionization in any simple solution of an 

 electrolyte can be determined by means of the method of 

 electrolytic conductivity devised by Kohlrausch. 1 The con- 

 ductivity is proportional to the number of free ions, and 

 hence, knowing the conductivity both at the given concen- 

 tration and at a concentration where ionization is complete, 

 we can calculate the amount of ionization. The conductivity 

 of many solutions has been determined by different authors 

 in different units. Of course, all are reducible to C. G. S. 

 units or to the conductivity of mercury, but it is immaterial 

 for the present purpose what units are used, so long as the 

 same ones are used throughout the same calculation. As the 

 solution becomes more and more dilute, the conductivity 

 approaches a limit. This limit is the conductivity at infinite 

 dilution, where ionization is complete; it is usually denoted 

 by X w . Allow X to denote the conductivity at the given 



concentration. Then = a , the fraction of the whole 



A oo 



IF. KOHLRAUSCH, Leitfaden der praktischen Physik, 7th ed. (Leipzig, 1892), 

 p. 301 ; also F. KOHLRAUSCH UNO L. HOLBORN, Das LeitvermOgen der Elektrolyte, 

 Leipzig, 1898; W. OSTWALD, "Ueber Apparate zur Bestimmung der Electrischen 

 Leitfahigkeit von Electrolyten, Zeitschr. f. physik. Chem., Vol. II (1888), pp. 561-67; 

 OSTWALD-WALKER, Manual of Physico-Chemical Measurements, London, 1894. 



