12 HYDROGEN ION CONCENTRATION 



liter. Since the degree of dissociation of water is extremely small, 

 the concentration of the undissociated H2O molecules does not show 

 any measurable decrease, and, therefore, the concentration of the 

 undissociated water is, within the limits of experimental error, equal 

 to the total concentration of the water, which is, of course, a constant 

 quantity. Then by transposing the term [H2O] to the right side 

 in equation (2) and designating [H2O] k = kw the above equilibrium 

 expression becomes: 



[H+] . [0H-] = kw (3) 



kw is the dissociation constant of water, and for every definite tem- 

 perature it has a definite value. 



The active masses indicated in the above formulation of the mass 

 law may be considered as being equal to their respective concentra- 

 tions only in very dilute solutions and in the ideal gaseous state. 

 The active mass of water indicated in (2) as [H2O] is not easily 

 defined. According to Nernst" its value is to be assumed as one 

 proportional to the vapor pressure of water. Thus, for instance, 

 the values for [H2O] in pure water and in a 1 M" sugar solution are 

 not quite identical, that of the latter being 2 per cent less. The 

 statement that the active mass of water in different solutions is 

 proportional to their respective vapor pressures is valid only for 

 solutions at the same temperature. The active mass of water in 

 solutions not at the same temperature cannot be assumed as being 

 simply proportional to the vapor pressures. 



The dissociation constant of water is then a constant value for 

 any given temperature. It varies with the temperature to a greater 

 extent than the dissociation constants of most electrolytes. This 

 depends on the fact that the value [H2O] varies greatly with the 

 temperature. While it is true, as was stated above, that [H2O] is 

 not to be assumed as being simply proportional to the vapor pres- 

 sures at varying temperatures, still this value is doubtlessly in 

 some, not as yet clearly understood, manner related to the vapor 

 pressure. The wide variations of kw depends not so much upon 

 any significant variations of the value k itself in equation (2) as 

 upon variations in the value of the factor [H2O]. A satisfactory 

 explanation of these relations has not as yet been derived either 

 from thermodynamic data or those of the molecular theory. Closely 



6 W. Nernst. Theoretische Chemie, 7th ed. P. 680. 



