14 2 THEORY CLF CONCENTRATED SOLUTIONS. 



independent of the degree of dissociation. The formula of a 

 compound cannot change gradually with temperature, and hence 

 the deviation curve at different temperatures must show a 

 maximum in exactly the same position. A confirmation of thi 

 deduction is seen in Fig. 5, which gives the viscosity deviation 

 curves for methyl alcohol — water mixtures at various tempera- 

 tures. The deviations become less as the temperature is 

 increased. This indicates that the amount of compound formed 

 is less at higher temperatures. Hence, again, the compound dis- 

 sociates on raising the temperature, and consequently the forma- 

 tion of the compound must generate heat. As is well known, the 

 mixing of alcohol and water evolves a considerable amount of 

 heat. 



It is evident, however, that the position of maximum devia- 

 tion is quite independent of temperature, as theory demands. 



This should remove one of the great difficulties in the 

 acceptance of the principle of deducing the formation of com- 

 pounds in binary mixtures from a study of their physical 

 properties. 



The evaluation of the dissociation constant K . 



The best test of any theory is to be found not merely in 

 qualitative but in the quantitative agreement of experiment with 

 the deductions from the theory. Further, in all cases of com- 

 bination in homogeneous systems, including hydrate formation 

 and dissociation, the great desideratum is, of course, the numeri- 

 cal value of the dissociation constant. This is often impossible 

 to determine by ordinary methods. It would seem that the ideas 

 developed in the present paper may offer a partial solution of 

 the problem. 



If we have a system composed of y mols total A and (i-y) 

 mols total B and x mols of a compound ABn are formed, then 

 the equilibrium is governed by the relationship 



(y - *)(i -y - «*)'" == KVx 



if active mass be taken as mols per unit volume, or 



[y — ,r)(i — v — ux)" = Kx{\ — iix)" 



if we put the active mass equal to the fraction of the total 

 number of mols present. 



Since the deviation from the straight line (8 ) is propor- 

 tional to the number of mols of compound formed we have 



x =-- p$' 



Substituting for x we obtain 



(r - ph){i - y - np8)"= KphV," 



and for another value of y 



(V -P8,)(i - y, - npi,y = Kph;V* 



