732 EARNED art. m 



dissociation product, mnWoH) may be determined if we know K, 

 and in this way we may study the effects of electrolytes on the 

 dissociation of the solvent.* 



We have still to determine K from the electromotive forces 

 of cells without liquid junction. Consider the cell, 



H2 1 MOB.{mi), MC1(W2) | AgCl | Ag.f 



Its electromotive force at 25° is given by 



E = Eq - 0.05915 logio mnwci - 0.05915 logio ThTci, (50) 



where Eq may be obtained from the cell containing hydrochloric 

 acid. If we substitute the value of m^ obtained from equation 

 (49), we obtain 



E = Eo- 0.05915 logio ^^^^'^^^ - 0.05915 logio thTci 



ThTohWoh 



= Eo- 0.05915 logio K - 0.05915 logio ' ^^^^^^^'" 



7H70H 



-0.05915 logio ^^. (51) 



moB. 



Eo is known. In dilute solutions the third term on the right is 

 very close to unity since it contains the ratio of activity coeffi- 

 cient products. Therefore, 



E + 0.05915 logio ^^^ 



moH 



in very dilute solutions has very nearly a constant value. Thus, 

 the extrapolation of this quantity to zero ionic concentration is a 

 simple matter, and its value at infinite dilution is equal to 

 [£'0 — 0.05915 logio K]. We have, therefore, an independent 

 measure of K. 



(b) The Ionic Activity Coefficients and Dissociation of Weak 

 Acids and Bases in Salt Solutions. By the application of the 



* Harned, /. Am. Chem. Soc, 47, 930 (1925). 

 t Roberts, J. Am. Chem. Soc, 62, 3877 (1930). 



