131 



or 



Substituting the value of tj^i given by (151) and (153) we have 



= Ml - XI, (160) 



or 



Xi 



t d{njt) \ 



n (161) 



(Compare equation (61).) 



20. The Activity. The potential of a solute, the relative 

 amount of which is very small, according to (128), is 



A« mi 



^^ = ^ + i^ ^°s 7- 



This relation can only be regarded as expressing the limiting 

 law of variation of the potential with the concentration at 

 infinite dilution, and the foregoing considerations give us no 

 guidance as to the modifications which may be necessary at 

 greater concentrations. In order to represent the values of 

 the potentials in actual solutions, G. N. Lewis has introduced a 

 quantity a, called the activity, which may be defined by the 

 equation 



At 



Ml = Ml" + ^^ log «i, (162) 



where /ii" is the potential in a chosen standard state, at the same 

 temperature and pressure, in which the activity is taken as 

 unity. The standard state may be chosen according to the 

 circumstances of different cases. 



