132 BUTLER AUT. d 



For example, in the case of a binary solution of the compo- 

 nents Si and S2, regarding Si as the solvent and *S2 as the solute, 

 we may adopt the following conventions: 



(1) The activity of the solvent is unity in the pure solvent 

 at the same temperature and pressure, i.e. 



ai = iVi, when A^i = 1, (163) 



where 



mi/Mi 



Ni = 



nil/ Ml + mil Ml 



is the molar fraction of the solvent. 



When the possible range of concentrations extends to 

 iV2 = 1, as is the case with two liquids which are miscible in 

 all proportions, the same convention may be adopted for *S2. 



(2) The activity of the solute is equal to its concentration when 

 the latter is very small. The concentration may be expressed 

 in any suitable way. If expressed as the molar fraction {N^, 

 we have 



as -^ A^2, when ATj -> 0. (164) 



In the case of dilute aqueous solutions the concentration is 

 often expressed as the number of mols {ui = nh/Mi), dissolved 

 in a given weight, say 1000 grams, of the solvent. The activity 

 may then be defined so that 



"2 —>■ ni, when n^ -^0* (165) 



21. Determination of Activities from the Vapor Pressure. 

 The potential of a volatile component of a solution is given, as 

 in (129), by the equation 



* The molecular weight to be employed in determining the activity 

 by (162) may have any appropriate value. But if the activity is deter- 

 mined from the partial vapor pressure according to the method of 

 Section 21 the molecular weight of the substance in the vapor state 

 must be used. Also when the activity is defined by convention (2) its 

 value can only be equal to the concentration in an infinitely dilute solu- 

 tion if the molecular weight is that in the solution. 



