Molecular Thermodynamics. 623 



order effects already discussed will generally enter and 

 invalidate equations (53). 



Appendix to Section IV. • 



Non-validity of " Mass-Action Equations" for Perfect 

 Solutes. 



It has been common to use equations of the form of (55) 

 with the " general terms'" omitted, or rather the equivalents 

 thereof, as the criteria of a " perfect solute." 



Ii may be suggested that this is incorrect, not being in 

 accord with the obviously rational definition of a perfect 

 solute as a solute which behaves like a constituent of a 

 mixture of perfect gases, — a solute, in fact, which does not 

 affect the "general terms'"' but only the linear, and for 

 which 



' U = ^-R[log fe -log(l + m 2e 1 )]. • ■ (57) 

 Now at very low concentrations (57) approximates closely to 



^— =(j) s — Rl0gG' s , (OS) 



and (58), or something equivalent to it, has hitherto generally 

 been the criterion of a perfect solute. 



The question arises, as an interesting example of what has 

 been said with regard to the "second approximation^ (56), 

 whether cases may not easily arise in which a solute remains 

 " perfect''' in accordance with the true criterion (57), within 

 the error of experiment even at concentrations for which 

 (57) does not, within the experimental error, approximate to 

 equation (58). 



We have seen that in aqueous solution when Xc { is half- 

 molar concentration the value of log(l-\-m Q 7iLci) is of the 

 order of '02. 



Now consider a chemical reaction between solutes which 

 are "perfect''' in accordance with the true criterion (57). 



We have from (21) for equilibrium with regard to the 

 virtual change %Sn s 



8^ = 0, 



that is 



