188 



GUGGENHEIM 



ART. E 



potentials ^t on the composition take a simpler form. Similarly 

 Vh denotes the increase in volume of a very large phase when 

 one adds to it one mol of the species Sh, keeping temperature 

 and pressure constant. Therefore Vh will be called the "partial 

 molar volume" of the species Sh- 



As already mentioned the potentials /zi, /i2, ... Mn will be 

 functions not only of t and p but also of the number of mols 

 mi, m2, . . . w„ of the various species in the phase. Actually 

 it is clear that each n will depend on the composition of the 

 phase but not on the absolute quantity of it. That is to say, 

 m, 1X2, ... iin will be functions of the quantities A^i, N2, . . . Nn 

 defined by 



(23) 



nil -\- nii . . . -\- rrin 



The quantities A^i, A''2, ■ • . Nn are called the mol fractions of 

 the species Si, S2, ... Sn- They are, of course, not mutually 

 independent but are subject to the identical relation 



A^i + A^2 . . . + A^. = 1, 

 from which it follows that 



dNi + dNi ... + dNn = 0. 



(24) 



(25) 



6. Ideal Solutions. A series of solutions in a given solvent 

 are said to be "ideal" if throughout a range of concentrations 

 extending continuously down to pure solvent the potential 

 Hh of each species Sh whether solvent or solute obeys the formula 



IJih = Hh\t, V) + ^t log A^;,, 



(26) 



where Hh^{t, p) is independent of the composition of the solution 

 and .4 is a universal constant known as the "gas constant." 



