372 



KEYES 



where vi is the partial volume 



(: 



dv\ 

 dmj 



p, t, m 



ART. J 



, and Xi the 



mol fraction. The analogue of Henry's law in terms of fugacity 

 becomes for dilute solutions /« = kmi", where m/' is the mass of 

 the dissolved gas in the liquid phase. A glance at the expres- 

 sion above for/e makes evident that a part of the deviations from 

 Henry's law will be found in the failure of the equihbrium gas 

 mixture to conform to the ideal gas laws. 



£6. RaoulVs Law of Vapor Pressure and the Thermodynamic 

 Theory of Dilute Solutions. Another principle in the same class 

 with Henry's law is Raoult's law, according to which the ratio 

 of the vapor pressure of a solution to the normal saturation 

 pressure is equal to the ratio of the number of molecules of the 

 solvent to the sum of those of the dissolved substance and the 

 solvent. Designate the salt with subscript 2 and the solvent 

 with subscript s. 



V 



Psat. 



n. 



Ua + n2 



or 



Psat. — P _ 



n2 



Psat. 



Psat. 



P 



P 



Us + ^2 

 W2 



ns 



(91) 



The relation of this result to the general Gibbs theory is easily 

 established for dilute salt solutions. A salt solution may be 

 regarded as a special case of a binary mixture in which the 

 component in smallest amount is non-volatile. The second of 

 the pair of equations in a, equation (83), vanishes and there 

 remains, since m^' = 0, 



Xi dp „ dr" 



7 = ^^^^+^'V- 



(92) 



Note in the first place that if m-l' jm-i' = r" is constant, and 

 we let 



Xo = ^ "^ (vi - v^ 



