300 Mr. F. Tinker on the Vapour Pressures 



according to the law of admixture in molecular proportions, 

 and if L denotes the molecular latent heat of vaporization 

 of the perfect mixture *, we have 



and p L=p 1 xL 1 +p 2 (l—x)L 2 , . . . [16] 



since the two components evaporate in the ratio of their 

 partial pressures. 

 Hence also 



/?i.rdL 1 +jt? 2 (l — #)dL 2 =jt? BL- 



Consequents , ,., >. dL 



p=pix+p 2 (l—x)—p ^fji 



e. 



P 



=^o(i-i); ["J 



or in words, " the total vapour pressure of a binary mixture 

 is equal to the vapour pressure calculated from the law of 

 admixture in molecular proportions, multiplied by a factor 



( 1— pTp), where ^L is the excess of the molecular latent 



heat of vaporization of the mixture over ithe [theoretical- 

 value calculated from the mixture rule." WlSWp'""'^^'^ 

 This simple relationship shows that the variation of the 

 total vapour pressure from the straight line law is deter- 

 mined almost entirely by abnormalities in the latent heat of 

 vaporization f. r -^"l 



Proceeding from equation [17] we have) three possible 

 cases : — 



(i.) If the actual molecular heat of vaporization of the 



* The actual molecular latent heat of the mixture would be the 

 heat of evaporation or condensation of one gm. molecule of vapour, 

 i. e. the heat required to produce 22-2 litres of the mixed vapour at 

 N.T.P. The theoretical value L is of course calculated from equation 



+ Volume changes on mixing will of course have a slight effect on the 

 vapour pressure also. Allowing for these, equation*[17] becomes 



^=^0(1—^) -CPi'ei+pa , «a). 



