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Mr. F. Tinker on the Vapour Pressures 



We may usually neglect the volume changes of liquids on 

 mixing, so that ei and e 2 are practically zero. In this case, 

 the total liquid pressure it becomes 



7r = 7r 1 ' + 7r 2 ' = 7r 1 # + '7r 2 (l —a?), . . . [11] 



Hence when there are no volume changes on mixing, the 

 total liquid pressure can be calculated from the two partial 

 liquid pressures by the law of admixture in molecular pro- 

 portions. For the partial liquid pressure of each component 

 is then proportional to its molar fraction. If, however, there 

 is a volume change, the law is only approximately accurate. 



But the ratio -^ — j- = (1 — e{) will in general be so nearly 



equal to unity that there will be an appreciable deviation 

 from the mixture law only in very abnormal cases. 



(b) Relation between the Partial and Total Vapour Pressures 

 in a binary mixture and the relative molecular concen- 

 trations of the two components. 



We can now apply Dieterici's equation proper to the 

 questions in hand. 



Let p 1 and p 2 = vapour pressures of pure components X 



and Y, 



Pi 



and p 2 ' = 



of X and Y in mixture, 



p=p{ +p 2 ' = total vapour pressure of mixture, 

 A 1 and A 2 = work done when a molecule of X or Y 



is evaporated from the pure liquids, 

 A/ and A 2 ' = work done when a molecule of X or Y 



is evaporated from the mixture. 

 N, n, x have same values as before. 



From equation [3] we have 



p 1 = 7Tl e RT , 



Hence 



lh , = «-i , *" M 





Pi 7Ti 



Combining this equation with equation [9] and neglecting e, 

 we have 



(A X '-A T ) 



p ± ' =p L xe ET [12] 



