of the Theory of Solutions. 



483 



Formula. 



M. 



Relative molecular 



lowering 

 of vapour-pressure. 



H 2 



18 



138 



76 



154 



120 



70 



78 



142 



32 



74 



200 



0185 



1-49 



0-8 



1-62 



1-3 



074 



0-83 



1-49 



0-33 



071 



2 (Gallium). 



PC1 3 



CS 2 



CC1 4 



CHCI3 







IOH3 



CH 4 



C A H 1n O 



Hg 





The Raising oj tlie Boiling -Point. — Fig. 4 contains portions 

 of the vapour-pressure curves for solvent and solution. The 



Fig. 4. 



T 



horizontal line ab corresponds to the atmospheric pressure, 

 a is the boiling-point of the solvent, b that of the solution, 

 ab the rise of boiling-point (dT), and ac the lowering of 

 vapour-pressure {dp). 



If, in the well-known equation 



dip q 

 ~ZT """2T 2 ' 

 where q represents the latent heat of evaporation per kilo- 

 gramme-molecule, the above expression of Raoult's law, 



— ±— vn ^s 



P 



m = 0'01M, 



is introduced, we obtain 



m 



AT = 



0-02 T 2 0-02 T 2 



<7/M 

 dt 



w 



or, rigorously, dt _ 2T 2 



m -~dn7~ W 

 which is the well-known van't Hoff-Beckmann-Arrhenius 

 expression for the molecular rise of boiling-point, the experi- 

 mental confirmation of which is contained in the following table 

 (W = the latent heat of evaporation per kilogramme). 



