RISE OF BOILING POINT 57 



is only strictly true for infinite dilution, so that the form 



dt _ 003 T 2 O'OiqST 2 



~ i/i/ TIT " or TT -r- 



da W W 



is to be preferred, where da is the dissolved substance in 

 100 parts. 



Molecular ^veight determination by the rise of boiling 

 point. Just as the solvent can be removed by freezing, so it 

 can be separated from the dissolved body by boiling, and the 

 lowering of vapour pressure already dealt with corresponds 

 to a rise in the boiling point. This may be determined in 

 magnitude by means of a cyclic process similar to that 

 referring to the freezing point, and amounts to 



0-02 T- 00108 T 2 

 - ir - or exactly- -^ , 



where t is the molecular rise of boiling point (i. e. -7- m, 



aa 



p. 56), T the absolute temperature of the boiling point, 

 W the latent heat of evaporation. The method of molecular 

 weight determinations ba"sed on this has also been worked 

 out by Beckmann. It is usually not so good as the freezing- 

 point method, because the molecular rise of boiling point 

 is, on account of the great latent heat of evaporation, 

 relatively small. 



This will be plain from the following table, which gives 

 the freezing point (Fp), latent heat of fusion (TF}) 3 boil- 

 ing point (Sp), and latent heat of evaporation at the 

 boiling point ( TT 8 ), together with the molecular change in 

 freezing point (Mf), and boiling point (M 8 }> calculated from 



the formula 



#\& OF TH 

 ,_ 0-0198 T 2 



~~ 



