48 MOLECULAR WEIGHT AND POLYMERISM 



be used in calculating N, as Raoult and Recoura l showed 

 to be the case for acetic acid. They measured, as explained 

 above, the relative molecular lowering of vapour pressure, 

 i.e. the lowering for a one per cent, solution multiplied by the 

 molecular weight of the dissolved substance, or in Raoult 's 

 nomenclature 



/-/' TO> 



where P is the percentage content of solution, / /' the 

 lowering of vapour pressure. This molecular diminution, 

 multiplied by 100 and divided by the molecular weight M 

 of the solvent, gives a number that averages unity, or 



f-f m_ 



We get the same result from the formula already given : 



= ooi M* 

 P 



w^hich coincides with the former if only p, the pressure of 

 the solvent, is put for /', that of the solution, which is 

 permissible in dealing with very dilute solutions. 



On the other hand, acetic acid gave 1-61 instead of I 

 when Jl/was taken as 60, according to the formula C V H 4 2 . 

 But to do so would be to choose too small a molecular 

 weight, for that, according to the vapour density, is 97. 

 Taking the correct value we again get a number close to 

 unity, as before, since 



60 

 i-oi x =i. 



97 



Hence, in determining the lowering of vapour pressure 

 lies another means of measuring the molecular weight of 

 the saturated vapour of the solvent, provided the molecular 

 weight of the dissolved body be known. 



1 Zeitschr.f. Phys. Chem. 5. 423 ; Berl. Ber. 29. Ref. 941. 



