CHANGE OF VAPOUR PRESSURE 45 



pressure, i.e. the value of /\p/p obtained for a one percent, 

 solution multiplied by the molecular weight of the dissolved 



substance, or 



Ap __ 7i 



<pT' 'K 11 



in which one part of dissolved substance is contained in 

 i oo parts of solvent : 



nm : NM = i : 100 or-^ = 1-01 M, 



whence 



A-0 

 m = ooi M: 



P 



in other words, the relative lowering of the vapour pressure 

 is equal to one hundredth of the molecular weight of the 

 solvent ; in which it must be borne in mind that the latter 

 quantity has the value derived from vapour density 

 measurements. 



The following table shows results obtained for moderately 

 dilute solutions * : 



Solvent M m 



P 



Water 18 0-185 



Phosphorus trichloride 137*5 i'49 



Carbon disulphide 76 0-8 



Carbon tetrachloride 154 1-62 



Chloroform 119-5 i-3 



Amylene 70 0-74 



Benzene 78 0-83 



Methyl iodide 142 1-49 



Methyl bromide 109 1-18 



Ether 74 0-71 



Acetone 58 0-59 



Methyl alcohol 32 0-33 



Mercury 2 200 2- 



It may be remarked that, as with all rules derived from 

 the theory of solutions, it is only strictly true for infinite 

 dilution. For that reason we will now repeat the proof 



1 Raoult, Compt. Rend. 87. 167. 



2 Ramsay, Zeitschr. f. Phys. Chem. 3. 359. 



