CHANGE OF VAPOUR PRESSURE 47 



osmotic work spent must be equal to the work of expansion 

 gained, i. e. 



zTdp m 



* T = TO- ' 

 p Mda 



or 



m = Mda, 

 P 



which again comes to Eaoult's law, if we remember that 

 the solution contains da to one or icoda per cent. Accord- 

 ingly, for a one per cent, solution, assuming the proportion- 

 ality between concentration and lowering of vapour pressure 

 which is true for infinite dilution to hold, we have 



Ap dp in 

 ^-m = - i 



p p looda 



In the same way as before we arrive at the expression 

 &p n 



~^ := F ' 



The meaning of this equation may be illustrated by 

 answering the question, ' What fall of vapour pressure does 

 i / o of sugar produce in water at 100 ? ' Here p = 760 mm. 

 and 



i 100 i 



n : 



-- -- - 



342 i 8 1900 



so that 



760 



- = 

 1900 



Abnormal values for abnormal vapour densities. There 

 is another essential point to be noticed, viz. that N, 

 the so-called number of molecules of solvent used in the 

 deduction, is not always the real number ; rather N is the 

 fictive number of molecules obtained by taking the mole- 

 cular weight of the saturated vapour at the temperature 

 considered as unity. If the vapour, then, has an abnormal 

 density, the abnormal molecular weight so obtained is to 



