52 MOLECULAR WEIGHT AND POLYMERISM 



molecular weight obtaining in the saturated vapour), so 

 now we have 



As 7i 



where s and As are the solubility and decrease of solubility 

 respectively, n and N the number of dissolved and dissolv- 

 ing molecules (the latter calculated from the molecular 

 weight obtaining in saturated solution, here therefore 74, 

 the molecular weight that ether gives in saturated aqueous 

 solution). Thus 



As : s = (6-1 1 4-86) : 34-967 = 0-0358 



0-00^2 1-88 



n:N= ^~ : -. ^ ^ ^ = 0.0358. 

 x 7 4 (=C 4 H 10 0) 



The value of x (molecular weight of benzene) so found 

 would be nearer to the expected value if we took into 

 account the solubility of water in ether, and the alteration 

 in that solubility brought about by addition of benzene. 

 Thus at the temperature of the experiment some 1 1 molecules 

 of water are dissolved in 100 of ether. 



On this account the so-called molecular displacement 



As 

 m = 



g 



should be determined by means of a body of known mole- 

 cular weight, and for a given mass of water and tempera- 

 ture, g being the mass of substance dissolved always in the 

 same mass of ether. Then a molecular weight desired may 

 be deduced from the value of C. 



Example : 0-0655 gram of benzene effected a displacement 

 of 0-45 cm.; the molecular weight of benzene is 78 (C 6 H G ), 

 so that 



0-0655 

 Then 0-1266 gram of naphthalene was taken, giving the 



