498 



Transactions. — Chemistry and Physics. 



Raoult's law states that the depression in the freezing- 

 point of a given solvent is directly proportional to the concen- 

 tration of the solution and inversely to the molecular weight 

 of the dissolved substance — i.e., 



D cc 



IV 



WM' 



ft 



where D = depression in freezing-point, W = weight of the 

 solvent, w = weight of the dissolved substance, and M = mole- 

 cular weight of the dissolved substance. So that, if K repre- 

 sent the depression which the molecular weight of any sub- 

 stance (in grams) will cause in 100 grams of the solvent, 



-. w x 100 T ^ 

 M = WD K « 



This method of determining molecular weights is in every-day 

 practice amongst research chemists, giving good results even 

 for substances with very high molecular weights. With such 

 substances the observed depression is so small that an exceed- 

 ingly sensitive, and therefore expensive, thermometer is re- 

 quired. For class purposes, however, we must make the 

 depression large enough to be easily registered on a common 

 thermometer. This is easily done by choosing a solvent 

 whose depression constant (K) is large, and dissolving in it 

 some substance whose molecular weight is small. 



Now, of all common substances water has 

 the lowest molecular weight, whilst phenol has 

 the highest depression constant (72) ; indeed, 

 1 per cent, of water depresses the melting-point 

 of phenol about 4° C. 



The apparatus needed is illustrated in the 

 figure. As it consists onlv of a test tube, com- 

 mon centigrade thermometer, cork, and brass- 

 wire stirrer, no explanation is necessary. 



To perform the experiment about 10 grams of 

 good carbolic acid is weighed into the test tube, 

 thoroughly melted by immersing for a few 

 moments in hot water, and the freezing-point 

 determined by thoroughly stirring until the 

 superfused liquid begins to crystallize and the 

 temperature indicated by the thermometer be- 

 comes steady. This operation should, of course, 

 be repeated. About 01 gram of water is now 

 added to the carbolic acid in the tube, and the 

 freezing-point again determined. The water is 

 conveniently added from a dropping-pipette, the 

 number of drops being carefully counted, and the number of 

 drops which make up a cubic centimeter being determined in 

 a separate experiment. 



; 



