180 CHEMICAL DISCOVERY AND INVENTION 



A glance at these figures is sufficient to show that the de- 

 pression produced by 1 gram of the substance is inversely as the 

 molecular weight, and hence that the product of the two is a 

 constant. Of course the product varies a little qwing to experi- 

 mental difficulties, but the rule has been since well established 

 by the results of hundreds of experiments made by other chemists 

 in all the laboratories in the world. And in fact determinations 

 of molecular weights by observation of the freezing-point of 

 solutions is now one of the commonest and most useful operations 

 in the course of research into the composition and character of 

 new compounds of all kinds. 



Suppose the weight P in grams of such a substance as sugar is 

 dissolved in 100 grams of water and the number of degrees below 

 at which the solution begins to freeze be expressed by C. 



C 



Then ^ will represent the depression which would be produced 



by 1 gram of substance dissolved in 100 grams of water. If this 

 expression is multiplied by the molecular weight of the substance 

 dissolved, in this case sugar, the product is the depression which 

 would be theoretically produced by the molecular weight M in 

 grams of sugar dissolved in 100 grams of the solvent. This 

 cannot be directly determined as it is necessary to work with 

 rather weak solutions which give a depression of only 1 or 2 C. 

 This molecular depression may be expressed by K, and is equal 



to 5 X M. 



When different substances of the same neutral character as 

 sugar are dissolved in the same solvent the value of K is found 

 to be the same. Hence a method is provided whereby if K is 

 known for the solvent chosen, and for compounds analogous to 

 the one under investigation, the molecular weight of the latter 

 can be determined, or rather, from the possible molecular weights 

 that value is chosen which comes nearest to the value of M in 



PxK 



the formula M= ~ . The solvents most commonly used are 

 u 



water, acetic acid, and benzene, for which the values of K now 

 adopted are respectively 19, 39, and 49. 



The method outlined here is the most accurate and the most 

 commonly used method for determining the molecular weight of 

 soluble substances which are not volatile without decomposition 

 and of which the vapour density cannot therefore be determined. 



