Identification of Organic Compounds. 43 



dissolved in 100 grammes of a liquid, from the deter- 

 mination of the depression of the freezing point of any 

 given solution of a substance of known molecular weight. 

 Suppose the solute taken is glucose, which has the formula 

 C 6 H 12 6 , and the solvent is water. Now the molecular 

 weight of glucose is 342 (that is, 6x12 + 12x1 + 6x16). 

 Suppose, furthermore, that 0'6 gramme of the substance 

 when dissolved in 100 c.c. produces a depression of 0'035. 

 Then the depression produced by dissolving 342 grammes 



342 

 in 100 c.c. should (theoretically) be 0'035 x -^p 19'9. 



This number is the so-called " molecular depression " of a 

 substance ; it is a constant for every given solvent which 

 can always be determined experimentally by ascertaining 

 the depression of freezing point of that solvent produced 

 by dissolving in it a given (not too large) amount of a 

 substance of known molecular weight. When the molec- 

 ular depression, k, of a solvent has once been determined, 

 it can be used for the estimation of molecular weights of 

 other substances which dissolve in it.* The method of 

 calculation is very simple, for if d be the depression 

 produced when w grammes of a solute are dissolved in W 

 grammes of the solvent, and m = the molecular weight of 



w 



the solute, then m k -7^ x 100. 

 dW 



The method employed for measuring the elevation of a 

 boiling point of a solvent by a dissolved substance is very 

 similar. Many forms of apparatus have been employed, one 

 of which is illustrated in the accompanying figure (Fig. 17). 

 The solvent in weighed amount (w) is introduced into a 

 tube A, with a side tube carrying a condenser. The Beck- 

 mann thermometer is inserted into the mouth of the tube A, 



* There are certain exceptions, especially when water is employed. Salts 

 give anomalous molecular depressions when dissolved in water. 



