42 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



D. DETERMINATION OF THE MOLECULAR WEIGHT. 



As will be seen later, several organic compounds can have the same 

 empirical formula, thus, for instance, lactic acid C 3 H 6 O 3 and glucose 

 C 6 H 12 O 6 have the same empirical formula, namely CH 2 O. 



In order to ascertain which of these formulae is the correct one, a 

 molecular weight determination is carried out, i.e. the weight of the 

 molecule of the substance compared with that of an atom of hydrogen 

 (Avogadro's law). 



The methods employed to determine the molecular weight are of 

 two kinds : (a) physical, (fr) chemical. 



(a) Physical Methods. 



I. Victor Meyer's Method. Of the physical methods, that by 

 Victor Meyer is the most frequently used when the substance can be 

 vaporised without decomposition. A known weight of the substance 

 is converted into vapour at a temperature 40-50 above its boiling- 

 point in a special apparatus. The air previously contained in the 

 apparatus is displaced by the vapour, collected in a graduated cylinder 

 and its volume measured ; this volume, after making corrections for 



temperature and pressure, corresponds to that 



occupied by the substance. 



Thus, if v c.c. are given by w grammes substance, 



, w x 22,400 

 .*. 22,400 c.c. are given by 



= mol. wt. 



The apparatus employed is shown in the 

 accompanying Fig. 21. A liquid, boiling 40-50 

 above the temperature at which the substance is 

 volatilised, is boiled in the round bulb of the 

 outer vessel. As soon as the temperature is 

 constant and no more air escapes from the inner 

 vessel by the side tube, the inverted graduated 

 cylinder, filled with water, is placed over the end 

 of the side tube, the cork is removed and a 

 known weight of substance, contained in a 

 small glass vessel, is dropped through the open- 

 ing into the inner vessel and the cork is quickly 

 FIG. 21. replaced The substance is rapidly vaporised 



and the vapour displaces an equal volume of air, which is driven out 

 and collected and measured in the graduated cylinder. 





