ELEMENTARY ANALYSIS. 447 



The best method for determining the actual number of atoms con- 

 tained in the molecule is the determination of the specific weight of 

 the gaseous compound, taking hydrogen as the unit. For instance : 

 Assume the analysis of a liquid substance gave the following result : 



Carbon 92.308 per cent. 



Hydrogen 7-692 " 



100.000 



From this result the empirical formula, CH, is deduced by apply- 

 ing the method stated above. If this formula were the molecular 

 formula, the density of the vapors of the substance would, when com- 

 pared with hydrogen (according to the law of Avogadro), be equal to 

 6.455, because a molecule of hydrogen weighs 2 and a molecule of the 

 compound CH weighs 12.91. 



Suppose, however, the density of the gaseous substance is found to 

 be 38.73, then the molecular formula would be expressed by C 6 H 6 , 

 because its molecular weight (6 X 11.91 -f 6 X 1) is equal to 77.46, 

 which weight, when compared with the molecular weight of hydrogen 

 = 2, gives the proportions 77.46 : 2, or 38.73 : 1. 



Not all organic compounds can be converted into gases or vapors 

 without undergoing decomposition, and the determination of the 

 molecular formulas of such compounds has to be accomplished by 

 other methods. If the substance, for instance, is an acid or a base, 

 the molecular formula may be determined by the analysis of a salt 

 formed by these substances. For instance : The empirical formula of 

 acetic acid is CH 2 O ; the analysis of the potassium acetate, however, 

 shows the composition KC 2 H 3 O 2 , from which the molecular formula 

 HC 2 H 3 O 2 is deduced for acetic acid. 



In many cases, however, it is as yet absolutely impossible to give 

 with certainty the molecular formula of some compounds. 



Rational, constitutional, structural, or graphic formulas. 

 These formulas are intended to represent the theories which have 

 been formed in regard to the arrangement of the atoms within the 

 molecule, or to represent the modes of the formation and decom- 

 position of a compound, or the relation which allied compounds bear 

 to one another. 



The molecular formula of acetic acid, for instance, is C 2 H 4 O 2 , but 

 different constitutional formulas have been used to represent the 

 structure of the acetic acid molecule. 



Thus, H.C 2 H 3 O 2 is a formula analogous to H.NO 3 , indicating that 

 acetic acid (analogous to nitric acid), is a monobasic acid, containing 

 one atom of hydrogen, which can be replaced by metallic atoms. 



