284 CONSIDERATION OF CARBON COMPOUNDS. 



of the composition of a substance. In the above case, the formula 

 C 6 H 10 O 5 would be the empirical formula. It might, however, be 

 possible that this formula did not represent the actual number of 

 atoms in the molecule, which might contain, for instance, twice or 

 three times the number of atoms given, in which case the true com- 

 position would be expressed by the formula C 12 H 20 O 10 or C 18 H 30 O 15 . 



If it could be proven that one of the latter formulas is the correct 

 one, it would be termed the molecular formula, because it expresses 

 not only the numerical relations existing between the atoms, but also 

 the absolute number of atoms of each element contained in the 

 molecule. 



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 deducted 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 

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

 compound CH weighs 13. 



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

 be 39, then the molecular formula would be expressed by C 6 H 6 , 

 because its molecular weight (6 X 12 + 6 X l)is equal to 78, which 

 weight, when compared with the molecular weight of hydrogen = 2,, 

 gives the proportions 78 : 2, or 39 : 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 deducted for acetic acid. 



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

 with certainty the molecular formula of some compounds. 



