244 CONSIDERATION OF CARBON COMPOUNDS. 



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 1) 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 accom- 

 plished 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 0; the analysis of 

 the potassium acetate, however, shows the composition KC 2 H 3 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 the molecular formula of a compound with certainty. 



Rational, constitutional, structural, or graphic formulas. These 

 formulas are intended to represent the theories which have been 

 formed in regard to the relative arrangement of the atoms 

 within the molecule, or to represent the modes of formation and 

 decomposition of a compound, or the relation which allied com- 

 pounds 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. 



CgHgOMIO 1 is a formula indicating that acetic acid is com- 

 posed of two univalent radicals which may be taken out of the 

 molecule and replaced by other atoms or groups of atoms. 

 This formula also indicates that acetic acid is analogous to 

 hydrates, the radical C 2 H 3 having replaced one atom of hydro- 

 gen in H 2 O. 



CH^.COgH 1 is a formula indicating that acetic acid is com- 

 posed of two other compound radicals. 



It may be finally said, that quite another number of rational 

 formulas has been applied, or, at least, has been proposed by 



