ELEMENTAR Y ANAL YSIS. 285 



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. 



CaHgO.OH 1 is a formula indicating that acetic acid is composed of 

 two univaleut radicals which may be taken out of the molecule and 

 replaced by other atoms or groups of atoms. This formula indicates 

 also that acetic acid is analogous to hydroxides, the radical C 2 H 3 O 

 having replaced one atom of hydrogen in H 2 O. 



CH 3 .CO 2 H l is a formula indicating that acetic acid is composed of 

 the two compound radicals, methyl and carboxyl. 



It may be said finally, that quite a number of other rational 

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

 different chemists and at different times, to represent the structure of 

 acetic acid, but it should be remembered that these formulas are not 

 intended to represent the actual arrangement of the atoms in space, 

 but only, as it were, their relative mode of combination, showing 

 which atoms are combined directly and which only indirectly, that 

 is, through the medium of others. 



QUESTIONS. 371. What is organic chemistry, according to modern views? 

 372. Mention the chief four elements entering into organic compounds, and 

 name the elements which may be made to enter into organic compounds by 

 artificial processes. 373. State the reason why the four elements, carbon, 

 hydrogen, oxygen, and nitrogen, are better adapted to form a large number of 

 compounds than most other elements. 374. State the general properties of 

 organic compounds. 375. Why does a qualitative analysis of an organic com- 

 pound, in most cases, not disclose its true nature ? 376. By what test may the 

 organic nature of a compound be established ? 377. By what tests may the 

 presence of carbon, hydrogen, and nitrogen be demonstrated in organic com- 

 pounds? 378. State the methods by which the elements carbon, hydrogen, 

 oxygen, sulphur, and phosphorus are determined quantitatively. 379. By 

 what general method may a formula be deducted from the results of a quanti- 

 tative analysis ? 380. What is meant by an empirical, molecular, and consti- 

 tutional formula; how are they determined; and what is the difference between 

 them ? 



