446 CONSIDERATION OF CARBON COMPOUNDS. 



Determination of atomic composition from results obtained 

 by elementary analysis. The elementary analysis gives the quan- 

 tity of the various elements present in percentages, and from these 

 figures the relative number of atoms may be found by dividing the 

 figures by the respective atomic weights. For instance : The analysis 

 above mentioned gave the composition of a compound, as carbon 

 44.444 per cent,, hydrogen 6.213 per cent., and oxygen 49.343 per 

 cent. By dividing each quantity by the atomic weight of the respec- 

 tive element, the following results are obtained : 



11.91 



= 3.731 



La 



15.88 



49 ' 34; =3.107 



The figures 3.731, 6.213, and 3.107 represent the relative number 

 of atoms present in a molecule of the compound examined. In order 

 to obtain the most simple proportion expressing this relation, the 

 greatest divisor common to the whole has to be found, a task which 

 is sometimes rather difficult on account of slight errors made in the 

 quantitative determination itself. In the above case, 0.6213 is the 

 greatest divisor, which gives the following results : 



3.731 6.213 _ . 3.107 



0.6213 ' 0.6213 ' 0.6213 



The simplest numbers of atoms are, accordingly, carbon 6, hydrogen 

 10, oxygen 5, or the composition is C 6 H 10 O 5 . 



Empirical and molecular formulas. A chemical formula is 

 termed empirical when it merely gives the simplest possible expression 

 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. 



