THE PRINCIPLES OP OKGANIC CHEMISTRY 43 



Similarly in every 18 parts H^O there are 2 parts H ; 

 therefore in 012 grm. HgO there will be :— 



0'12 X 2 „ „, 



— jg = 0013 parts H 



Together the C and H make up 0-079 + 0-013 = 0-092 of the 

 total weight, 0-2 grm., of substance taken ; the remainder, 

 O'lOS, is assumed to be oxygen. 



Converting these proportions to percentages we have : — 



»»™ X ™ = 39-5 per cent. c„b„.. 



0-2 

 0013 X 100 



0-2 

 0-108 X 100 



= 6-5 „ „ hydrogen. 

 = 54-0 „ „ oxygen. 



0-2 



From the percentage composition we can readily calculate 

 the empirical formula of the substance, i.e., the simple ratio of 

 the number of atoms of each element to each other, by cal- 

 culating how many times 12 parts by weight of carbon, 

 1 part by weight of hydrogen or 16 parts of oxygen, etc., 

 are contained in the percentage amounts, viz. : — 



=3-3 parts of carbon. 



12 ^ 



6-5 



— = 6-5 parts of hydrogen. 



= 3-4 parts of oxygen. 



The lowest ratio of these numbers, i.e., the empirical 

 formula, is obviously CHgO, the slight errors in the experi- 

 mental results being neglected. 



In order now to determine the molecular formula of a com- 

 pound we need to know its molecular weight ; this is readily 

 obtained by determining the weight of a known volume of its 



