100 PRINCIPLES OF CHEMISTRY. 



To establish a molecular formula requires a careful quantitative determina- 

 tion of the proportions of the constituents of a compound; conversely, if we 

 know the molecular formula of a compound, we can calculate from it the rela- 

 tive quantities of the elements, or the percentage composition of the com- 

 pound. Also, if we know the formula, and the weight of one constituent, we 

 can calculate the weight of the other constituents, and the total weight of the 

 compound. Let us consider the red oxide of mercury, which has the molec- 

 ular formula, HgO. In the molecule there is an atom of mercury weighing 

 198.5 times as much as an atom of hydrogen, and an atom of oxygen weighing 

 15.88 times as much as an atom of hydrogen. It is evident that the weights 

 of the mercury and oxygen in a molecule are to each other as 198.5 : 15.88. 

 What is true of one molecule must be true of any number of molecules, that 

 is, of any quantity of oxide of mercury; hence we can conclude that in the 

 oxide there are 198.5 parts of mercury to every 15.88 parts of oxygen, making 

 198.5 + 15.88 = 214.38 parts of oxide. To calculate the percentage composi- 

 tion, or parts per hundred, we use the proportions, 



214.38 HgO : 198.5 Hg : : 100 HgO : x Hg and 

 214.38 HgO : 15.88 O : : 100 HgO : x O. 



Of course, in all cases where there are only two constituents, and we find the 

 per cent, of one, the other need not be calculated. 



If we had a quantity of oxide of mercury that we knew contained, say, 30 

 parts of mercury, and we wanted to know the weight of the oxygen present or 

 the total weight of oxide, the following proportions would give us the answers: 



198.5 Hg : 15.88 O : : 30 Hg : x O. 

 198.5 Hg : 214.38 HgO : : 30 Hg : x HgO. 



We learn from the discussion above that the elements enter into combina- 

 tion in quantities represented by their atomic weights, or multiples of these, 

 to produce a quantity of compound represented by the molecular weight. 



The law of chemical combination by volume, or the Law of 

 Gay-Lussac, may be stated as follows : When two or more gaseous 

 constituents combine chemically to form a gaseous compound, the volumes 

 of the individual constituents bear a simple relation to the volume of the 

 product. The law may be divided into two laws, thus : 1. Gases 

 combine by volume in a simple ratio. 2. The resulting volume of 

 the compound, when in the form of a gas, bears a simple ratio to the 

 volumes of the constituents. For instance : 1 volume of hydrogen 

 combines with 1 volume of chlorine, forming 2 volumes of hydro- 

 chloric acid gas ; 2 volumes of hydrogen combine with 1 volume of 

 oxygen, forming 2 volumes of water-vapor ; 3 volumes of hydrogen 

 combine with 1 volume of nitrogen, forming 2 volumes of ammonia. 



If the different combining volumes of the gases mentioned are 



