HYDROCHLORIC ACID. CALCULATIONS 133 



(p. 104) is an action which belongs to both of the first two classes, 

 because it is reversible. For example, 



2H 2 <= 2H 2 + 2 . 



Again oxidation (p. 40) and reduction (p. 57) are connected with 

 the particular substances, such as oxygen, which are concerned 

 in the action. The first classification (Nos. 1 to 4) paid no at- 

 tention to the kinds of elements which were present. Thus, 

 every decomposition is a decomposition. If it is reversible, then 

 it is also a dissociation. If oxygen is set free, then it is a reduction 

 os well. 



CALCULATIONS 



Calculations Connected with Formulae. In a previous 

 chapter (p. 99) we have seen that formulae represent the com- 

 position of substances, and we have seen how the formula of 

 each substance is worked out from the data obtained by experi- 

 ment. Some ways in which the information contained in formulae 

 can be used may now be taken up. 



The Composition from the Formula. Take, for example, 

 the formula for silver nitrate, AgN0 3 . To learn the composition 

 of this compound, we look up the atomic weights (see rear cover 

 of this book). We find Ag = 107.88 parts of silver, N = 14.008 

 parts of nitrogen, O 3 = 3 X 16 or exactly 48 parts of oxygen. 

 The proportions of the constituents, in the same order, there- 

 fore, are 107.88 : 14.008 : 48. 



What is the proportion of oxygen to nitrogen alone? It is 

 48 : 14.008, or 3.427 : 1. 



Significant Figures. The division of 14.008 into 48 really 

 gives the quotient 3.426613. But no atomic weights have been 

 measured so accurately that we know the values of the numbers 

 beyond the third place of decimals. For many elements, we do 



