106 SMITH'S INTERMEDIATE CHEMISTRY 



Similar confirmation of the formulae H 2 and Cl2 will be found 

 on p. 148. 



Molecular Formulas of Compounds. The need of atten- 

 tion to making our formulae molecular comes out also in the cases 

 of many compounds. Thus, formaldehyde (a disinfectant) has 

 the composition CH 2 0, and its molecular weight is 30, so that 

 CH 2 O (= 12 + 2 + 16) is the correct formula. But acetic acid 

 (the sour substance in vinegar) has the same composition, CH 2 0, 

 only its molecular weight is 60, and the formula is therefore written 



For gaseous and volatile substances the correct molecular for- 

 mulae are always used. Thus, for phosphorus pentoxide (p. 72), 

 P 4 0io is preferred to P 2 O 3 because the molecular weight of the 

 substance in the state of vapor is 284 and not 142. 



The correct equation for reaction 5, p. 72, is therefore: 



P 4 + 5O 2 - P 4 Oi . 



Only by bearing in mind the true molecular formulae can we 

 include the volume and molecular proportions of the reacting sub- 

 stances in the vapor state, as well as their weight and atomic 

 proportions (p. 100), in our condensed statement of a reaction. 



Often, however, when a reaction takes place wholly between 

 solids and liquids, we use for convenience the simplest possible 

 formulae throughout. Thus for the combination of iron and sul- 

 phur (p. 15) we write: Fe + S -* FeS. 



Warnings. Always place the formulae of the products on the 

 right-hand side of the equation, and the formulae of the reacting 

 substances on the left. 



Point the arrow in the direction of the reaction ; that is, towards 

 the products. 



Use the molecular formulae for elementary substances (0 2 , H 2 , 

 N 2 , etc.). The molecular formulae, when they are known, are the 

 only ones given in the text. The symbols of the elements, as 



