ELECTROLYTIC DISSOCIATION 251 



The value of n, the number of ions into which a 

 molecule of the solute dissociates, is obtained from a 

 study by weight of the substances liberated as gases or 

 deposited on the electrodes, when different solutions 

 transfer equal quantities of electricity. In the case 

 of hydrochloric acid, a molecule dissociates into one 

 anion and one cation and n = 2. Each of these ions 

 serves to transfer one electron. Whatever the division 

 may be, the quantity of electricity transferred by each 

 ion must be an integral number of electrons. 



Other types of dissociation are possible giving higher 

 values of n. Thus n may be 3, as is the case for barium 



hydroxide Ba(OH) 2 which separates into the cation, 



++ 



Ba, deficient by two electrons, and two anions of the 



composition, OH, each with a single excess electron. 

 The converse may also happen ; for example sulphuric 



acid, H 2 S0 4 , separates intojbwo cations of H and a 

 single anion of the form SO 4 which carries two extra 

 electrons. 



In the case of those elements which have more than 

 one valence, ions may be formed differing in their 

 number of electrons, depending upon the compound 

 from which they are derived. Thus iron may be 



divalent and form the ferrous chloride FeCl 2 , in which 



++ _ 



case the ions are Fe and two Cl ions. On the other 



hand, in ferric chloride FeCl 3 the iron atom is trivalent, 



and this molecule in solution splits into an iron ion 



+++ 



Fe, which is deficient by three electrons, and three 



ions of chloride, Cl, each with an extra electron. For 

 this case n=4. 

 The same element may appear in one substance as 



