188 SMITH'S INTERMEDIATE CHEMISTRY 



contains 6.06 X 10 23 molecules. If ionization in solution were 

 complete, therefore, a liter of a normal solution (p. 121) of hydro- 

 chloric acid would contain 6.06 X 10 23 hydrogen ions, and the 

 same number of chlorine ions. These, moving at the rates given 

 above, would give a definite, calculable conductivity. The 

 conductivity actually obtained by experiment with a normal 

 solution of HC1 at 18, however, is only 78 per cent of this cal- 

 culated value. The conclusion has been drawn that, in normal 

 solution at 18, hydrochloric acid contains only 78 per cent of 

 free ions, the remaining 22 per cent of solute contributing nothing 

 towards the conductivity. We may express this conclusion by 

 writing the reversible ionic dissociation in the form: 



(22%) HC1 ^ H+ + Cl- (78%). 



Equilibrium is reached in a normal solution of hydrochloric acid 

 at 18, therefore, when 78 per cent of the molecules are broken 

 up into free ions. 



At other concentrations different degrees of dissociation would 

 be indicated. For example, a 10 N solution gives only 17 per 

 cent of the calculated conductivity, a 0.1 TV solution 92 per cent, 

 a 0.01 N solution 97 per cent. At very high dilutions, therefore, 

 the ionization becomes practically complete. 



All electrolytes are not ionized to equal extents at the same 

 concentrations. Thus, a normal solution of acetic acid at 18 

 shows only 0.4 per cent of the calculated conductivity for com- 

 plete ionization. Even a 0.001 N solution is only 12.5 per cent 

 ionized. A fundamental point in the ionic hypothesis of Ar- 

 rhenius, however, is that ionization always approaches com- 

 pletion as the solution becomes more and more dilute. 



The following table shows the approximate degrees of ioniz- 

 ation of a number of typical electrolytes in tenth-normal solution 

 in water at 18. In the case of acids and bases containing more 

 than one displaceable unit of hydrogen or hydroxyl, the kind of 

 ionization on which the figure is based should be particularly noted. 



