REACTIONS IN HOMOGENEOUS SYSTEMS 35 



In other words the electrical conductivity does not diminish 

 so rapidly as the concentration of the solution, therefore the 

 ratio given by the electrical conductivity divided by the num- 

 ber of molecules in the solution increases as the solution is 

 made more dilute. The molecular conductivity at a given 

 concentration divided by the molecular concentration at 

 infinity, that is when complete dissociation occurs, gives the 

 same value that is obtained by calculating the degree of 

 dissociation from the osmotic pressure of the solution, i.e. 

 conductivity^ any concentration ^ d o f dissociation 



conductivity at infinity 



or ~ = A x . 



A oo 



The ionic dissociation theory thus outlined explains many 

 facts both qualitatively and quantitatively which are difficult 

 to explain on any other hypothesis. 



It has been found that different ions conduct electricity 

 to a greater or less extent and the difference is ascribed to 

 the rate of movement of the various ions. The conductivity 

 of a solution is the algebraical sum of the conductivity of the 

 various ions in the solution. 



TABLE VII.* 

 Table of Ionic Velocities. 



H.' K.' Na.' OH.~* Cl." a*. 

 18 313 64-4 42-7 174 65-2 67 

 25 34<> 73-8 50*5 196 75'2 77 



Substances which in moderately concentrated solution are 

 nearly completely dissociated into ions are called strong 

 electrolytes whilst those which are only slightly dissociated 

 are called weak electrolytes. The similar behaviour of differ- 

 ent substances which by dissociation can- set free a common 

 ion is thus explained : all chlorides set free chlorine 

 ions and all acids hydrogen ions. The reactions of a given 

 substance may depend on the ions formed from it or on the 

 undissociated molecules. 



The strength of an acid will depend upon the degree of its 

 dissociation into ions as the more dissociated the acid the 

 greater the number of hydrogen ions (H + ) for the same con- 



* Abbreviated from Oswald Luther, Physiko-Chemische Messungen, 

 Leipzig, 1910, p. 481. 



