24 PHYSICAL CHEMISTRY 



electrode gives the transport number of the ion. By following 

 the above reasoning backward, the ionic speed may be calculated 

 (Hittorf). 



These values of ionic speed are relative. The actual speed 

 of ions is slow, as is shown by electrolyzing a solution containing 

 colored anions around the cathode, and observing the time neces- 

 sary for them to reach the anode. 



The relative speeds of ions may be more conveniently deter- 

 mined by comparing the molecular conductivities of a series of 

 salts that are completely dissociated. The molecular conductivity 

 of a salt is proportional to the sum of the ionic speeds of anion 

 and cation (v-j-u). Thus, the ratio of the molecular conductiv- 

 ities of NaCl and KC1 is the ratio of the ionic speeds of Na 

 and K. 



The relative ionic speeds at i8° according to Kohlrausch 

 (1907) are as follows: 



H 318. OH 174. With organic ions, the longer 



In general we may divide electrolytes into two classes, the 

 strong and the weak. Strong electrolytes are greatly dissociated, 

 whereas weak electrolytes are poorly dissociated. So-called non- 

 electrolytes may be to a minute extent dissociated. Water is 

 a non-electrolyte; its ions are the most rapid, but it is poorly 

 dissociated. Whereas the concentration of water is about 55 

 formal (55 X 18 g per liter), only io _r mol is dissociated at 22°. 

 [Owing to the partial association of H 2 into (H 2 2 ) its con- 

 centration is less than 55 mol.] 



Strong electrolytes have not yet been brought entirely under 

 the law of mass action. Weak electrolytes obey the law, and 

 therefore their ions must obey it. Take, for example, the disso- 

 ciation of acetic acid at different dilutions. The equation for 



