ELECTROLYTIC DISSOCIATION 249 



measurements upon the osmotic pressure, the freezing 

 point, and the boiling point of such solutions. 



The amount of the dissociation may be conveniently 

 calculated from such observations. Suppose that there 

 are N molecules of solute and that a fraction a of these 

 are rendered abnormal by dissociation. Then there 

 are (la)N normal molecules and aN dissociated 

 molecules of the solute. Let n represent the number 

 of ions into which each dissociated molecule has sepa- 

 rated, then the total of molecular particles introduced 

 into the solution by the solute is not N but is 



(l-a)N+naN=N(na-a+l) = N[(n-l)a+l] (1) 



A study of the conductivity of an electrolytic solu- 

 tion enables us to determine a in a direct and very con- 

 venient manner. If the two electrodes of such an elec- 

 trolytic cell as was described above are maintained at 

 a constant potential difference while the concentration 

 in moles per liter of the solution is altered, it is found 

 that the current varies, indicating a variation in the 

 ability of the solution to conduct electricity. The 

 conductivity is the ratio of the current in amperes to 

 the potential difference in volts. A solution would 

 then have unit conductivity if it carried electricity at 

 the rate of one ampere l under a potential difference 

 of one volt. 



For any given electrolyte the conductivity will de- 

 pend upon the dissociation a. At great dilution we may 

 consider a to be unity. Or, as it is usually expressed, 

 the dissociation at infinite dilution is complete. Under 

 these conditions, the ions, which are formed by the ac- 

 1 One coulomb per second. Cf. p. 178. 



