Conductometric methods:

Along With this background now we will see how dissociation of a weak electrolyte could be determined by using conductometric methods. The Ostwald derived a relationship among the molar conductivity and limiting molar conductivity. The molar conductivity of weak electrolyte can be expressed as the product of degree of dissociation of the electrolyte and it's limiting molar conductivity.

Λ_m = αΛ^∝       or α   = Λ_m/Λ ^∝                                                                

 This relationship is known as Ostwald relation. Substituting this in Eq. 6.3 gives.

 K_a = ((Λ_m/ Λ ^∝ )² c)/1 - (Λ_m/ Λ^∝ )

Rearrange Eq. (6.10) gives

Λ_m c= K _a [(Λ ^∝ )²/ Λm] -K a  Λ^∝                                                     

When we plot Λ mc against 1/ Λ _m, we will get a linear plot with slope equal to K_a ( Λ ^∝ )². Therefore, K_a can be determined provided Λ^∝ is known. Therefore, we could use this method for the determination K_a. of weak acids and bases.  To further understand this, let us consider the dissociation of nitric acid in methanol over a wide range of concentration (see Table 6.1). In methanol nitric acid acts as a weak electrolyte and therefore, we can use Eq. (6.12) to determine the dissociation constant K_a.

Table:  Molar Conduvtivity values of Nitric Acid in Methanol