Kohlrausch’s law

Aug 26, 2022, 16:45 IST

About Kohlrausch law

EFFECT OF DILUTION ON CONDUCTANCE

(i) When we increase the dilution of electrolyte, magnitude of specific conductance decreases, because number of ions in 1 cm3 volume of electrolyte decreases.

(ii) When dilution of electrolyte increases, magnitude of molar conductance and equivalent conductance increases, because number of ions increases after dilution.

(iii)The molar conductivity of strong electrolytes is found to vary with concentration according to the equation given in the following pdf .

The maximum ( limiting) value of equivalent conductivity ( value obtained at infinite dilution  or zero concentration) for a strong electrolyte can be obtained by extrapolating the equivalent conductance graph to zero concentration.  

Kohlrausch’s law

However, in weak electrolytes the value of equivalent conductance at infinite dilution () or at zero concentration (0) cannot be obtained by extrapolating the concerned graphs. The value of for weak electrolytes is actually obtained by an indirect method based on Kohlrausch’s law.  According to this law “the equivalent conductance of an electrolytes at inifinite dilution is the sum of two values , one depending upon the cation and the other upon the anion”  

Application of Kohlrausch’s Law

(i) Determination of   of a weak electrolyte:In order to calculate   of a weak electrolyte say CH3COOH, we determine experimentally   values of the following three strong electrolytes:

(a) A strong electrolyte containing same cation as in the test electrolyte, say HCl

(b) A strong electrolyte containing same anion as in the test electrolyte, say CH3COONa

(c) A strong electrolyte containing same anion of (a) and cation of (b) i.e. NaCl.

(ii) Determination of degree of dissociation ():

(iv) Determination of ionic product of water:

Find solved example of Ko​hlrausch’s law.