Question

State the Kohlrausch’s law of independent migration of ions. Why does the conductivity of a solution decrease with dilution?

Answer 1

Hint:Independent migration of ions means that each individual ion migrates on its own independent of its co-ion at infinite dilution and it makes its own contribution to total molar conductivity of the electrolyte irrespective of the nature of the other ion with which it is being associated.

Complete answer:
Kohlrausch’s law of independent migration of ions states that equivalent conductivity of an electrolyte is equal to the sum of conductance of anions and cations at infinite dilution.
The molar conductivity of any solution at a given concentration refers to the conductance of volume of solution consisting of one mole of electrolyte placed between the two electrodes with unit area of cross-section and unit distance length. Molar conductivity of any solution increases with decline in the concentration owing to the increase in total volume consisting of one mole of electrolyte. When the concentration of electrolyte approaches zero, molar conductivity is called limiting molar conductivity, ${E_m}^ \circ$. Important applications of Kohlrausch law are:
1. The Kohlrausch law of independent migration of ions helps in determining limiting molar conductivities for the electrolyte. Weak electrolytes possess lower molar conductivities as well as lower degree of dissociation at higher concentrations.
2. Kohlrausch law of independent migration of ions also helps in the determination of the value of dissociation constant from molar conductivity and limiting molar conductivity of weak electrolyte at a specified concentration.
\[\alpha = \Lambda {E_m}^ \circ \]
Here $\alpha$ is dissociation constant, $\Lambda$ is molar conductivity and ${E_m}^ \circ$ is limiting molar conductivity
The conductivity of a solution decreases with dilution owing to the presence of less ions available for conduction.

Note:
Kohlrausch’s law is generally used for the estimation of degree of dissociation, solubility of sparingly soluble salt, dissociation Constant for weak electrolytes and molar Conductivity of weak electrolytes at infinite dilution.