Question

Define conductivity and molar conductivity of an electrolyte solution. Discuss the change in their value with the change in concentration of electrolyte.

Hint:

Conductivity = conductance of 1 cm³ of solution (S cm^-1); molar conductivity \(\Lambda_m = \kappa \times 1000 / C\) (S cm² mol^-1). On dilution kappa falls but molar conductivity rises.

Answer 1

Step 1: Conductivity (specific conductance) is the conductance of a unit cube of the solution, 1 cm on each side. It tells how easily current passes through that fixed volume of solution. SI unit S m^-1, common unit S cm^-1.
Step 2: Molar conductivity ties conductivity to the amount of electrolyte. We divide the conductivity by the molar concentration so that the same one mole of electrolyte is compared at every dilution: \(\Lambda_m = \dfrac{\kappa}{c}\), with the working unit S cm² mol^-1.
Step 3: Why conductivity falls on dilution. Conductivity counts the ions inside a fixed 1 cm³. Adding water spreads the same ions over a larger volume, so fewer ions sit in each cm³ and the value drops.
Step 4: Why molar conductivity rises on dilution. Molar conductivity always follows one mole of electrolyte however much water is added. As water increases, that one mole spreads out, weak electrolytes ionise more and strong-electrolyte ions feel less mutual drag, so the conduction per mole grows and approaches the limiting value \(\Lambda_m^{0}\) at infinite dilution.