Question

What is meant by the 'molar conductivity' of a solution? Describe how for weak and strong electrolytes, molar conductivity changes with the concentration of the solution. How is such change explained?

Accepted Answer

Molar conductivity of a solution at a dilution $V$ is the conductance of all the ions produced from one mole of the electrolyte dissolved in $V {cm}^{3}$ of the solution when the electrodes are one $cm$ apart and the area of the electrodes is so large that the whole solution is contained between them. It is usually represented by $Λ_{m}$ . The molar conductivity is related to the specific conductivity $(κ)$ as $Λ_{m} = \frac{κ \times 1000}{Molarity}$ .

The molar conductivity of strong electrolytes is found to vary with concentration according to the Debye Huckel-Onsagar equation : ${Λ^{c}}_{m} = Λ °_{m} - A \sqrt{c}$ where $A$ is constant depending upon the type of the electrolyte, the nature of the solvent and the temperature, and $Λ °_{m}$ is the molar conductivity at infinite dilution, called limiting molar conductivity. Thus, if $Λ_{m}$ is plotted against $\sqrt{c}$ , a linear graph is obtained for low concentration but it is not linear for higher concentration like $KCl$ .

The graph represents variation of molar conductivity of a strong electrolyte and a weak electrolyte with concentration

The curve shows that there is a small increase in conductance with dilution because a strong electrolyte is completely dissociated in solution and so the number of ions remains constant. At higher concentration, the greater inter-ionic attractions retard the motion of ions and therefore, the conductance falls with increasing concentration, i.e., with dilution, the ions are far apart and, hence, the interionic attractions decrease due to which conductance increases with dilution and approaches a maximum limiting value at infinite dilution, designated as $Λ °_{m}$ or ${Λ^{\infty}}_{m}$ .

Weak electrolyte dissociates to a much lesser extent as compared to a strong electrolyte, therefore the conductance of a weak electrolyte is much lower than that of a strong electrolyte at the same concentration. This is shown in the obtained curve, in which acetic acid, a weak electrolyte shows a very large increase in conductance with dilution especially near infinite dilution because as the concentration of the weak electrolyte is reduced, more of it ionises. Thus, increase in conductance with decrease in concentration is due to the increase in the number of ions in the solution.

What is meant by the 'molar conductivity' of a solution? Describe how for weak and strong electrolytes, molar conductivity changes with the concentration of the solution. How is such change explained?

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