Question

Define Molar Conductivity and explain its variation with concentration for strong and weak electrolytes.

Hint:

Strong electrolytes show a slight increase in molar conductivity on dilution, while weak electrolytes show a sharp increase due to increased ionization.

Answer 1

Concept: Molar conductivity measures the ability of an electrolyte solution to conduct electricity per mole of electrolyte. It depends on the number of ions and their mobility in solution.
Step 1: Definition of Molar Conductivity. Molar conductivity ($\Lambda_m$) is defined as the conductance of the volume of solution containing one mole of electrolyte placed between two electrodes. \[ \Lambda_m = \frac{\kappa}{c} \] where $\kappa$ = conductivity and $c$ = molar concentration. Unit: S cm$^2$ mol$^{-1}$.
Step 2: Variation with concentration. Molar conductivity increases on dilution because:

• Interionic attractions decrease
• Ionic mobility increases
• Degree of ionization may increase

Step 3: Strong electrolytes. Strong electrolytes (e.g., HCl, NaCl) are almost completely ionized even at higher concentrations.

• $\Lambda_m$ increases slightly with dilution
• Increase is mainly due to reduced interionic interactions
• Variation follows Kohlrausch’s law: \[ \Lambda_m = \Lambda_m^\circ - A\sqrt{c} \]

Step 4: Weak electrolytes. Weak electrolytes (e.g., CH$_3$COOH, NH$_4$OH) are partially ionized.

• $\Lambda_m$ increases sharply with dilution
• Ionization increases significantly at low concentration
• At infinite dilution, they approach $\Lambda_m^\circ$

Conclusion: Molar conductivity increases with dilution for all electrolytes, but the rise is small for strong electrolytes and large for weak electrolytes due to increased ionization.