Question

The molar conductivity of a weak electrolyte when plotted against the square root of its concentration, which of the following is expected to be observed?

• A. Molar conductivity decreases sharply with increase in concentration.
• B. A small increase in molar conductivity is observed at infinite dilution.
• C. Molar conductivity increases sharply with increase in concentration.
• D. A small decrease in molar conductivity is observed at infinite dilution.

Hint:

At infinite dilution, the molar conductivity of weak electrolytes approaches a limiting value due to the complete dissociation of ions.

Answer 1

The Correct Option is A

The molar conductivity of a weak electrolyte typically decreases sharply with an increase in concentration. This behavior can be explained using the following reasoning:

1. Definition of Molar Conductivity: Molar conductivity (Λ_m) is defined as the conductivity of a solution divided by the molar concentration of the electrolyte. It is expressed as:
Λ_m = κ/C
where κ is the conductivity and C is the concentration of the electrolyte.

2. Weak Electrolytes and Ionization: Weak electrolytes only partially ionize in solution. With higher concentrations, the degree of ionization decreases because the ions are closer together and experience more interaction, leading to reduced mobility of ions.

3. Concentration and Ion-Interaction: At higher concentrations, ion-ion interactions increase, leading to electrostatic forces that hinder the movement of ions. Therefore, fewer ions contribute to conductivity, reducing the molar conductivity.

4. Limit at Infinite Dilution: At infinite dilution, the ions are farthest apart, and their interaction is minimized, allowing for maximum mobility and thus the highest possible value of molar conductivity for the electrolyte.

In summary, as the concentration of a weak electrolyte increases, the molar conductivity decreases sharply due to enhanced ion interactions and reduced ion mobility.