Choose the correct statement about molar conductivity, \( Λ_m \)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → As the concentration of a weak electrolyte approaches zero, there is a sharp increase in the molar conductivity of the solution
Molar conductivity, \( \Lambda_m \), is the conductance of the solution containing one mole of electrolyte. It increases on dilution for every electrolyte, but for different reasons:
A strong electrolyte is already fully dissociated. On dilution the ions are farther apart and pull on one another less, so \( \Lambda_m \) rises slowly and linearly with \( c^{1/2} \): \( \Lambda_m = \Lambda_m^\circ - A\,c^{1/2} \).
A weak electrolyte (such as acetic acid) is only slightly dissociated at higher concentrations. On dilution its degree of dissociation increases, so the number of ions from one mole rises, and \( \Lambda_m \) increases steeply, especially near zero concentration.
So as the concentration of a weak electrolyte approaches zero, there is a sharp increase in its molar conductivity.
Why the other options are incorrect
Option (1) Molar conductivity of a weak electrolyte decreases gradually as the concentration of the electrolyte decreases in the higher concentration ranges — Incorrect. On dilution \( \Lambda_m \) of a weak electrolyte increases, not decreases; at higher concentrations the rise is small, but it is a rise.
Option (2) Molar conductivity of a strong electrolyte increases linearly with the increase in the concentration of the electrolyte — Incorrect. \( \Lambda_m \) of a strong electrolyte decreases slightly as the concentration increases, linearly with \( c^{1/2} \), because the ions attract one another more.
Option (4) The limiting molar conductivity of a weak electrolyte can be determined directly from the plot of \(\Lambda_m\) against \(c^{1/2}\) — Incorrect. The plot for a weak electrolyte is a steep curve, so it cannot be extrapolated to zero concentration; \( \Lambda_m^\circ \) is obtained from Kohlrausch’s law of independent migration of ions.
NCERT (2026-27 edition):
Weak Electrolytes
