Molar conductivity of ______ increases drastically on dilution.

The correct answer is Option (3) → \(CH_3COOH\)

Molar conductivity (Λ_m) is the conductivity of the volume of solution that contains one mole of electrolyte. It increases with dilution for every electrolyte, but in two very different ways:

• Strong electrolytes (salts such as NaCl, \( \ce{NH4Cl} \), \( \ce{CH3COONa} \)) are already completely dissociated. On dilution only the attraction between the ions decreases, so Λ_m increases slowly, following \( \Lambda_m = \Lambda_m^\circ - A\sqrt{c} \).

• Weak electrolytes (such as acetic acid) are only slightly dissociated. On dilution the degree of dissociation increases, so the number of ions in the solution containing one mole of electrolyte increases. Λ_m therefore rises steeply, especially at low concentration.

    \( \displaystyle \ce{CH3COOH(aq) <=> CH3COO-(aq) + H+(aq)} \)

So the molar conductivity of the weak acid CH₃COOH increases drastically on dilution.

Why the other options are incorrect

• Option (1) \(NH_4Cl\) — Incorrect. A salt, hence a strong electrolyte; its Λ_m rises only slowly.

• Option (2) \(NaCl\) — Incorrect. A strong electrolyte, like every salt here.

• Option (4) \(CH_3COONa\) — Incorrect. Although it is the salt of a weak acid, the salt itself is completely dissociated in water, so it behaves as a strong electrolyte.

NCERT (2026-27 edition):
Weak Electrolytes