Λ0m for NaCl, HCl and NaAc are 126.4, 425.9 and 91.0 S cm2 mol−1, res…

The correct answer is Option (1) → 390.5 S cm² mol⁻¹

By Kohlrausch’s law of independent migration of ions, the limiting molar conductivity of an electrolyte is the sum of the contributions of its cation and anion. So Λ°_m of a weak electrolyte such as acetic acid (HAc) can be found from strong electrolytes that together supply the same ions:

    \( \displaystyle \Lambda^{\circ}_m(\mathrm{HAc})=\Lambda^{\circ}_m(\mathrm{HCl})+\Lambda^{\circ}_m(\mathrm{NaAc})-\Lambda^{\circ}_m(\mathrm{NaCl}) \)

This works because the \( \ce{Na^{+}} \) and \( \ce{Cl^{-}} \) contributions cancel:

    \( \displaystyle =[\lambda^{\circ}_{H^+}+\lambda^{\circ}_{Cl^-}]+[\lambda^{\circ}_{Na^+}+\lambda^{\circ}_{Ac^-}]-[\lambda^{\circ}_{Na^+}+\lambda^{\circ}_{Cl^-}] \)

    \( \displaystyle =\lambda^{\circ}_{H^+}+\lambda^{\circ}_{Ac^-} \)

Substituting the values:

    \( \displaystyle \Lambda^{\circ}_m(\mathrm{HAc})=425.9+91.0-126.4 \)

    \( \displaystyle =390.5\ \mathrm{S\,cm^2\,mol^{-1}} \)

Note 1: This question is from NCERT (2026-27). NTA sets numerical questions from NCERT too, so do not ignore NCERT while practising numericals.

NCERT (2026-27 edition):
Variation of Conductivity and Molar Conductivity with Concentration – …

Note 2: The same/similar question has been asked in different years by NTA as per the details given below: