0.01 M aqueous solutions of following salts were prepared. Their cond…

The correct answer is Option (4) → CsCl

All four salts are 1:1 chlorides at the same concentration, so the difference in conductivity comes from the mobility of the cation. The conductivity depends on the size of the ions and their solvation.

Going down group 1, the bare ions get larger: \( \ce{Li^{+}} \) < \( \ce{Na^{+}} \) < \( \ce{Rb^{+}} \) < \( \ce{Cs^{+}} \). A small ion has a high charge density and binds many water molecules, so its hydrated size is the largest. The order of hydrated size is therefore reversed:

    Li⁺(aq) > Na⁺(aq) > Rb⁺(aq) > Cs⁺(aq)

Cs⁺(aq) is the smallest hydrated ion, moves fastest and has the highest ionic mobility, so CsCl conducts best.

Why the other options are incorrect

• Option (1) LiCl — Incorrect. \( \ce{Li^{+}} \) is the most heavily hydrated ion and moves slowest; LiCl has the lowest conductivity of the four.

• Option (2) RbCl — Incorrect. Rb⁺(aq) is larger than Cs⁺(aq), so it moves a little more slowly.

• Option (3) NaCl — Incorrect. Na⁺(aq) is more strongly hydrated than \( \ce{Rb^{+}} \) and \( \ce{Cs^{+}} \), so its mobility is lower.

NCERT (2026-27 edition):
Conductance of Electrolytic Solutions – “… The conductivity of electrolytic (ionic) solutions depends on:
(i) the nature of the electrolyte added
(ii) size of the ions produced and their solvation
(iii) the nature of the solvent and its viscosity
(iv) concentration of the electrolyte
(v) temperature (it increases with the increase of temperature).”