Question

Correct order of limiting molar conductivity for cations in water at 298 K is :

• A. \( H^+>K^+>Ca^{2+}>Mg^{2+} \)
• B. \( H^+>Ca^{2+}>Mg^{2+}>K^+ \)
• C. \( Mg^{2+}>H^+>Ca^{2+}>K^+ \)
• D. \( H^+>Na^+>Ca^{2+}>Mg^{2+}>K^+ \)

Hint:

The limiting molar conductivity depends on the size and charge of the ion. Smaller and highly charged ions are more hydrated, leading to lower mobility and thus lower conductivity. However, \(H^+\) has exceptionally high conductivity due to its movement through a proton hopping mechanism.

Answer 1

The Correct Option is B

Limiting Molar Conductivities of ions :

$\lambda^0_{H^+} : 349.8 \text{ Sem}^2 \text{mol}^{-1}$

$\lambda^0_{Na^+} : 50.11 \text{ Sem}^2 \text{mol}^{-1}$

$\lambda^0_{K^+} : 73.52 \text{ Sem}^2 \text{mol}^{-1}$

$\lambda^0_{Ca^{2+}} : 119 \text{ Sem}^2 \text{mol}^{-1}$

$\lambda^0_{Mg^{2+}} : 106.12 \text{ Sem}^2 \text{mol}^{-1}$

Therefore correct order of limiting molar conductivity of cations will be -

$H^+>Ca^{2+}>Mg^{2+}>K^+>Na^+$