Correct order of limiting molar conductivity for cations in water at 298 K is : [2025]

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Solution

Q.
Correct order of limiting molar conductivity for cations in water at 298 K is : [2025]

1 $H^{+} > {Na}^{+} > K^{+} > {Ca}^{2 +} > {Mg}^{2 +}$

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

3 ${Mg}^{2 +} > H^{+} > {Ca}^{2 +} > K^{+} > {Na}^{+}$

4 $H^{+} > {Na}^{+} > {Ca}^{2 +} > {Mg}^{2 +} > K^{+}$

Ans.
(2)

Among cations $H^{+}$ has highest molar conductivity in water.

This is because $H^{+}$ is conducted by transfer between water molecules.

For other ions, $Λ_{m}^{\circ} \propto charge$ . Thus

$Λ_{m}^{\circ} ({Ca}^{2 +}), Λ_{m}^{\circ} ({Mg}^{2 +}) > Λ_{m}^{\circ} (K^{+}), Λ_{m}^{\circ} ({Na}^{+})$

For ions carrying same charge, smaller ions are more hydrated, hence their limiting molar conductivity is lesser as $Λ_{m}^{\circ} \propto size .$

Thus $Λ_{m}^{\circ} ({Ca}^{2 +}) > Λ_{m}^{\circ} ({Mg}^{2 +}) and Λ_{m}^{\circ} (K^{+}) > Λ_{m}^{\circ} ({Na}^{+})$

Thus correct order of limiting molar conductivity is:

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

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