Concept used: limiting molar conductivity

At infinite dilution, the limiting molar conductivity of an ion depends on its ionic mobility in water.

• $\mathrm{H^+}$ has exceptionally high conductivity due to the proton hopping mechanism (Grotthuss mechanism).
• For other cations, mobility depends on size and extent of hydration.
• Strongly hydrated ions move more slowly.

Known relative ionic conductivities in water at $298\,\text{K}$

The standard limiting ionic molar conductivities are approximately:

$\lambda^\circ(\mathrm{H^+}) \gg \lambda^\circ(\mathrm{Ca^{2+}}) > \lambda^\circ(\mathrm{Mg^{2+}}) > \lambda^\circ(\mathrm{K^+}) > \lambda^\circ(\mathrm{Na^+})$

More specifically, typical values are:

$\lambda^\circ(\mathrm{H^+}) \approx 349.6$ $\lambda^\circ(\mathrm{Ca^{2+}}) \approx 119$ $\lambda^\circ(\mathrm{Mg^{2+}}) \approx 106$ $\lambda^\circ(\mathrm{K^+}) \approx 73.5$ $\lambda^\circ(\mathrm{Na^+}) \approx 50.1$

Hence,

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

Check each option

• Option A: $\mathrm{H^+} > \mathrm{Na^+} > \mathrm{K^+} > \mathrm{Ca^{2+}} > \mathrm{Mg^{2+}}$

  This is incorrect because $\mathrm{K^+} > \mathrm{Na^+}$, and also $\mathrm{Ca^{2+}}, \mathrm{Mg^{2+}}$ have higher limiting molar conductivity than $\mathrm{K^+}, \mathrm{Na^+}$.

• Option B: $\mathrm{H^+} > \mathrm{Ca^{2+}} > \mathrm{Mg^{2+}} > \mathrm{K^+} > \mathrm{Na^+}$

  This matches the correct order.

• Option C: $\mathrm{Mg^{2+}} > \mathrm{H^+} > \mathrm{Ca^{2+}} > \mathrm{K^+} > \mathrm{Na^+}$

  Incorrect because $\mathrm{H^+}$ must be the highest.

• Option D: $\mathrm{H^+} > \mathrm{Na^+} > \mathrm{Ca^{2+}} > \mathrm{Mg^{2+}} > \mathrm{K^+}$

  Incorrect because $\mathrm{K^+} > \mathrm{Na^+}$, and also $\mathrm{Ca^{2+}}, \mathrm{Mg^{2+}} > \mathrm{K^+}, \mathrm{Na^+}$.

Final answer

The correct order is:

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

So the correct option is B.

Comparison with stored correct answer

Stored correct answer = B

My derived answer = B

Therefore, they agree.