Q64 / 200 · NEET 2006 · Chemistry
The correct order of the mobility of the alkali metal ions in aqueous solution is:
- Correct answerA$\mathrm{Rb}^{+}>\mathrm{K}^{+}>\mathrm{Na}^{+}>\mathrm{Li}^{+}$
- B$\mathrm{Li}^{+}>\mathrm{Na}^{+}>\mathrm{K}^{+}>\mathrm{Rb}^{+}$
- C$\mathrm{Na}^{+}>\mathrm{K}^{+}>\mathrm{Rb}^{+}>\mathrm{Li}^{+}$
- D$\mathrm{K}^{+}>\mathrm{Rb}^{+}>\mathrm{Na}^{+}>\mathrm{Li}^{+}$
Solution
As Charge density $\propto$ hydration. Therefore hydrated size of $\mathrm{Li}^{+}$is large and having less mobility and down the group degree of hydration decreases. So, among alkali metal ions, lithium has the lowest mobility.
$$
\mathrm{Rb}^{+}>\mathrm{K}^{+}>\mathrm{Na}^{+}>\mathrm{Li}^{+}
$$
Related Theory
The magnitude of hydration enthalpy depends on the charge density of the ions. The charge density is more for smaller ions and hence the smaller ions have higher values of hydration enthalpy. The higher the charge density the higher will be the force of attraction between the ion and the water polar end.
Hence, the correct answer is option: 'A'.