Alkali metal salts ionic and soluble in water. The solubility of an ionic compound depends on (i) lattic ethalpy and (ii) hydration enthalpy. These two factor oppose each other. If hydration ethalpy is high, the ions will have greater tendency to be hydrated and therefore the solubility will be high. The smaller the cation, the greater is the degree of hydration. The reducing behaviour of alkali metals in solution is also dependent on the hydration enthalpy besides other factors.
The ionic mobility of `Li^(o+)` is less than of the `Na^(o+)` ion in solution because

Options

A. `Li^(o+)` ion has a high charge density.
B. `Li^(o+)` ion has the highest hydration tendency.
C. `Li^(o+)` ion has the highest ionisation enthalpy.
D. `Li^(o+)` ion has two electrons.

Explanation

To answer the question regarding the ionic mobility of `Li^(+)` being less than that of `Na^(+)`, we will analyze the factors affecting ionic mobility, particularly focusing on charge density and hydration.

### Step-by-Step Solution:

1. **Understand Ionic Mobility**: Ionic mobility refers to the ability of an ion to move through a solution. It is influenced by the size of the ion and the extent to which it is hydrated by water molecules.

2. **Compare Size of Ions**: Lithium ion (`Li^(+)`) is smaller in size compared to sodium ion (`Na^(+)`). This size difference is crucial because it affects how many water molecules can surround each ion.

3. **Charge Density**: The charge density of an ion is defined as the charge of the ion divided by its volume. Since `Li^(+)` is smaller than `Na^(+)`, it has a higher charge density. Higher charge density leads to stronger interactions with water molecules.

4. **Hydration Enthalpy**: Smaller ions like `Li^(+)` have a higher hydration enthalpy because they can attract more water molecules due to their higher charge density. This results in a greater degree of hydration.

5. **Effect of Hydration on Mobility**: When `Li^(+)` is surrounded by more water molecules, it becomes more "hydrated." This hydration creates a larger effective radius, making it harder for the ion to move freely through the solution. In contrast, `Na^(+)`, being larger, has fewer water molecules surrounding it, allowing it to move more freely.

6. **Conclusion**: Therefore, the ionic mobility of `Li^(+)` is less than that of `Na^(+)` because the higher hydration of `Li^(+)` (due to its smaller size and higher charge density) restricts its movement in solution.

### Final Answer:
The ionic mobility of `Li^(+)` is less than that of `Na^(+)` because `Li^(+)` has a higher charge density, resulting in a greater degree of hydration, which restricts its movement in solution.