The correct order of mobility of alkali ions in aqueous solution is :

Options

A. `Rb^(+)gt K^(+)gt Na^(+)gt Li`
B. `K^(+)gt Rb^(+)gt Na^(+)gt Li`
C. `Li^(+)gt Na^(+)gt K^(+)gt Rb^(+)`
D. `K^(+)gt Na^(+)gt Li^(+)gt Rh^(+)`

Answer

To determine the correct order of mobility of alkali ions in aqueous solution, we will analyze the factors affecting their mobility, particularly focusing on hydration and size.

### Step-by-Step Solution:

1. **Understanding Alkali Ions**: The alkali metals include Lithium (Li), Sodium (Na), Potassium (K), and Rubidium (Rb). These metals form cations (Li⁺, Na⁺, K⁺, Rb⁺) when dissolved in water.

2. **Hydration of Cations**: When alkali ions are dissolved in water, they become surrounded by water molecules, a process known as hydration. The extent of hydration depends on the size of the cation; smaller cations can attract more water molecules.

3. **Size of Alkali Ions**: The size of alkali metal cations increases as we move down the group from Li to Rb:
- Li⁺ < Na⁺ < K⁺ < Rb⁺
This means that lithium is the smallest and rubidium is the largest among these cations.

4. **Effect of Size on Hydration**: The degree of hydration is inversely proportional to the size of the cation. Smaller cations (like Li⁺) are more heavily hydrated because they can attract more water molecules due to their higher charge density.

5. **Mobility of Cations**: The mobility of cations in an aqueous solution is affected by their hydration. Heavily hydrated ions are less mobile because the surrounding water molecules create a solvation sphere that hinders their movement. Conversely, larger cations that are less hydrated can move more freely.

6. **Order of Mobility**: Based on the size and hydration:
- Rb⁺ (largest, least hydrated, most mobile)
- K⁺ (next largest, less mobile than Rb⁺)
- Na⁺ (smaller than K⁺, less mobile than K⁺)
- Li⁺ (smallest, most hydrated, least mobile)

7. **Final Order**: Thus, the correct order of mobility of alkali ions in aqueous solution is:
- Rb⁺ > K⁺ > Na⁺ > Li⁺

### Conclusion:
The correct order of mobility of alkali ions in aqueous solution is Rb⁺ > K⁺ > Na⁺ > Li⁺.

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