The solubility order for alkali metal fluoride in water is :

Options

A. `LiF lt RbF lt KF lt NaF`
B. `RbF lt KF lt NaF lt LiF`
C. `LiF lt RbF lt NaF lt KF `
D. `LiF lt NaF lt KF lt RbF `

Answer

To determine the solubility order of alkali metal fluorides (LiF, NaF, KF, RbF) in water, we can follow these steps:

### Step 1: Understand the Compounds
We are dealing with alkali metal fluorides, which include:
- Lithium fluoride (LiF)
- Sodium fluoride (NaF)
- Potassium fluoride (KF)
- Rubidium fluoride (RbF)

### Step 2: Identify the Anion
All the compounds have the same anion, which is fluoride (F⁻). Therefore, the solubility will depend on the cation (Li⁺, Na⁺, K⁺, Rb⁺).

### Step 3: Analyze Cation Sizes
The cations are alkali metals, and their ionic sizes increase as we move down the group in the periodic table:
- Li⁺ < Na⁺ < K⁺ < Rb⁺

### Step 4: Relate Ionic Size to Solubility
The solubility of ionic compounds in water is influenced by the difference in size between the cation and the anion. A larger difference in size typically leads to higher solubility. Since the anion (F⁻) remains constant, the solubility will increase with increasing cation size.

### Step 5: Determine the Solubility Order
As we move from LiF to RbF, the size of the cation increases:
- LiF (smallest cation, least solubility)
- NaF
- KF
- RbF (largest cation, highest solubility)

Thus, the order of solubility from least to most soluble is:
**LiF < NaF < KF < RbF**

### Final Answer
The solubility order for alkali metal fluoride in water is:
**RbF > KF > NaF > LiF**