Which among the following is the least soluble in water

Options

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

Answer

To determine which alkali metal fluoride is the least soluble in water, we can follow these steps:

### Step 1: Identify the Compounds
The question involves the fluorides of alkali metals. The alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). Therefore, we are considering the following fluorides:
- LiF (Lithium fluoride)
- NaF (Sodium fluoride)
- KF (Potassium fluoride)
- RbF (Rubidium fluoride)
- CsF (Cesium fluoride)

### Step 2: Understand Solubility Factors
The solubility of ionic compounds in water is influenced by the nature of the cation and anion. In this case, we are focusing on the cation (the alkali metal) and its ability to polarize the anion (fluoride ion).

### Step 3: Analyze the Polarizing Power of Cations
The polarizing power of a cation is determined by its size; smaller cations have a higher polarizing power. This means they can distort the electron cloud of the anion more effectively, leading to a greater covalent character in the compound.

### Step 4: Compare the Sizes of Alkali Metal Cations
- Lithium (Li⁺) is the smallest cation among the alkali metals.
- Sodium (Na⁺) is larger than Li⁺.
- Potassium (K⁺) is larger than Na⁺.
- Rubidium (Rb⁺) is larger than K⁺.
- Cesium (Cs⁺) is the largest.

### Step 5: Determine Covalent Character
According to Fajan's rule, the smaller the cation, the greater its polarizing power and hence the higher the covalent character of the ionic compound formed. Therefore, LiF will have the highest covalent character among the fluorides of alkali metals.

### Step 6: Conclude on Solubility
Compounds with higher covalent character tend to be less soluble in water. Since LiF has the highest covalent character due to the small size of Li⁺, it is expected to be the least soluble in water compared to the other alkali metal fluorides.

### Final Answer
Thus, the least soluble compound in water among the given alkali metal fluorides is **LiF (Lithium fluoride)**.

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