Which is least soluble

Options

A. `Na_(2)S`
B. `MgS`
C. `MgCl_(2)`
D. `NaCl`

Explanation

To determine which compound is least soluble in water among the given options, we can follow these steps:

### Step 1: Identify the Compounds
We need to identify the compounds in question. The compounds mentioned in the video include sodium chloride (NaCl), magnesium chloride (MgCl2), sodium sulfide (Na2S), and magnesium sulfide (MgS).

### Step 2: Analyze the Solubility of Chlorides
Chlorides of alkali and alkaline earth metals are generally soluble in water.
- **NaCl**: Sodium chloride is highly soluble in water.
- **MgCl2**: Magnesium chloride is also soluble in water, though slightly less than NaCl.

### Step 3: Analyze the Solubility of Sulfides
Sulfides are generally less soluble in water compared to chlorides.
- **Na2S**: Sodium sulfide is more soluble in water compared to magnesium sulfide.
- **MgS**: Magnesium sulfide is less soluble in water.

### Step 4: Apply Fajan's Rule
To compare the solubility of sodium sulfide and magnesium sulfide, we can use Fajan's rule, which states that the covalent character of ionic compounds increases with the polarizing power of the cation and the polarizability of the anion.
- Sodium ion (Na⁺) has a +1 charge.
- Magnesium ion (Mg²⁺) has a +2 charge, which gives it a higher polarizing power.

### Step 5: Determine the Least Soluble Compound
Since magnesium ion has a higher polarizing power, it will lead to a greater covalent character in magnesium sulfide (MgS). This increased covalent character results in decreased solubility in water compared to sodium sulfide (Na2S).

### Conclusion
Based on the above analysis, we can conclude that magnesium sulfide (MgS) is the least soluble compound in water among the options provided.

### Final Answer
The least soluble compound is **MgS (Magnesium Sulfide)**.

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