Which of the following metal hydroxide is least basic ?

Options

A. `Ca(OH)_2`
B. `Mg(OH)_2`
C. `Ba(OH)_2`
D. `Sr(OH)_2`

Answer

To determine which metal hydroxide is least basic among magnesium hydroxide (Mg(OH)₂), calcium hydroxide (Ca(OH)₂), barium hydroxide (Ba(OH)₂), and strontium hydroxide (Sr(OH)₂), we can follow these steps:

### Step 1: Understand Basicity of Metal Hydroxides
Basicity in metal hydroxides is related to the ability of the hydroxide ion (OH⁻) to dissociate from the metal cation. The easier the hydroxide ion can dissociate, the more basic the hydroxide is.

### Step 2: Analyze the Group Trends
All the given hydroxides belong to Group 2 of the periodic table (alkaline earth metals). As we move down the group from magnesium to barium, the size of the metal cations increases due to the addition of electron shells.

### Step 3: Consider Bond Strength
The bond strength between the metal cation and the hydroxide ion decreases as the size of the metal cation increases. This is because the larger cations have a greater distance between their nucleus and the hydroxide ion, leading to weaker bonds.

### Step 4: Compare the Hydroxides
- **Magnesium Hydroxide (Mg(OH)₂)**: Smallest cation, strongest bond with OH⁻, least dissociation, hence least basic.
- **Calcium Hydroxide (Ca(OH)₂)**: Larger than Mg but smaller than Ba and Sr, moderate basicity.
- **Strontium Hydroxide (Sr(OH)₂)**: Larger than Ca, weaker bond, more basic than Mg.
- **Barium Hydroxide (Ba(OH)₂)**: Largest cation, weakest bond, most basic.

### Step 5: Conclusion
Based on the analysis, magnesium hydroxide (Mg(OH)₂) has the strongest bond with the hydroxide ion and therefore is the least basic among the given options.

### Final Answer
**Magnesium hydroxide (Mg(OH)₂) is the least basic.**

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