Weak acids dissociate partially in aqueous medium. If any common ion is present in the solution, the degree of dissociation of the acid is suppressed however the dissociation constant value remains constant.
In which of the following the degree of dissociation of water is maximum '?

Options

A. `0.1 M NH_(3)`
B. `0.1 M HCOOH`
C. `0.1 M HCN`
D. Pure water

Answer

To determine in which scenario the degree of dissociation of water is maximum, we need to analyze the effect of common ions on the dissociation of water. Here’s a step-by-step solution:

### Step 1: Understanding Water Dissociation
Water (H₂O) dissociates into hydrogen ions (H⁺) and hydroxide ions (OH⁻):
\[ H_2O \rightleftharpoons H^+ + OH^- \]
This dissociation is very slight, indicating that water is a weak electrolyte.

### Step 2: Effect of Common Ions
When a common ion is present in the solution, it can suppress the dissociation of weak acids or bases. This is known as the "common ion effect." The presence of H⁺ ions or OH⁻ ions from other sources will shift the equilibrium of water dissociation to the left, reducing the degree of dissociation.

### Step 3: Analyzing the Given Options
1. **Pure Water**: No common ions present, so the dissociation of water will be at its maximum.
2. **Ammonium Hydroxide (NH₄OH)**: Dissociates to give NH₄⁺ and OH⁻ ions. The presence of OH⁻ will suppress water dissociation.
3. **Formic Acid (HCOOH)**: Dissociates to give HCOO⁻ and H⁺ ions. The presence of H⁺ will suppress water dissociation.
4. **Hydrogen Cyanide (HCN)**: Dissociates to give CN⁻ and H⁺ ions. The presence of H⁺ will also suppress water dissociation.

### Step 4: Conclusion
Since the degree of dissociation of water is suppressed in the presence of any common ions (H⁺ or OH⁻), the only scenario where there are no common ions is in pure water. Therefore, the degree of dissociation of water is maximum in pure water.

**Final Answer**: The degree of dissociation of water is maximum in **pure water** (Option 4).