The `pH` values of aqueous solutions of which of the following compounds does not change on dilution?

Options

A. `PhCOONH_(4)`
B. `NH_(4)CN`
C. `HCOONa`
D. `NH_(4)CI`

Explanation

To determine which of the given compounds has a pH value that does not change upon dilution, we need to analyze the nature of the salts provided. Here's a step-by-step solution:

### Step 1: Understand the Concept of pH and Dilution
The pH of a solution is a measure of the hydrogen ion concentration. When a solution is diluted, the concentration of solute decreases, which can affect the pH. However, for certain types of solutions, particularly buffer solutions or salts of weak acids and bases, the pH remains relatively constant upon dilution.

### Step 2: Identify the Types of Salts
We need to classify the salts based on their components:
1. **Salt of weak acid and weak base**: pH does not change on dilution.
2. **Salt of weak acid and strong base**: pH changes on dilution.
3. **Salt of strong acid and weak base**: pH changes on dilution.
4. **Buffer solutions**: pH remains constant on dilution.

### Step 3: Analyze Each Option
Let's analyze the given salts:

1. **NH4Cl**: This is the salt of NH4OH (weak base) and HCl (strong acid). The pH will change upon dilution because the concentration of H⁺ ions increases.

2. **NH4COO**: This is the salt of HCOOH (weak acid) and NH4OH (weak base). The pH will change upon dilution as both components are weak.

3. **NaHCOO**: This is the salt of HCOOH (weak acid) and NaOH (strong base). The pH will change upon dilution because it involves a strong base.

4. **NH4CN**: This is the salt of NH4OH (weak base) and HCN (weak acid). The pH remains constant upon dilution since it is a salt of a weak acid and a weak base.

### Step 4: Conclusion
From the analysis, we conclude that the pH of **NH4CN** does not change upon dilution.

### Final Answer
The compound whose pH value does not change on dilution is **NH4CN**.