Assertion (A): On addition of `NH_(4)CI` to `NH_(4)OH, pH` decreases but remains greater than `7`.
Reason (R) : Addition of `overset(o+)NH_(4)` ion decreases ionisation of `NH_(4)OH`, thus `[overset(Theta)OH]` decreases and also `pH` decreases.

Options

A. If both (A) and (R) are correc, and (R) is the correct explanation of (A).
B. If both (A) and (R) are correct but (R) is not the correct explanation of (A).
C. If (A) is correct, but (R) is incorrect.
D. If (A) is incorrect, but (R) is correct.

Explanation

To solve the given question, we will analyze the assertion and reason step by step.

### Step 1: Understand the Components
We have two components in the question:
- **Assertion (A)**: On addition of NH₄Cl to NH₄OH, pH decreases but remains greater than 7.
- **Reason (R)**: Addition of NH₄⁺ ion decreases ionization of NH₄OH, thus [OH⁻] decreases and also pH decreases.

### Step 2: Analyze the Reaction
When NH₄OH dissociates, it can be represented as:
\[ \text{NH}_4\text{OH} \rightleftharpoons \text{NH}_4^+ + \text{OH}^- \]

When NH₄Cl is added, it dissociates into:
\[ \text{NH}_4\text{Cl} \rightarrow \text{NH}_4^+ + \text{Cl}^- \]

### Step 3: Identify the Common Ion Effect
The addition of NH₄Cl introduces NH₄⁺ ions into the solution, which is a common ion with NH₄OH. According to the common ion effect, the presence of a common ion (NH₄⁺) suppresses the ionization of NH₄OH, shifting the equilibrium to the left:
\[ \text{NH}_4\text{OH} \rightleftharpoons \text{NH}_4^+ + \text{OH}^- \]

### Step 4: Determine the Effect on [OH⁻]
As the equilibrium shifts to the left, the concentration of OH⁻ ions decreases. Since pOH is inversely related to the concentration of OH⁻ ions, a decrease in [OH⁻] leads to an increase in pOH.

### Step 5: Relate pH and pOH
Since pH and pOH are related by the equation:
\[ \text{pH} + \text{pOH} = 14 \]
An increase in pOH implies a decrease in pH. Therefore, as [OH⁻] decreases, the pH of the solution decreases.

### Step 6: Conclusion on Assertion and Reason
- The assertion states that the pH decreases but remains greater than 7. This is true because the solution remains basic due to the presence of NH₄OH, even though the pH decreases.
- The reason correctly explains why the pH decreases: the addition of NH₄⁺ ions decreases the ionization of NH₄OH, leading to a decrease in [OH⁻] and thus a decrease in pH.

### Final Answer
Both the assertion (A) and reason (R) are correct, and the reason (R) is a correct explanation for the assertion (A).

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