The pH of a weak solution of ammonium hydroxide has been measured . If ammonium chloride is now added

Options

A. the pOH will decrease
B. the pH will increase
C. the pH will decrease
D. the acidity will decrease

Explanation

To solve the problem, we need to analyze what happens when ammonium chloride (NH4Cl) is added to a weak solution of ammonium hydroxide (NH4OH). Here’s a step-by-step breakdown of the solution:

### Step 1: Understand the Initial Solution
- Ammonium hydroxide (NH4OH) is a weak base that partially dissociates in solution:
\[ NH4OH \rightleftharpoons NH4^+ + OH^- \]
- The pH of this solution is determined by the concentration of hydroxide ions (OH^-).

### Step 2: Adding Ammonium Chloride
- When ammonium chloride (NH4Cl) is added to the solution, it dissociates completely in water:
\[ NH4Cl \rightarrow NH4^+ + Cl^- \]
- This introduces additional ammonium ions (NH4^+) into the solution.

### Step 3: Common Ion Effect
- The addition of NH4^+ ions from NH4Cl causes a shift in the equilibrium of the dissociation of NH4OH due to the common ion effect:
\[ NH4OH \rightleftharpoons NH4^+ + OH^- \]
- According to Le Chatelier's principle, the equilibrium will shift to the left to counteract the increase in NH4^+, resulting in a decrease in the concentration of OH^- ions.

### Step 4: Effect on pH
- As the concentration of OH^- decreases, the concentration of H^+ ions (which are inversely related to OH^-) must increase to maintain the water dissociation equilibrium:
\[ [H^+][OH^-] = 10^{-14} \]
- Therefore, as OH^- decreases, H^+ increases, leading to a decrease in pH.

### Step 5: Conclusion
- The overall effect of adding NH4Cl to the solution of NH4OH is that the pH of the solution decreases due to the common ion effect, which reduces the concentration of hydroxide ions.

### Final Answer
- The pH of the solution decreases upon the addition of ammonium chloride.

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