Question:
At 25°C \(pK_a\) of CH\(_3\)COOH is 4.76 and \(pK_b\) of NH\(_4\)OH is 4.75. The pH of 1 M CH\(_3\)COONH\(_4\) is found to be 7.005. What will be the pH of 2 M CH\(_3\)COONH\(_4\) solution at the same temperature?
At 25°C \(pK_a\) of CH\(_3\)COOH is 4.76 and \(pK_b\) of NH\(_4\)OH is 4.75. The pH of 1 M CH\(_3\)COONH\(_4\) is found to be 7.005. What will be the pH of 2 M CH\(_3\)COONH\(_4\) solution at the same temperature?
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For salts of weak acid + weak base, pH is independent of concentration when \(pK_a \approx pK_b\).
Updated On: Jun 20, 2026
- 6.900
- 7.100
- 6.995
- 7.005
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The Correct Option is D
Solution and Explanation
Step 1: Understand the nature of salt CH\(_3\)COONH\(_4\).
CH\(_3\)COONH\(_4\) is a salt of a weak acid (CH\(_3\)COOH) and a weak base (NH\(_4\)OH). Such salts generally show hydrolysis of both ions, but the pH depends on relative strengths of acid and base.
Step 2: Use standard formula for weak acid–weak base salt.
For a salt of weak acid and weak base: \[ \text{pH} = \frac{1}{2}(pK_w + pK_a - pK_b) \] This expression does not depend on concentration because both ions hydrolyze equally.
Step 3: Identify key observation from given data.
It is explicitly given that for 1 M solution, pH = 7.005. This indicates near neutrality since \(pK_a \approx pK_b\), meaning hydrolysis balance is almost equal.
Step 4: Effect of concentration.
For weak acid–weak base salts, pH is independent of concentration because both hydrolysis equilibria shift equally with dilution or concentration change. Hence, increasing concentration does not change pH significantly.
Step 5: Apply conclusion.
Since pH is independent of concentration for CH\(_3\)COONH\(_4\), the pH of 2 M solution remains the same as that of 1 M solution.
Step 6: Final verification.
Given symmetry of \(pK_a\) and \(pK_b\), system remains neutral-like even at different concentrations. Therefore, pH remains unchanged.
Final Answer: \[ \boxed{7.005} \]
CH\(_3\)COONH\(_4\) is a salt of a weak acid (CH\(_3\)COOH) and a weak base (NH\(_4\)OH). Such salts generally show hydrolysis of both ions, but the pH depends on relative strengths of acid and base.
Step 2: Use standard formula for weak acid–weak base salt.
For a salt of weak acid and weak base: \[ \text{pH} = \frac{1}{2}(pK_w + pK_a - pK_b) \] This expression does not depend on concentration because both ions hydrolyze equally.
Step 3: Identify key observation from given data.
It is explicitly given that for 1 M solution, pH = 7.005. This indicates near neutrality since \(pK_a \approx pK_b\), meaning hydrolysis balance is almost equal.
Step 4: Effect of concentration.
For weak acid–weak base salts, pH is independent of concentration because both hydrolysis equilibria shift equally with dilution or concentration change. Hence, increasing concentration does not change pH significantly.
Step 5: Apply conclusion.
Since pH is independent of concentration for CH\(_3\)COONH\(_4\), the pH of 2 M solution remains the same as that of 1 M solution.
Step 6: Final verification.
Given symmetry of \(pK_a\) and \(pK_b\), system remains neutral-like even at different concentrations. Therefore, pH remains unchanged.
Final Answer: \[ \boxed{7.005} \]
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