Question:

Equal volumes of \(0.5 \, N\) acetic acid and \(0.5 \, N\) sodium acetate are mixed. What is the pH of resultant solution?
\[ (pK_a \text{ of acetic acid } = 4.75) \]

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For a buffer solution: \[ pH=pK_a+\log\frac{[\text{Salt}]}{[\text{Acid}]} \] If concentrations of acid and salt are equal, then: \[ pH=pK_a \]
Updated On: Jun 15, 2026
  • \(4.85\)
  • \(4.65\)
  • \(4.75\)
  • \(7.0\)
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The Correct Option is C

Solution and Explanation

Step 1: Identify the type of solution formed.
Acetic acid is a weak acid and sodium acetate is its conjugate salt.
Mixing them forms a buffer solution.
For an acidic buffer, pH is calculated using Henderson-Hasselbalch equation.

Step 2: Write Henderson-Hasselbalch equation.
\[ pH=pK_a+\log\frac{[\text{Salt}]}{[\text{Acid}]} \]

Step 3: Compare concentrations after mixing.
Equal volumes of \(0.5\,N\) acetic acid and \(0.5\,N\) sodium acetate are mixed.
Hence, concentration ratio becomes
\[ \frac{[\text{Salt}]}{[\text{Acid}]}=1 \]

Step 4: Substitute the values.
\[ pH=4.75+\log(1) \] Since,
\[ \log(1)=0 \] therefore,
\[ pH=4.75 \]

Step 5: Final conclusion.
Hence, the pH of the resultant solution is
\[ \boxed{4.75} \]
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