Question

Find the concentration of sodium acetate when added to \(0.1\ \text{M}\) solution of acetic acid to form a buffer solution of \(pH = 5.5\). (Given: \(pK_a\) of \(\text{CH}_3\text{COOH} = 4.5\))

• A. \(0.1\ \text{M}\)
• B. \(0.01\ \text{M}\)
• C. \(1.0\ \text{M}\)
• D. \(\text{None of these}\)

Hint:

If \(pH - pK_a = 1\), then: \[ \frac{[\text{Salt}]}{[\text{Acid}]} = 10 \]

Answer 1

The Correct Option is C

Concept: Henderson-Hasselbalch equation: \[ pH = pK_a + \log \left(\frac{[\text{Salt}]}{[\text{Acid}]}\right) \]

Step 1: Substituting the values.
\[ 5.5 = 4.5 + \log \left(\frac{[\text{Salt}]}{0.1}\right) \]

Step 2: Simplifying the equation.
\[ 1 = \log \left(\frac{[\text{Salt}]}{0.1}\right) \]

Step 3: Solving for concentration.
\[ 10 = \frac{[\text{Salt}]}{0.1} \] \[ [\text{Salt}] = 1.0\ \text{M} \]