Question

Find the concentration of sodium acetate when added to 0.1 M solution of acetic acid to form a buffer solution of pH = 5.5 ? (pK$_a$ of CH$_3$COOH = 4.5)

• A. 0.1 M
• B. 0.01 M
• C. 1.0 M
• D. 10.0 M

Hint:

When $pH = pK_a + 1$, the ratio of $[\text{Salt}]/[\text{Acid}]$ is exactly $10$.

Answer 1

The Correct Option is C

Step 1: Henderson-Hasselbalch Equation
$pH = pK_a + \log\left(\frac{[\text{Salt}]}{[\text{Acid}]}\right)$

Step 2: Calculation
$5.5 = 4.5 + \log\left(\frac{[\text{Salt}]}{0.1}\right)$
$1.0 = \log\left(\frac{[\text{Salt}]}{0.1}\right)$
$10^1 = \frac{[\text{Salt}]}{0.1} \implies [\text{Salt}] = 10 \times 0.1 = 1.0\text{ M}$.

Step 3: Conclusion
The concentration of sodium acetate required is 1.0 M.
Final Answer: (C)