Question

What is the pH of buffer solution formed by mixing 0.01 M acetic acid and 0.05 M sodium acetate? $(\text{pK}_\text{a} = 4.7447)$

• A. 2.80
• B. 3.00
• C. 5.44
• D. 6.5

Hint:

If the salt concentration is 5 times the acid concentration, the pH will be $pK_a + 0.7$. If they are equal, $pH = pK_a$.

Answer 1

The Correct Option is C

Step 1: Concept
The pH of an acidic buffer solution is calculated using the Henderson-Hasselbalch equation: $pH = pK_a + \log_{10} \frac{[\text{Salt}]}{[\text{Acid}]}$.

Step 2: Meaning
Acetic acid is the weak acid and sodium acetate is its conjugate base (salt).

Step 3: Analysis
Given: $pK_a = 4.7447$, $[\text{Salt}] = 0.05 \text{ M}$, $[\text{Acid}] = 0.01 \text{ M}$. $pH = 4.7447 + \log_{10} \left( \frac{0.05}{0.01} \right)$. $pH = 4.7447 + \log_{10}(5)$. Since $\log_{10}(5) \approx 0.6990$. $pH = 4.7447 + 0.6990 = 5.4437$.

Step 4: Conclusion
The pH of the buffer solution is approximately 5.44. Final Answer: (C)