Question

What is the pH of buffer solution prepared by mixing 0.01 M weak acid and 0.02 M salt of weak acid with strong base? (\( \text{pK}_{\text{a}} = 4.680 \))

• A. 4.379
• B. 2.379
• C. 4.981
• D. 2.981

Hint:

If salt concentration is double the acid, \(\text{pH} = \text{pK}_{\text{a}} + 0.3\).

Answer 1

The Correct Option is C

Step 1: Concept
The pH of an acidic buffer is calculated using the Henderson-Hasselbalch equation.

Step 2: Meaning
\(\text{pH} = \text{pK}_{\text{a}} + \log_{10}\left(\frac{[\text{Salt}]}{[\text{Acid}]}\right)\).

Step 3: Analysis
\(\text{pH} = 4.680 + \log_{10}\left(\frac{0.02}{0.01}\right) = 4.680 + \log_{10}(2)\).
Since \(\log_{10}(2) \approx 0.301\).

Step 4: Conclusion
\(\text{pH} = 4.680 + 0.301 = 4.981\). Final Answer: (C)