Question

What is the pH of a weak dibasic acid that is 2% dissociated in its M/100 solution at 298 K?

• A. 1.6990
• B. 2.3979
• C. 3.3970
• D. 4.6990

Hint:

For a dibasic acid, remember to multiply by 2: $[H^+] = 2C\alpha$. For a tribasic acid, multiply by 3, and so on.

Answer 1

The Correct Option is C

Step 1: Concept
For a weak dibasic acid ($H_2A$), each molecule releases two protons, so $[H^+] = 2 \times C \times \alpha$.

Step 2: Meaning
Concentration $C = \dfrac{M}{100} = 0.01\ \text{M} = 10^{-2}\ \text{M}$ and $\alpha = 2% = 0.02 = 2\times 10^{-2}$.

Step 3: Analysis
\[[H^+] = 2 \times 10^{-2} \times 2 \times 10^{-2} = 4 \times 10^{-4}\ \text{M}.\] \[\text{pH} = -\log(4 \times 10^{-4}) = 4 - \log 4 = 4 - 0.6020 = 3.3980 \approx 3.3970.\]

Step 4: Conclusion
The pH is approximately 3.3970. Final Answer: (C)