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: Understanding the Concept:
A weak dibasic acid dissociates partially.
The total \( [H^+] \) concentration depends on the concentration and the degree of dissociation (\( \alpha \)).
Step 2: Detailed Explanation:
Given:
Concentration \( C = M/100 = 0.01 \) M.
Dissociation \( \alpha = 2% = 0.02 \).
For a dibasic acid:
\[ [H^+] = 2 \times C \times \alpha = 2 \times 0.01 \times 0.02 = 0.0004 \text{ M} \] \[ [H^+] = 4 \times 10^{-4} \text{ M} \] Calculate \( pH \):
\[ pH = -\log(4 \times 10^{-4}) = 4 - \log 4 \] \[ pH = 4 - 0.6020 = 3.3980 \] Closest option is 3.3970.
Step 3: Final Answer:
The pH is approximately 3.3970.
This matches Option (C).