Question

The following diagram is obtained in a pH-metric titration of a weak dibasic acid (H₂A) with a strong base. The point that best represents [HA⁻]=[A ⁻² ] is

• A. P
• B. q
• C. r
• D. s

Answer 1

The Correct Option is C

Concept 

A weak dibasic acid is: $$\text{H}_2\text{A} \;\rightleftharpoons\; \text{H}^+ + \text{HA}^-$$ $$\text{HA}^- \;\rightleftharpoons\; \text{H}^+ + \text{A}^{2-}$$ The point where $$[\text{HA}^-] = [\text{A}^{2-}]$$ corresponds to the **second half-equivalence point**, i.e., $$\text{pH} = pK_{a2}.$$

Working

In a pH-metric titration curve of a dibasic acid, the characteristic points are:

• P → first buffer region
• Q → first equivalence point
• R → second buffer region (pH = pK_a2)
• S → second equivalence point

The condition $$[\text{HA}^-] = [\text{A}^{2-}]$$ occurs in the **second buffer region**, i.e., at **R**.

Answer

Correct point = R ✔ This corresponds to Option 3.