Consider the following information about the diprotic acid ascorbic acid (H₂As for short, molar mass = 176.1) . The titration curve for disodium ascorbate, Na₂As, with standard HCl is shown below:
-What is the pH at point I (V₁/2 HCl added) ?

Question

Consider the following information about the diprotic acid ascorbic acid (H₂As for short, molar mass = 176.1) .

Consider the following information about the diprotic acid ascorbic acid (H<sub>2</sub>As for short, molar mass = 176.1) . The titration curve for disodium ascorbate, Na<sub>2</sub>As, with standard HCl is shown below: -What is the pH at point I (V<sub>1</sub>/2 HCl added) ? A) 7.95 B) 12.39 C) 10 D) 11.79 E) none of these

The titration curve for disodium ascorbate, Na₂As, with standard HCl is shown below: Consider the following information about the diprotic acid ascorbic acid (H2As for short, molar mass = 176.1) . The titration curve for disodium ascorbate, Na2As, with standard HCl is shown below: -What is the pH at point I (V1/2 HCl added) ? A) 7.95 B) 12.39 C) 10 D) 11.79 E) none of these

-What is the pH at point I (V₁/2 HCl added) ?

Quizplus · Accepted Answer

At point I, all of the ₂As has reacted with half of the added HCl to form ascorbic acid H2₂As. The solution contains H2₂As and NaCl, so midpoint of the buffer region. The pKa values for H₂As are 4.1 and 11.6. Since the pH is above the first pKa, the acid dissociation constant for the first proton can be ignored. The pH of the solution is then determined by the second dissociation constant Ka2: Ka2 = [H+][₂As-] / [H₂As] Since the solution is at the midpoint of the buffer region, [₂As-] = [H₂As], so: Ka2 = [H+] pKa2 = pH = 11.6 Therefore, the answer is D) 11.79.

Consider the Following Information About the Diprotic Acid Ascorbic Acid