Consider the following information about the diprotic acid ascorbic acid (H 2 As for short, molar mass = 176.1). The titration curve for disodium ascorbate, Na 2 As, with standard HCl is shown below:
-What is the pH at point III?
A) 7.95
B) 11.79
C) 12.39
D) 4.10
E) none of these

Question

Consider the following information about the diprotic acid ascorbic acid (H 2 As for short, molar mass = 176.1).  The titration curve for disodium ascorbate, Na 2 As, with standard HCl is shown below: 
-What is the pH at point III?
A) 7.95 
B) 11.79 
C) 12.39 
D) 4.10 
E) none of these

Quizplus · Accepted Answer

At point III, all of the ascorbic acid has been converted to H2As-. H2As- is a diprotic acid, so it will have two ionization steps. The first ionization step will result in the formation of HAs1U1B12S1U1B0- and H+. The second ionization step will result in the formation of As1U1B12S1U1B0S1U1B12- and H+. The pH at point III will depend on which ionization step has a greater equilibrium constant. Since ascorbic acid is a stronger acid than HAs1U1B12S1U1B0- and has a higher equilibrium constant for its first ionization step, the pH at point III will be determined by the first ionization step of H2As-. At this point, [HAs1U1B12S1U1B0-]=[H+], so pH=pKa1=4.10.

Consider the Following Information About the Diprotic Acid Ascorbic Acid