Consider the following information about the diprotic acid, ascorbic acid. (H_2A

Question

Consider the following information about the diprotic acid, ascorbic acid. (H_2AS for short, molar mass 176.1) H_2As Has^- + H^+ pK_a1 = 4.10 (K_a1 = 7.9 times 10^-5) HAs^- As^2- + H^+ pK_a2 = 11.79 (K_a2 = 1.6 times 10^-12) The titration curve for disodium ascorbate, Na_2 As, with standard HCI is shown below: a.) What major species is (are) present at point III?(Has^- and H_2As) b.) What is the pH at point I (V_1/2 HCl added)? (11.79) c.) What is the pH at point III? (4.10) d.) Which of the following is a major species present at point IV? (H As)

Explanation / Answer

13) Titration of Na2As with standard HCl

Equation,

As^2- + H+ <==> HAs-

HAs- + H+ <===> H2As

a) major species at point III (second half-equivalence point) would be,

equal amounts of HAs- and H2As

b) pH at point I (first half-equivalence point) would be,

= pKa3

= 11.79

c) pH at point III (second half-equivalence point) would be,

= pKa1

= 4.10

d) Major species at point IV (second equivalence point) would be,

H2As

All of Na2As has been neutralized to give H2As species.

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