3. Ascorbic acid (H2C6H6O6 or HAsc), known as vitamin C, is a diprotic acid (Ka1

Question

3. Ascorbic acid (H2C6H6O6 or HAsc), known as vitamin C, is a diprotic acid (Ka1 = 1.0 x 10-5 and Ka2 = 5 x 10-12) found in citrus fruit. Calculate [H2Asc], [HAsc-], [Asc2-], and the pH of 0.050 M H2Asc.

Explanation / Answer

Ka1 >> Ka2 so the first dissociation produces virtually all of the H3O+. Ka1 is small, thus [H2Asc]initial ­ [H2Asc]eq After finding concentrations of the various species for the first dissociation, use them as initial concentrations for the second dissociation. H2Asc(aq) + H2O(l) HAsc-(aq) + H3O+(aq) Ka1 = [HAsc-][H3O+] [H2Asc] = 1.0 x 10-5 HAsc-(aq) + H2O(l) Asc2-(aq) + H3O+(aq) Ka2 = [Asc2-][H3O+] [HAsc-] = 5 x 10-12

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