SAMPLE PROBLEM 18.8
Calculating equilibrium concentrations for a polyprotic acid
PROBLEM:
Ascorbic acid (H2C6H6O6; abbreviated H2Asc), 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 a 0.050 M aqueous solution of H2Asc.
SOLUTION:
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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