Calculations for Polyprotic Weak Acids

• H2S(aq) + H2O(l) ? H3O+(aq) + HS-(aq) Ka1 = 5.7 x 10-8

• HS- (aq) + H2O(l) ? H3O+(aq) + S2-(aq) Ka2 = 1.2 x 10-13

• 0.100 M H2S (saturated solution often used in qualitative analysis where [S2-] controls the precipitation of metal sulfide salts)

• [H2S] = 0.100 M

• [H3O+] = [HS-] = 7.55 x 10-5 M

• [S2-] = 1.2 x 10-13 M

• Is the value of [S2-] dependent on pH?

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