Question

# An aqueous solution contains 0.10 M H2S and 0.20 M HCl. If the equilibrium constants for the formation of HS−from H2S is 1.0×10−7 and that of S2− from HS− ions is 1.2×10−13, then the concentration of S2− ions in aqueous solution is:

A
5×1019
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B
5×108
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C
3×1020
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D
6×1021
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Solution

## The correct option is C 3×10−20According to the given data, H2S⇌HS−+H+; K1=1×10−7 ...(1) HS−⇌S2−+H+; K2=1.2×10−13...(2) Adding (1) and (2) we get, H2S⇌S2−+2H+ Keq=K1×K2=1.2×10−20 Since HCl is a strong acid, we can assume all the H+ ions to come from HCl. Thus, [H+]=0.2 M Ka=[S2−][H+]2[H2S] [S2−]=1.2×10−20×[H2S][H+]2 =1.2×10−20×10−14×10−2 = 3×10−20M

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