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
5×10−19
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B
5×10−8
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C
3×10−20
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D
6×10−21
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Solution
The correct option is C3×10−20 According 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