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Question

Chemistry Question on Equilibrium

An aqueous solution contains 0.10MH2S0.10 \, M \, H_2S and 0.20MHCl0.20 \, M \, HCl . If the equilibrium constants for the formation of HSHS^- from H2SH_2S is 1.0×1071.0 \times 10^{-7} and that of S2S^{2- } from HSHS^- ions is 1.2×1013 1.2 \times 10^{-13} then the concentration of S2S^{2-} ions in aqueous solution is -

A

5×1085 \times 10^{-8}

B

3×10203 \times 10^{-20}

C

6×10216 \times 10^{-21}

D

5×10195 \times 10^{-19}

Answer

3×10203 \times 10^{-20}

Explanation

Solution

In presence of external H+H ^{+},

H2S2H++S2,Ka1Ka2=KeqH _{2} S \rightleftharpoons 2 H ^{+}+ S ^{2-}, K _{ a _{1}} \cdot K _{ a _{2}}= K _{ eq }
[H+]2[S2][H2S]=1×107×1.2×1013\therefore\,\,\, \frac{\left[ H ^{+}\right]^{2}\left[ S ^{2-}\right]}{\left[ H _{2} S \right]}=1 \times 10^{-7} \times 1.2 \times 10^{-13}
[0.2]2[S2][0.1]=1.2×1020\frac{[0.2]^{2}\left[ S ^{2-}\right]}{[0.1]}=1.2 \times 10^{-20}
[S2]=3×1020{\left[ S ^{2-}\right]=3 \times 10^{-20}}