JEE Main On 08 April 2018 Question 20
Question: An aqueous solution contains 0.10 M $ {H_2}S $ and 0.20 M $ HCl $ . If the equilibrium constants for the formation of $ H{S^{-}} $ from $ {H_2}S $ is $ 1\text{.0}\times 1{0^{-7}} $ and that of $ {S^{2-}} $ from $ H{S^{-}} $ ions is $ 1\text{.2}\times 1{0^{-13}} $ then the concentration of $ {S^{2-}} $ ions in aqueous solution is: [JEE Main Online 08-04-2018]
Options:
A) $ 6\times {10^{-21}} $
B) $ 5\times {10^{-19}} $
C) $ 5\times {10^{-8}} $
D) $ 3\times {10^{-20}} $
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Answer:
Correct Answer: D
Solution:
$ \text{HCl(aq)}\xrightarrow{{}}\underset{0\text{.2M}}{\mathop{{H^{+}}\text{(aq)+}}},C{l^{-}}\text{(aq)} $
$ {H_2}S(aq){H^{+}}(aq)+H{S^{-}}(aq),k _1=1\times {10^{-7}} $
$ H{S^{-}}(aq){H^{+}}(aq)+{S^{-2}}(aq),k _2=1.2\times {10^{-13}} $
$ {H_2}{S^{-}}(aq)2{H^{+}}(aq)+{S^{-2}}(aq),K _{eq}=k _1\times k _2 $
$ 0\text{.1M} $ $ {K _{eq.}}=1.2\times {10^{-20}}=\frac{{{[0.2]}^{2}}[{S^{-2}}]}{0.1} $
$ [{S^{\text{-2}}}]\text{ =3 }\times\text{ 1}{0^{\text{-20}}}M $
