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What is the concentration of $\mathrm{[S^{2-}]}$ in a solution containing $\mathrm{0.1\ M\ H_2S}$ and $\mathrm{0.1\ M\ HCl}$ ?

$\mathrm{(Given\ for\ H_2S, k_{a_1}= 10^{-7}, k_{a_2}=10^{-14})}$
Option: 1
$10^{-15}$

Option: 2
$10^{-17}$

Option: 3
$10^{-18}$

Option: 4
$\mathrm{10^{-20}}$

Answers (1)

best_answer

$\mathrm{HCl}$ is a strong acid and $\mathrm{[H^+]_{HCl}=0.1\ M}$

Now, dissociation of $\mathrm{H_2S}$ ,

$\mathrm{H_2S\ \ \rightleftharpoons\ \ 2\ H^+ +\ \ S^{2-},\ \ K= k_{a_1}\times k_{a_2}=10^{-21} }$

$0.1$ $0.1$ $-$

$\mathrm{(0.1-x)}$ $\mathrm{(0.1+2x)}$ $\mathrm{(x)}$

$\mathrm{\approx 0.1}$ $\mathrm{\approx 0.1}$

Writing the expression for equilibrium constant,

$\mathrm{\frac{x \times (0.1)^2}{0.1}=10^{-21}}$

$\Rightarrow\mathrm{x=10^{-20}}$

$\mathrm{\therefore [S^{2-}]=10^{-20}\ M}$

Thus, the correct answer is Option (4)

Posted by

shivangi.bhatnagar

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