Discussion Forum

1)

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 S^2- from HS^- ions is 1.2×10^-13 then the concentration of S^2- ions in aqueous solution is

 

Answer:

Option B

Explanation:

Given

     [H₂S]=0.10 M

    [HCl]=0.20 M

    [H⁺]=0.20 M

So,

$H_{2}S\rightleftharpoons H^{+}+HS^{-},K_{1}=1.0\times 10^{-7}$

$HS^{-}\leftrightarrows H^{+}+S^{2-},K_{2}=1.2\times 10^{-13}$

It means for,

  $H_{2}S\rightleftharpoons 2H^{+}+S^{2-}$

$K=K_{1}\times K_{2}$

$1.0\times 10^{-7} \times 1.2\times 10^{-13}$

=1.2× 10^-20

Now      $[S^{2-}]=\frac{K\times [H_{2}S]}{[H^{+}]^{2}}$  [according to the final equation]

   =  $\frac{1.2\times 10^{-20}\times 0.1 M}{(0.2M)^{2}}$

  = $\frac{1.2\times 10^{-20}\times 1 \times 10^{-1}M}{4\times 10^{-2}M}$

   = 3× 10^-20M