Question

In an alkaline solution, the following equilibrium exists.

$S^{2-}+S\leftrightharpoons S_2^{2-},K_1=12$
$S_2^{2-}+S\leftrightharpoons S_3^{2-},K_2=11$

What is the equilibrium constant for the given equilibrium?

$S_3^{2-}\leftrightharpoons S^{2-}+2S$

• A. $23$
• B. $132$
• C. $1/132$
• D. $1/32$

Answer 1

Answer: (C)

$1/132$

$K_1=\frac{[S_2{]}^{2-}}{\left[S\right]\left[S^{2-}\right]}=12$

$K_2=\frac{[S_3{]}^{2-}}{\left[S\right][S_2{]}^{2-}}=11$

For the required equation, we have to add equation number $1$ and $2$ and reverse it.

Now, adding equation means to multiply equilibrium constant.

But after adding equation we get the reverse reaction of the required one. Hence we have to reverse the reaction. Now, reversing the reaction means to find the reciprocal of the equilibrium constant.

Hence, take the reciprocal to get the required $K$.

$K=\frac{1}{K_1\times K_2}=\frac{1}{132}$