Question

In alkaline solution, the following equilibria exist:
a. $S^{2-}+S\to S_2^{2-}$ equilibrium constant $K_1$
b. $S_2^{2-}+S\to S_3^{2-}$ equilibrium constant $K_2$
$K_1$ and $K_2$ have values 12 and 11, respectively.
$S_3^{2-}\to S^{2-}+2S$. What is equilibrium constant for the reaction

• A. $132$
• B. $7.58\times {10}^{-3}$
• C. $1.09$
• D. $0.918$

Answer 1

The correct option is B $7.58\times {10}^{-3}$

i. $S^{2-}+S\to S_2^{2-}$ ......$K_1$
ii. $S_2^{2-}+S\to S_3^{2-}$...... $K_2$

The required reaction is:
iii. $S_3^{2-}\to S^{2-}+2S$,

Since eq. (iii) is obtained by reversing and adding equations (i) and (ii).

Hence, $K=\frac{1}{K_1\times K_2}=\frac{1}{12\times 11}=7.58\times {10}^{-3}$