Question

Sulphide ions in alkaline solution react with solid sulphur to form polyvalent sulphide ions. The equilibrium constant for the formation of $S_2^{2-}$ and $S_3^{2-}$ from. S and $S^{2-}$ ions are 1.7 and 5.3 respectively. Calculate equuilibrium constant for the formation of $S_3^{2-}$ from $S_2^{2-}$ and $S$.

• A. 0.311
• B. 3.11
• C. 31.3
• D. 0.0313

Answer 1

Answer: (B)

3.11

The given equilibrium expressions are $S\left(s\right)+S^{2-}\iff S_2^{2-},K_1=\frac{\left[S_2^{2-}\right]}{\left[S\right]\left[S^{2-}\right]}=1.7$ ......(1)

$2S\left(s\right)+S^{2-}\iff S_3^{2-},K_2=\frac{\left[S_3^{2-}\right]}{\left[S{]}^2\right[S^{2-}]}=5.3$ ......(2)

$S_2^{2-}+S\iff S_3^{2-},K=\frac{\left[S_3^{2-}\right]}{\left[S_2^{2-}\right]\left[S\right]}$ ......(3)

The relationship between three equilibrium constants is $K=\frac{K_2}{K_1}=\frac{5.3}{1.7}=3.11$.