For the following reaction, the equilibrium constant K _ e at 298K...

{\text{Fe}}^{\text{2+}} + {\text{S}}^{\text{2-}} ⇌ \text{Fe}{\text{S}}_{( \text{s} )}$.....${\text{K}}_{\text{c}} = 1.6 \times {10}^{17}$<br />$\text{t} = 0$:$0.03\text{M}\left(0.1\text{M}\right)$<br />$\text{t} = \text{t;}\left(0.03 - \text{H}\right)\left(0.1\text{H}\right)$<br />Since${\text{K}}_{\text{c}} > > 1,$<br />$n ≃ 0.03$<br />$∴ [ {\text{S}}^{\text{2-}} ] = 0.1 - 0.031 = 0.07\text{M}$<br />${\text{K}}_{\text{c}} = \left[{\text{Fe}}^{2 +}\right]\left[{\text{S}}^{\text{2-}}\right] = \dfrac{1}{1.6 \times {10}^{17}}$<br />\left[{\text{Fe}}^{2 +}\right] = \dfrac{1}{1.6 \times 0.07} \times {10}^{- 17}$
$\text{y} = \dfrac{1}{1.6 \times 0.07} ≃ 8.93$