The K _ sp of FeS = 4 × 10 ^ - 19 at 298K. The minimum...

\left[{Fe}^{2 +}\right]\left[S^{– 2}\right] = 4 \times {10}^{– 19} ⇒ \left[S^{– 2}\right] = \dfrac{4 \times 1{0}^{– 19}}{1 \times 1{0}^{– 2}} = 4 \times {10}^{– 17}M$<br/><br/>In order to precipitate$FeS, \left[S^{– 2}\right]$required is$4 \times {10}^{– 17}M$from$0.01M{Fe}^{2 +}$salts.<br/><br/>Now$\dfrac{{\left[H^{+}\right]}^{2}\left[4 \times 1{0}^{– 17}\right]}{0.1} =1 \times {10}^{– 21}$<br/><br/>$⇒ {\left[H^{+}\right]}^{2} = 2.5\times {10}^{– 6}

⇒ \left[ H^{+}\right] = 1.6 \times {10}^{– 3}