Question

Explain the following:
(i) When ${NH}_{4}Cl$ and ${NH}_{4}OH$ are added to a solution containing both, ${Fe}^{3+}$ and ${Ca}^{2+}$ ions,which ion is precipitated first and why?
(ii) Dissociation of $H_{2}S$ is suppressed in acidic medium.

Answer 1

(i) When ${NH}_{4}Cl$ and ${NH}_{4}OH$ are added to the solution containing both ${Fe}^{3+}$ and ${Ca}^{2+}$ ions, ${Fe}^{3+}$ ions are precipitated first. ${NH}_{4}OH$ is a weak electrolyte. Presence of ${NH}_{4}Cl$ suppresses dissociation of ${NH}_{4}OH$ further die to common ion effect.
at low concentration of ${OH}^{-}$ ions the ionic product of ${Fe}^{+++}$ and ${OH}^{-}$ is eough to precipitate ${Fe}^{3+}$ ions as $Fe{\left(OH\right)}_3$ as $K_{sp}$ of $Fe{\left(OH\right)}_3$ is quite low as compared to that of $Ca{\left(OH\right)}_2$.
(ii) $H_{2}S⟶2H^{+}+S^{2-}$
$K_{dissociation}=\frac{{\left[H^{+}\right]}^2\left[S^{2-}\right]}{\left[H_{2}S\right]}$
In acidic medium, the concentration of $H^{+}$ ions increases. As a result, due to common ion effect the dissociation of $H_{2}S$ gets suppressed.