Question

Why ammonium chloride is and ammonium hydroxide is added before passing h2s gas?

Answer 1

Ammonium chloride is and ammonium hydroxide is added before passing h2s gas because of common ion effect.

Common ion effect : it is the reduction in the ionisation of a weak acid/ base due to the presence of a common ion in the solution.

NH4Cl is a strong electrolyte and so ionise completely while NH4OH being a weak electrolyte does not. NH4CL provides the common Ion NH4+

which further suppresses the ionisation of NH4OH. so as to decrease the OH- ion concentration so higher group cations precipitated.