Question

in the reaction h2s 2h+ + s2- when nh4oh is added

Answer 1

The solubility product (Ksp) of the sulphides of second group is very low. As a result, we need less concentration of sulphide ions to precipitate them. If the concentration is not kept very low then the sulphides of all the group 4 cations will also be precipitated due to which we won't be able to judge whether group 2 metal ion is present in salt or not. So to add sulphide (S2-) ions in our water extract we add H2S but H2S dissociates more than required and result in a solution with excess of sulphide ions which is not favourable for our analysis. So to reduce the concentration of sulphide ions, we need to make sure that the solution already contains some H+ ions so that by COMMON ION EFFECT, H2S dissociates less and we get less concentration of sulphide ions to precipitate the required group 2 sulphides for analysis. Hence, we add did HCl in our water extract!

HCl → H+ + Cl-

H2S → 2H+ + S2-

Common ion effect of H+ ions is thus, required for less concentration of sulphide ions.

Hope this helps!