Question

Reagent used in the qualitative analysis of IVth group is:
A.\[HCl\]
B.\[{H_2}S\] (alkaline)
C.\[{\left( {N{H_4}} \right)_2}S\]
D. None of these

Answer 1

Hint:In qualitative inorganic analysis, the cations are classified into six analytical groups. Each group has a common reagent used to separate the cations of that group. The cations are usually separated through precipitation.
Complete step-by-step answer:
The qualitative inorganic analysis is a method in analytical chemistry that aims to determine the elemental composition of a given inorganic compound (or a mixture of compounds). This form of analysis mostly focuses on the detection of the ions of the given inorganic compound in an aqueous solution. If the given compound is in any other form, it must be made into an aqueous solution by dissolving it in water before the analysis can proceed.
Anions and cations are tested separately in this method. Both anions and cations are classified into analytical groups based on their properties and their reactions to certain reagents.
The cations are divided into six analytical groups. Each group has a common reagent which is used to separate the cations of that group from the aqueous solution of the given inorganic compound.
Cations of the 4^th analytical group include \[C{o^{2 + }},Z{n^{2 + }},N{i^{2 + }},M{n^{2 + }}\] etc. These cations precipitate as sulphides that are insoluble at high sulphide ion (\[{S^{2 - }}\]) concentrations therefore, the reagent for the qualitative analysis of the 4^th group is hydrogen sulphide gas (\[{H_2}S\]) passed in the presence of ammonium hydroxide (\[N{H_4}OH\]).
The purpose of the ammonium hydroxide is to increase the concentration of the sulphide ions furnished by the hydrogen sulphide gas. Hydrogen sulphide dissociates in an aqueous medium as follows:
\[{H_2}S \rightleftharpoons 2{H^ + } + {S^{2 - }}\]
As the concentration of\[{H^ + }\] ions increases, the equilibrium tends to shift to the left inhibiting any further dissociation of\[{H_2}S\] and the concentration of the\[{S^{2 - }}\]ions decreases. Thus, excess \[{H^ + }\] ions need to be removed from the solution. This is where the ammonium hydroxide comes in. Ammonium hydroxide dissociates in an aqueous medium as follows:
\[N{H_4}OH \rightleftharpoons N{H^{4 + }} + O{H^ - }\]
The \[O{H^ - }\] ions take up the excess \[{H^ + }\] ions to form water.
\[O{H^ - } + {H^ + } \to {H_2}O\]
Thus, option B is correct.

Note:To obtain reliable results when testing for 4^th group cations, it must be ensured that the hydrochloric acid (\[HCl\]) used in the analysis of 2^nd group cations is very dilute. This is because the 2^nd group cations also precipitate as sulphides. If the HCl used is not dilute, any 4^th group cations that might be present will precipitate during the 2^nd group analysis and create misleading results.