Question

In IV group analysis $N{H_4}OH$ is added before passing ${H_2}S$ gas because:
(A) The sulphides of IV group are insoluble in $N{H_4}OH$
(B) The sulphides of other metals are soluble in $N{H_4}OH$
(C) The concentration of ${S^{2 - }}$ ions is high enough to precipitate the sulphides of the IV group
(D) The sulphides of the second group are soluble in $N{H_4}OH$

Answer 1

To answer this question, you should recall the qualitative analysis of group 4 cations. Qualitative analysis of a salt is a chemical technique used to identify the ions present in a salt by analyzing its physical and chemical properties and hence determine the identity of the salt.

Complete step by step solution:
Solubility products of sulphides of group (II) radicals are less than solubility products of sulphides of (IV) group. So to precipitate (II) group radicals as sulphides ${S^{2 - }}$ are sufficient. In presence of Dil $HCl{S^{2 - }}$ are released due to the common ion effects. In (IV) group the ionisation of ${H_2}S$ is favoured by the use of $N{H_4}OH$
Hence, the correct option is C.

Additional information
We know that ${H_2}S$ is identified by its odour and its precipitation of coloured sulphides of various metal ions. Sulphides or hydrogen sulphide also are oxidized to elemental sulphur giving black precipitate and sulphate by oxidizing agents such as permanganate, nitric acid, sulfuric acid, ferrous, etc.
The reaction can be represented as: $3{H_2}S\left( {aq} \right){\text{ }} + {\text{ }}2{H^ + }\left( {aq} \right){\text{ }} + {\text{ }}2N{O_3}^ - \left( {aq} \right){\text{ }} \to {\text{ }}2NO\left( g \right){\text{ }} + {\text{ }}4{H_2}O{\text{ }} + {\text{ }}3S\left( s \right)$ . On passing ${H_2}S$ the salt of metal ion converts to its sulphide

Note:
We should know the colour of important cations and also remember the reactions of important anions: $C{O_3}^{2 - }{\text{ }},{\text{ }}C{H_3}CO{O^-},{\text{ }}{C_2}{O_4}^{2 - },P{O_4}^{3 - },{\text{ }}S{O_4}^{2 - }$ as they are most commonly asked in competitive examinations.

Colour of salt	Cation Present
Deep Green or purple	${\text{C}}{{\text{r}}^{{\text{3 + }}}}$
Whitish pink	${\text{M}}{{\text{n}}^{2 + }}$
Deep red	${\text{C}}{{\text{o}}^{2 + }}$
Green	${\text{F}}{{\text{e}}^{{\text{3 + }}}}$
Brown or yellow	${\text{F}}{{\text{e}}^{2 + }}$
Dark blue	${\text{C}}{{\text{o}}^{2 + }}$
Green	${\text{N}}{{\text{i}}^{2 + }}$
Proper blue	${\text{C}}{{\text{u}}^{2 + }}$
Green or blue	${\text{C}}{{\text{u}}^{2 + }}$