Question

The cation that will not be precipitated by ${{H}_{2}}S$ in the presence of dil. $HCl$ is:
(A) $A{{s}^{3+}}$
(B) $P{{b}^{2+}}$
(C) $C{{u}^{2+}}$
(D) $C{{o}^{2+}}$

Answer 1

Think about the inorganic qualitative salt analysis. Think about the cations which do not get precipitated out in acidic medium. Take a look at the options and find out the groups of each action. We know, in initial groups acidic medium is used and then in later and last groups, basic medium is used. Choose the cation which precipitates in the basic medium on action of ${{H}_{2}}S$ gas.

Complete step by step solution:
- Inorganic qualitative salt analysis is the method used to detect the presence of cations and anions in an unknown salt mixture.
- Let’s take a look at the cations given in the options.
- $A{{s}^{3+}}$, $P{{b}^{2+}}$ and $C{{u}^{2+}}$ are the cations which easily precipitate out in the presence of dil. $HCl$ and in action of ${{H}_{2}}S$ gas. These cations are group II cations. These cations will precipitate out as chlorides or sulphides.
- $C{{o}^{2+}}$ is a cation which will not precipitate out in an acidic medium and in action of ${{H}_{2}}S$ gas as well. It is a group IV cation which will precipitate out of action of ${{H}_{2}}S$ gas in a basic medium that is, in the presence of ammonium hydroxide.
- Dilute $HCl$ will suppress the ionization of hydrogen sulphide, ${{H}_{2}}S$ gas and so, $C{{o}^{2+}}$ will not precipitate out.
- $C{{o}^{2+}}$ will only precipitate out as a sulphide in a strongly basic medium.

Therefore, the correct answer is option (D).

Note: Remember lead, arsenic and copper are group II cations and cobalt is a group IV cation. Dilute $HCl$ suppresses the ionization of hydrogen sulphide whereas ammonium hydroxide will enhance the ionization of ${{H}_{2}}S$ and therefore, in alkaline medium, group IV cations will precipitate out as sulphides.