Question

Of the following sulphides which one is insoluble in dilute acids but soluble in alkalies:
A. \[PbS\]
B. \[CdS\]
C. \[FeS\]
D. \[S{b_2}{S_3}\]

Answer 1

Hint: Metal cations are placed into certain groups based on their properties in qualitative inorganic analysis. The cations of the 2nd analytical group precipitate as sulphides that are insoluble in acids.

Complete Step by Step Solution:
Inorganic qualitative analysis is an analytical chemistry method that involves analysing a given salt (or a mixture of salts) sample to determine its elemental composition. The elemental composition of the salt sample is determined by testing for the presence of their constituent ions. This testing is usually conducted in an aqueous solution therefore, the sample must be brought into an aqueous solution first.

Various reagents are then used to treat this aqueous solution to test for certain characteristic reactions of the ions present. These characteristic reactions usually cause a colour change, precipitation or some other type of visual change.

Anions and cations are analysed separately. They are classified into groups based on their properties. Cations are divided into six analytical groups (which are different from the Groups we study in the Periodic Table). Each analytical group has a common reagent that can be used to separate the cations of that group from the aqueous solution of the sample.

All cations belonging to the 2nd analytical group precipitate as sulphides which are insoluble in acids. The common reagents can be any substance that can furnish sulphide ions (\[{S^{2 - }}\]) in an aqueous medium. Usually, hydrogen sulphide (\[{H_2}S\]) gas is used as the common reagent.

Among the sulphides given in the options, those of lead (\[PbS\]), cadmium (\[CdS\]) and antimony (\[S{b_2}{S_3}\]) will be insoluble in acids since these cations belong to the 2nd analytical group. Among these three sulphides, antimony trisulphide (\[S{b_2}{S_3}\]) is the only one that is soluble in alkalis such as sodium hydroxide (\[NaOH\]) because of the following reaction:
\[2S{b_2}{S_3}(s) + 4NaOH(aq) \to NaSb{O_2}(aq) + 3NaSbS(aq) + 2{H_2}O\]
Thus, option D is correct.

Note: Apart from being soluble in sodium hydroxide, antimony trisulphide is also soluble in concentrated hydrochloric acid. It is used in the manufacture of safety matches, fireworks and explosives, manufacture of ruby-coloured glasses etc.