A mixture consists of A (yellow solid) and B (colourless solid) which gives lilac colour in flame.
(a) Mixture gives black precipitate C on passing H₂S(g) through its aqueous solution.
(b) C is soluble in aqua-regia and on evaporation of aquaregia and adding SnCl₂ gives greyish black precipitate D.
The salt solution with NH₄OH gives a brown precipitate.
(i) The sodium carbonate extract of the salt with CCl₄/FeCl₃ gives a violet layer.
(ii) The sodium carbonate extract gives yellow precipitate with AgNO₃ solution which is insoluble in NH₃.
Identify A and B, and the precipitates C and D.
Answers
Aqua regia dissolves gold, though neither constituent acid will do so alone, because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a virtually undetectable amount of gold, forming gold ions (Au3+). The hydrochloric acid provides a ready supply of chloride ions (Cl−), which react with the gold ions to produce tetrachloroaurate(III) anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction which favors formation of chloroaurate anions (AuCl4−). This results in a removal of gold ions from solution and allows further oxidation of gold to take place. The gold dissolves to become chloroauric acid. In addition, gold may be dissolved by the free chlorine present in aqua regia. Appropriate equations are:
Au+3HNO3+4HCl↽−⇀AuCl4−+3NO2+H3O++2H2O
Au+HNO3+4HCl↽−⇀AuCl4−+NO+H3O++H2O
If the aqua regia solution only contains gold, solid tetrachloroauric acid may be prepared by boiling off excess aqua regia, and removing residual nitric acid by repeatedly heating with hydrochloric acid.