Question

The composition of aqua regia is:
(A) Dil.HCl (3) : Conc. $HN{O_3}$(1)
(B) Conc.HCl (3) : Dil. $HN{O_3}$(1)
(C) Conc.HCl (3) : Conc. $HN{O_3}$(1)
(D) Dil.HCl (3) : Dil. $HN{O_3}$(1)

Answer 1

Hint:. Aqua regia is fuming liquid that is used as an electrolyte in refining of the highest quality gold, to prepare chloroauric acid and sometimes to clean the glassware of organic compounds. Firstly, we have to think about what makes aqua regia special and significant from other acid compositions.

Complete step by step answer:
Firstly in order to understand what makes aqua regia special and significant we should know about noble metals. Noble metals are the metals which have high resistance towards oxidation, action with acids, etc. Basically we can say it is almost inert chemically. These noble metals are rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold. Among which the most inert noble metals are gold and platinum. The gold and platinum are easily dissolved in aqua regia solution. This makes the aqua regia significant. Aqua regia is the combination of concentrated hydrochloric acid and concentrated nitric acid in a molar ratio of 3:1 that is the combination of 3 parts of hydrochloric acid and 1 part of concentrated nitric acid.
The reaction involved: \[3HC{l_{(aq)}} + HN{O_{{3_{(aq)}}}} \to NOC{l_{(g)}} + C{l_{{2_{(g)}}}} + 2{H_2}{O_{(aq)}}\]

The concentrated nitric acid is a good oxidizing agent. The chlorine formed in the above reaction is due to oxidation of hydrochloric acid due to nitric acid and the chlorine formed is known as nascent chlorine, which actually reacts with gold to form salt that is gold chloride.
So, the correct answer is “Option C”.

Note: From the option given we can figure out that aqua regia is the combination of hydrochloric acid and nitric in molar ratio 3 :1. Now, a question arises: the acids used are concentrated or diluted. Some noble metals react with hot concentrated acids, but none of the noble metals reacts with dilute acids in the first place. Therefore option C is the only option where both hydrochloric acid and nitric acids are given as concentrated.