Question

What reference to aqua regia, choose the correct option(s):
(A) Reaction of gold with aqua regia produces $N{O_2}$ in the absence of air.
(B) The yellow color of aqua regia is due to the presence of $NOCl$ and $C{l_2}$ .
(C) Aqua regia is prepared by mixing conc. $HCl$ and conc. $HN{O_3}$ in 3: 1 (v/v) ratio.
(D) Reaction of gold with aqua regia produces an anion having $Au$ in +3 oxidation state.

Answer 1

For solving this question, let’s first understand the basics of aqua regia. Then, we can easily choose for the correct options. Aqua regia is a mixture of concentrated hydrochloric acid and concentrated nitric. Also, aqua regia is a highly corrosive liquid that is able to attack on gold and other water-resistant substances.

Complete step by step answer:
As we know that aqua regia is formed by mixing concentrated hydrochloric acid and concentrated nitric acid in a molar ratio of 3:1. So, option C is true that states that when we mix 3 parts of concentrated hydrochloric acid with one part of concentrated nitric acid then aqua regia is obtained.
Also, we know that aqua regia is used to purify or wash gold. So, when it reacts with aqua regia then an anion is formed that has $Au$ in +3 oxidation state.
So, the reaction when $Au$ reacts with aqua regia becomes:
$Au + 4{H^ + } + N{O_3}^ - + 4C{l^ - } \to {[AuC{l_4}]^ - } + NO + 2{H_2}O$
In this reaction $N{O_3}^ -$ has +5 oxidation states and $Au$ in ${[AuC{l_4}]^ - }$ has oxidation state of +3. Thus, option D in reference to aqua regia is also true.
Now, let’s go to option A which says that when gold is reacted with aqua regia then $N{O_2}$ is produced in the absence of air. But when we check this reaction, then we get to see that the reaction produces $NO$ and not $N{O_2}$. So, option A is invalid.
$Au + 4{H^ + } + N{O_3}^ - + 4C{l^ - } \to {[AuC{l_4}]^ - } + NO + 2{H_2}O$
Finally, we will check option B. here, we need to know that the color of $NOCl$ is yellow. But when we form aqua regia the reaction becomes:
$3HCl + HN{O_3} \to NOCl + C{l_2} + 2{H_2}O$
So, the yellow color of $NOCl$ and $C{l_2}$ turns the color of aqua regia formed to yellow. Thus, this option is also correct.
So, we conclude that option B, C, and D are correct with respect to aqua regia.

Note:
We must know that the aqua regia can dissolve noble metals like gold and platinum. We are using the aqua regia to produce chloroauric acid, the electrolyte In the Wohlwill process for refining the highest quality gold. Also we can use aqua regia in etching processes and special analytical procedures. In some laboratories, it is also used to clean organic compounds and metal particles from glassware.