Question

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

Answer 1

Aqua regia is a mixture of HCl and $HN{{O}_{3}}$. Aqua Regia is a highly corrosive solution. This mixture can dissolve such salts that are not soluble in concentrated HCl or concentrated $HN{{O}_{3}}$ solution.

Complete step by step solution:
-Aqua regia is a mixture of concentrated nitric and concentrated hydrochloric acid. It was given another name of “royal water” by alchemists. Below is the reaction of how Gold reacts with aqua regia.
$Au+3HN{{O}_{3}}+4HCl\rightleftharpoons {{[AuC{{l}_{4}}]}^{-}}+3[N{{O}_{2}}]+{{[{{H}_{3}}O]}^{+}}+2{{H}_{2}}O$
-Aqua regia or royal water has the ability to dissolve gold and other so-called noble metals.
-Now, we will know the reason why gold metal can dissolve in aqua regia. We know that nitric acid is a powerful oxidizer, which will dissolve an undetectable amount of gold. We should know that it will form gold ions ($A{{u}^{3+}}$) and also we should know that the hydrochloric acid provides a ready supply of chloride ions ($C{{l}^{-}}$) which react with the gold ions to produce tetrachloroaurate (III) anions. The reaction with hydrochloric acid is an equilibrium reaction that favours the formation of chloroauric anions ($AuC{{l}^{4-}}$). This results in the removal of gold ions from solution and allows further oxidation of gold to take place. The gold dissolves to become chloroauric acid. We can represent the above paragraph in a chemical equation.
-Aqua regia is a mixture of conc. nitric acid and conc. hydrochloric acid. And we use it in the ratio of 1:3. It means that to form aqua regia we should take three parts of hydrochloric acid and one part of nitric acid.
-Now, let’s know about the manufacturing of aqua regia. We should note that when we mix hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile products nitrosyl chloride and chlorine gas:
$HN{{O}_{3(aq)}}+3HC{{l}_{(aq)}}\to NOC{{l}_{(g)}}+C{{l}_{2(g)}}+2{{H}_{2}}{{O}_{(l)}}$
We know that the above reaction will make a fuming mixture and characteristic yellow colour of aqua regia. As the volatile products escape from solution, the aqua regia loses its potency. Nitrosyl chloride can further decompose into nitric oxide and chlorine:
$2NOCl\left( g \right)\to 2NO\left( g \right)+\text{C}{{l}_{2}}\left( g \right)$
For example: if we want to make 40ml aqua regia then we should take 30ml hydrochloric acid and 10ml nitric acid. They both should be concentrated.

So, from this, we can say that option (C) is the correct answer.

Note: Remember that there is no water content present in aqua regia. So, do not get confused about whether its constituents are concentrated acids or diluted acids. Aqua can dissolve some noble metals but it cannot dissolve all metals.