Question

A mixture is known to contain \[{\text{N}}{{\text{O}}_3}^ - {\text{ and N}}{{\text{O}}_2}^ - \]. Before performing ring test for \[{\text{N}}{{\text{O}}_3}^ - \], the aqueous solution should me made free of \[{\text{N}}{{\text{O}}_2}^ - \]. This is done by heating aqueous extract with:
A.\[{\text{conc}}{\text{. HN}}{{\text{O}}_3}\]
B.\[{\text{dil}}{\text{. HN}}{{\text{O}}_3}\]
C.Urea
D.Zinc dust

Answer 1

Hint: We use this method in the laboratory while element detection tests. The compound that is used to remove \[{\text{N}}{{\text{O}}_2}^ - \] ions are widely used as fertilizer. It is an organic compound containing carbonyl functional groups.

Complete step by step answer:
A very common test used for the detection of nitrate ions is known as brown ring test. Iron sulphate is added to a solution containing nitrate io, and then concentrated sulphuric acid is poured from the sides of the test tube. A brown ring is seen being formed at a junction between the two layers. The formation of brown rings confirms the presence of nitrate ions.
The basic principle is that in the nitrate ion, the nitrogen is in maximum oxidation state and hence cannot further oxidize, so it gets reduced itself and acts as an oxidizing agent. The problem arises when some other oxidizing ion is also present and hence interferes in the process of detection which may give incorrect results, because since the nitrate ion is not present but the result is still positive because of other ions. So we have to remove the nitrite ion before we start testing for nitrate, this is by reacting it with urea.
\[{\text{HN}}{{\text{O}}_2} + {\text{CO}}{({\text{N}}{{\text{H}}_2})_2} \to {\text{C}}{{\text{O}}_2} + {{\text{H}}_2} + 3{{\text{H}}_2}{\text{O}}\]

Hence the correct option is Urea.

Note:
The product formed urea has a molecular formula \[{\text{CO}}{({\text{N}}{{\text{H}}_2})_2}\]. It has two amide groups and one carbonyl group. There are certain side effects of using Urea. It causes irritation in eyes, skin and respiratory system. Lower concentration of urea inhaled is not harmful for the human body but the higher concentration can cause damage to blood.