Question

A salt, when warmed with zinc powder and an $NaOH$ solution, gives a gas that turns a filter paper soaked with an alkaline solution of ${{K}_{2}}[Hg{{I}_{4}}]$ brown. The salt responds to the brown ring test when acetic acid is used in place of sulphuric acid. The anion present in the salt is:
A.$NO_{3}^{-}$
B.$NO_{2}^{-}$
C.$B{{r}^{-}}$
D.None of these

Answer 1

Acetic acid is also known as ethanoic acid, which is a colorless liquid, and has a chemical formula of $C{{H}_{3}}COOH$ . In this question, a brown ring test is used in which a brown ring appears at the junction of two liquids.

Complete step by step answer:
In this question, we can see that the given salt is responding to the brown ring test. This test is basically performed to detect the presence of nitrate ions.
In brown ring test, when a nitrate solution is added to concentrated solution of iron sulphate, which is acidified with concentrated acetic acid, it results in the formation of brown ring at the junction of the two liquids due to the formation of $\left[ Fe{{\left( {{H}_{2}}O \right)}_{5}}NO \right]S{{O}_{4}}$ .
$2HN{{O}_{3}}+3{{H}_{2}}S{{O}_{4}}+6FeS{{O}_{4}}\to 3F{{e}_{2}}{{(S{{O}_{4}})}_{3}}+2NO+4{{H}_{2}}O$
$\left[ Fe{{({{H}_{2}}O)}_{6}} \right]S{{O}_{4}}+NO\to \left[ Fe{{\left( {{H}_{2}}O \right)}_{5}}NO \right]S{{O}_{4}}+{{H}_{2}}O$
In the overall reaction, the nitrate ion is reduced to nitric oxide by iron $(II)$ , that is further oxidized to iron $(III)$ , after that, nitric oxide and remaining iron $(II)$ react with each other to form nitrosyl complex, that is the brown ring complex.
Here, $\left[ Fe{{\left( {{H}_{2}}O \right)}_{5}}NO \right]S{{O}_{4}}$ results in the appearance of brown ring.
Therefore, the anion present in the salt is $NO_{2}^{-}$
Hence, the correct option is (B) $NO_{2}^{-}$ .

Additional information:
The nitrate ion can also be identified when copper turnings are heated along with concentrated sulphuric acid. A pungent smell is evolved which turns brown litmus to red.
Potassium tetraiodomercurate $({{K}_{2}}[Hg{{I}_{4}}])$ consists of potassium cations and tetraiodomercurate anion. It is an inorganic compound, which is used as Nessler’s reagent. It is used for the detection of ammonia.
Nitrate anion is also an oxidizer and the test conducted for the nitrate anion depends on its property. Almost all nitrates are soluble in water.

Note: In the conclusion, we can say that the brown ring test is basically performed to detect the presence of nitrate anion in any sample. It is verified by the brown ring formed at the junction of two solutions.
Filter paper is a semi – permeable paper, which is used to filter out impurities from the sample. It allows certain molecules or ions to pass through it.