Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

A salt on treatment with dil. HCl gives a pungent smelling gas and a yellow precipitate. The salt gives green flame when tested. The solution gives a yellow precipitate with potassium chromate. The salt is (a) \(\mathrm{BaS}_{2} \mathrm{O}_{3}\) (b) \(\mathrm{CuSO}_{4}\) (c) \(\mathrm{PbS}_{2} \mathrm{O}_{3}\) (d) \(\mathrm{NiSO}_{4}^{*}\)

Step-by-Step Solution

Answer

The salt is (c) PbS₂O₃.

Step 1: Analyze the Reaction with Dilute HCl

The salt gives a pungent smelling gas and a yellow precipitate on treatment with dilute HCl. The pungent gas could be identified as sulfur dioxide (SO₂) or hydrogen sulfide (H₂S), both known for their characteristic odors. The yellow precipitate in conjunction with HCl suggests the formation of lead(II) chloride (PbCl₂), which is known to be yellow under certain conditions.

Step 2: Examine the Flame Test

The salt burns with a green flame. This color change is indicative of the presence of copper ions. Copper compounds are known to impart a green flame when burned.

Step 3: Consider Precipitation with Potassium Chromate

The appearance of a yellow precipitate with potassium chromate implies the presence of lead ions, as lead chromate ( ext{PbCrO}_4) is a yellow solid. This points to the possibility of lead being part of the original compound.

Step 4: Combine Observations to Identify the Salt

Based on the clues: a pungent gas, a yellow precipitate with both HCl and potassium chromate, and the green flame, the likely salt is one that contains lead and copper, points to the salt as containing lead ions capable of forming lead(II) chloride and lead chromate precipitates. Thus, combining all observations, the most consistent salt is ext{PbS}_2 ext{O}_3, which matches the given reactions and observations.

Key Concepts

Flame Test

The flame test is a simple and effective method used to identify the presence of certain metal ions based on the color they emit when heated. When a salt is introduced to a flame, the heat excites the electrons in the metal ions that are part of the compound. Upon returning to their ground state, these electrons release energy in the form of light.

Each element typically emits light at characteristic wavelengths, resulting in specific colors. For instance, copper ions are known to impart a green color to the flame. This is because the excited state of copper ions releases light in the range that appears green to the human eye. Other metals like sodium and potassium emit different colors, such as yellow and lilac respectively.

In the context of salt analysis, observing a green flame when testing with fire can strongly suggest the presence of copper ions. This information helps eliminate certain compounds and focus on those that contain copper.

Precipitation Reactions

Precipitation reactions occur when two solutions containing soluble salts are mixed, and an insoluble solid, called a precipitate, forms. This type of reaction is common in salt analysis because it is easy to observe visually based on the formation of colored solids.

For instance, mixing a solution containing lead ions with potassium chromate results in the formation of lead chromate. Lead chromate is a classic yellow precipitate, indicating the presence of lead ions in the original salt solution. Reactions like these are essential for confirming the presence of certain ions in a sample.

Such precipitations are not random but follow rules of solubility. Knowing these rules can assist in predicting the outcome when different salts are combined. In practical applications, analyzing the color and texture of the precipitate further aids in identifying the specific ions present in the solution.

Chemical Reactions with HCl

When a salt reacts with dilute hydrochloric acid (HCl), several observations can be made that help identify the chemical composition of the salt. One key reaction is the evolution of a pungent gas, which is a significant identifier in qualitative analysis.

In this exercise, the salt gives off a pungent smelling gas upon treatment with dilute HCl. This gas is typically hydrogen sulfide (H₂S) or sulfur dioxide (SO₂). Both gases are known for their distinctive and strong odors, which provide clues about the presence of sulfide or sulfite ions in the salt.

Moreover, a yellow precipitate forming can indicate the presence of lead ions when soluble lead compounds react with chloride ions to form lead(II) chloride, a solid under specific conditions. Understanding these chemical behaviors helps in deciphering what compounds are present.

Pungent Gas Identification

Identifying gases by their smell is a technique often used in laboratory settings to confirm the presence of specific chemicals. A pungent odor can be a strong indicator of certain volatile compounds.

When a compound releases a pungent gas upon reaction with an acid like HCl, it's usually indicative of gases such as hydrogen sulfide (H₂S) or sulfur dioxide (SO₂). These gases, although often not pleasant, are quite revealing in terms of their chemical nature.

Hydrogen sulfide has a characteristic "rotten egg" smell, easily recognizable, whereas sulfur dioxide has a sharp, suffocating odor. These distinct odors help chemists quickly narrow down potential sources of certain ions in the original compound.

In practical salt analysis, the quick identification of such gases aids in piecing together the identity of the unknown salt alongside other tests and reactions, leading to a coherent conclusion.