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) PhS O (d) NiSO

Step-by-Step Solution

Answer

The salt is (a) \( \mathrm{BaS}_{2} \).

Step 1: Analyze the Gaseous Product

The salt is treated with dilute HCl and produces a pungent smelling gas. Hydrogen sulfide (H₂S) is known for having a characteristic pungent odor. Therefore, the gas produced is likely H₂S.

Step 2: Examine the Yellow Precipitate

The formation of a yellow precipitate suggests the presence of a sulfide, such as the precipitation of sulfur (S) from H₂S reacting with HCl.

Step 3: Investigate the Flame Test

A green flame indicates the presence of a barium compound. Barium salts are known to produce green flames.

Step 4: Consider the Reaction with Potassium Chromate

A yellow precipitate with potassium chromate indicates the presence of barium ions. Potassium chromate reacts with barium ions to form barium chromate, which is a yellow precipitate.

Step 5: Determine the Identity of the Salt

Combining all observations, the salt is a barium compound that can produce H₂S, a yellow precipitate occurs in reactions with chromate, and a green flame, pointing towards barium sulfide, BaS.

Key Concepts

Sulfide Detection

When a salt reacts with dilute hydrochloric acid (HCl) and releases a pungent gas, it's often a sign that hydrogen sulfide (\( \text{H}_2\text{S} \)) is present. This gas is notorious for its 'rotten egg' smell. To detect sulfides, you can perform a simple test using paper dipped in a lead acetate solution. If the paper turns black, it confirms the presence of sulfide ions, as lead sulfide (PbS) is formed, which is black in color.
Detection of \( \text{H}_2\text{S} \) is particularly useful in qualitative analysis to confirm the presence of sulfide ions in a sample. When \( \text{H}_2\text{S} \) reacts with another chemical, such as HCl, it can often produce a yellow precipitate, consisting of sulfur. This process helps identify specific elements and compounds in the lab.

Flame Test

The flame test is a simple but revealing procedure used to detect the presence of certain metal ions based on the characteristic color they emit when heated. When a compound is exposed to a flame, the heat excites the electrons in the metal ions, causing them to emit visible light at specific wavelengths, which correspond to specific colors.

• Green Flame: The appearance of a green flame typically indicates the presence of barium ions. Barium compounds are well-known for producing a bright green color when introduced into a flame, due to their unique emission spectrum.

For students and chemists, flame tests are a quick and fascinating way to identify certain elements. While not all elements can be detected this way, flame tests are effective for identifying metals like lithium (red), sodium (yellow), and barium (green). Always remember to take precautions and wear safety equipment when conducting a flame test.

Barium Compounds

Barium compounds play a significant role in both qualitative analysis and industrial applications. Two key properties of barium compounds are their ability to form vivid green flames in a flame test and their tendency to form precipitates when reacted with certain ions.
A classic example is barium chromate, which precipitates as a yellow solid when barium ions encounter chromate ions.

• Reactivity: Barium compounds are highly reactive. When they react with sulfur sources, like sulfides, they can release hydrogen sulfide gas (\( \text{H}_2\text{S} \)), which is crucial for sulfide detection.
• Applications: Beyond the lab, barium compounds have real-world uses. Barium sulfate is used in X-ray imaging and in manufacturing due to its opaque nature, while other barium salts are used in fireworks for their green coloration.

Barium's unique properties make it an essential element in both scientific research and various industries. When studying barium compounds, it's essential to understand both their chemical interactions and practical applications.