Problem 111 - A Complete Resource Book in Chemistry for JEE Main

Question

A green crystalline compound shows the following tests: (i) Its aqueous solution gives a dense white precipitate with Barium chloride solution (ii) Its aqueous solution gives a rose-red precipitate with dimethyl glyoxime and \(\mathrm{NH}_{4} \mathrm{OH}\). The compound is (a) \(\mathrm{NiSO}_{4}\) (b) \(\mathrm{FeSO}_{4}\) (c) \(\mathrm{Cr}_{2}\left(\mathrm{SO}_{4}\right)_{3}\) (d) \(\mathrm{FeCl}_{3}\)

Step-by-Step Solution

Answer

The compound is \( \text{NiSO}_4 \) (option a).

Step 1: Analyze Precipitate with Barium Chloride

The test with Barium chloride indicates the presence of sulfate ions. When a compound reacts with Barium chloride to form a white precipitate, it forms Barium sulfate \( (\text{BaSO}_4) \). Therefore, the compound must contain sulfate ions \( (\text{SO}_4^{2-}) \).

Step 2: Examine the Reaction with Dimethyl Glyoxime

The rose-red precipitate formed with dimethyl glyoxime indicates the presence of nickel ions \( (\text{Ni}^{2+}) \). Dimethyl glyoxime is a specific reagent for nickel, forming a characteristic rose-red complex when nickel ions are present.

Step 3: Compare Results with Given Options

From steps 1 and 2, the compound must contain both nickel ions and sulfate ions. Among the given options, only \( \text{NiSO}_4 \) contains both these ions.

Step 4: Confirm the Correct Choice Based on Tests

\( \text{NiSO}_4 \) is the only compound that contains both sulfate ions that form a white precipitate with Barium chloride and nickel ions that form a rose-red precipitate with dimethyl glyoxime. Thus, \( \text{NiSO}_4 \) matches both experimental observations.

Key Concepts

Sulfate Ion Test

The sulfate ion test is pivotal in identifying the presence of sulfate ions in a compound. When an aqueous solution of a test compound is mixed with Barium chloride \((\text{BaCl}_2)\), a characteristic dense white precipitate can form. This precipitate is Barium sulfate \((\text{BaSO}_4)\).

The reaction occurs because sulfate ions \((\text{SO}_4^{2-})\) in the solution react with barium ions \((\text{Ba}^{2+})\) to produce the insoluble Barium sulfate. The equation representing this reaction is:

• \[ \text{SO}_4^{2-} + \text{Ba}^{2+} \rightarrow \text{BaSO}_4 \downarrow \]

This dense white precipitate confirms the presence of sulfate ions in the solution.

This test is a straightforward indication of sulfate ions, often used in qualitative analysis to determine sulfate in various compounds. It is an essential step to consider if you are trying to confirm the presence of sulfates in an unknown solution.

Nickel Ion Identification

Identifying nickel ions \((\text{Ni}^{2+})\) in a compound can be accomplished using dimethyl glyoxime. This is a specific reagent for nickel.

In the presence of nickel ions, dimethyl glyoxime forms a complex that is rose-red. This distinctive color change is very helpful in qualitative analysis.

• The reaction can be represented as: \[ \text{Ni}^{2+} + 2 \text{C}_4\text{H}_8\text{N}_2\text{O}_2 \rightarrow \text{Ni(C}_4\text{H}_7\text{N}_2\text{O}_2)_2 \]

Dimethyl glyoxime is not only effective in detecting nickel but is also commonly employed because of its specificity.

By adding this reagent and observing the reaction, one can confidently conclude the presence of nickel ions when this rose-red complex forms. This test is especially suitable for quick visual verification of nickel in a sample.

Chemical Reactions and Precipitates

Chemical reactions that form precipitates are common in qualitative analysis. A precipitate is a solid that forms when two solutions react and produce an insoluble product.

Such reactions often help identify specific ions present in a compound.

• A precipitate can confirm the presence of certain ions when they react with specific reagents.
• For example, a white precipitate formed with Barium chloride confirms sulfate ions.
• A rose-red precipitate with dimethyl glyoxime confirms nickel ions.

Precipitation reactions are key in the separation, purification, and identification of chemical ions.

By understanding which precipitates form with which ions, chemists can deduce the possible composition of an unknown sample. This method remains an essential tool for chemists working on qualitative chemical analysis.