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 Formation with BaCl2

The green crystalline compound forms a dense white precipitate with Barium chloride solution (BaCl2). This indicates the presence of sulfate ions \((\text{SO}_4^{2-})\) in the compound. When BaCl2 is added to a solution with sulfate ions, Barium sulfate \((\text{BaSO}_4)\) forms, which is a dense white precipitate. Thus, the compound is likely to be a sulfate.

Step 2: Analyze Precipitate Formation with Dimethyl Glyoxime and NH4OH

The compound also gives a rose-red precipitate with dimethyl glyoxime and \(\text{NH}_4\text{OH}\). This is a classic test for the presence of nickel ions \((\text{Ni}^{2+})\), where dimethyl glyoxime reacts with nickel in the presence of ammonia to form a red precipitate.

Step 3: Identify the Compound

From Step 1, we inferred the presence of sulfate ions, and from Step 2, we confirmed the presence of nickel ions. Hence, the compound that contains both nickel as well as sulfate ions is \(\text{NiSO}_4\).

Key Concepts

Sulfate Ion Test

The sulfate ion test is a straightforward and important qualitative analysis method in chemistry for detecting the presence of sulfate ions (\(\text{SO}_4^{2-}\)) in a solution. The procedure involves adding a barium chloride solution (\(\text{BaCl}_2\)) to the sample. If sulfate ions are present, a chemical reaction occurs, producing a white precipitate of barium sulfate (\(\text{BaSO}_4\)).

This process can be summarized in the simple chemical equation: \[\text{BaCl}_2 (\text{aq}) + \text{SO}_4^{2-} (\text{aq}) \rightarrow \text{BaSO}_4 (\text{s}) + 2\text{Cl}^- (\text{aq})\]To make sure the test is reliable, it's essential to observe the distinct white color and density of the precipitate. These characteristics are typical indicators of barium sulfate. Incorporating this test is crucial in analytical chemistry due to its specificity for sulfate ions.

Usually, chemists use this test to confirm the presence of sulfate in unknown samples and to assure the purity and completeness of other chemical analyses.

Nickel Ion Test

Testing for nickel ions (\(\text{Ni}^{2+}\)) is a well-established process in qualitative analysis. The test uses dimethyl glyoxime, which, when combined with nickel ions in an ammoniacal medium (such as \(\text{NH}_4\text{OH}\)), forms a distinctive rose-red precipitate.

The reaction can be depicted as:\[\text{Ni}^{2+} (\text{aq}) + 2 \text{DMG} (\text{dimethyl glyoxime}) + 2 \text{OH}^- \rightarrow \text{[Ni(DMG)_2]} (\text{s}) + 2 \text{H}_2\text{O} (\text{l})\]This bright red complex formed is insoluble and exhibits a stunning color change, which is a reliable indication of the presence of nickel.

Ensure that the solution is properly ammoniated to promote the optimal formation of the precipitate, making the test both visual and simple to execute. This test is valuable in laboratories to identify nickel content in various samples and verify nickel plating in metalworks.

Chemical Precipitation Reactions

Chemical precipitation reactions play a pivotal role in qualitative analysis. These reactions occur when ions in solution combine to form an insoluble solid, or precipitate, which can sometimes be seen as a solid deposit in a liquid matrix. Such reactions are utilized to isolate and identify the presence of specific ions within a solution.

Here’s how they typically work:

• Two aqueous solutions containing different anions and cations are mixed.
• The combination results in a compound that is insoluble in water.
• This insoluble compound forms a precipitate that falls to the bottom of the solution or causes the solution to become cloudy.

For instance, the reaction of barium chloride (\(\text{BaCl}_2\)) with a solution containing sulfate ions (\(\text{SO}_4^{2-}\)) results in the formation of barium sulfate (\(\text{BaSO}_4\)), as explained before. Similarly, when nickel ions react with dimethyl glyoxime in an ammoniacal medium, a characteristic rose-red precipitate forms.

These reactions are vital for identifying constituents in a mixture and are frequently used in analytic chemistry for separation and purification processes.