Problem 88 - A Complete Resource Book in Chemistry for JEE Main

Question

A light greenish coloured salt was soluble in water. On passing \(\mathrm{H}_{2} \mathrm{~S}\) into the solution, a black ppt. was obtained which dissolved readily in HCl. The metal ion present is (a) \(\mathrm{Fe}^{2+}\) (b) \(\mathrm{Mn}^{2+}\) (c) \(\mathrm{Ni}^{2+}\) (d) \(\mathrm{Co}^{2+}\)

Step-by-Step Solution

Answer

The metal ion present is \( \mathrm{Ni}^{2+} \).

Step 1: Identify Potential Metal Ions

Review the color of the given salt. The light greenish color suggests that the metal ion could be either \( \mathrm{Fe}^{2+} \), \( \mathrm{Ni}^{2+} \), or \( \mathrm{Co}^{2+} \). Mn salts are generally pale pink in color, so \( \mathrm{Mn}^{2+} \) is less likely.

Step 2: Reaction with \( \mathrm{H}_2 \mathrm{S} \)

Consider the reaction with \( \mathrm{H}_2 \mathrm{S} \) which results in a black precipitate. This indicates the formation of a metal sulfide. Both \( \mathrm{FeS} \) and \( \mathrm{NiS} \) can form black precipitates, but Co salts usually form a different colored precipitate.

Step 3: Solubility in HCl

Determine which metal sulfide dissolves in HCl. Among \( \mathrm{FeS} \), \( \mathrm{MnS} \), \( \mathrm{NiS} \), and \( \mathrm{CoS} \), only \( \mathrm{NiS} \) is known to dissolve readily in HCl. \( \mathrm{FeS} \), \( \mathrm{MnS} \), and \( \mathrm{CoS} \) do not dissolve as easily.

Step 4: Conclusion of the Metal Ion

Considering the color indication, reaction with \( \mathrm{H}_2 \mathrm{S} \), and solubility behavior, the metal ion in the light greenish salt is \( \mathrm{Ni}^{2+} \).

Key Concepts

Transition Metal Ions

Transition metal ions are the building blocks of many interesting and colorful reactions in inorganic chemistry. They are the elements found in the d-block of the periodic table, known for their partially filled d-orbitals. This unique characteristic allows them to form various oxidation states and complex ions.

These ions often exhibit fascinating colors, which are a result of the d-d electron transitions when they absorb specific wavelengths of light. For example, the greenish color of the salt in the exercise suggests the presence of a transition metal, helping to narrow down the possibilities. Metals like nickel ( Ni^{2+}), iron ( Fe^{2+}), and cobalt ( Co^{2+}) can exhibit such a color under certain conditions. Understanding these colors provides a valuable tool in identifying unknown metal ions in a solution.

Additionally, transition metal ions are known for forming compounds with interesting magnetic, catalytic, and electrical properties. This makes them vital in a wide array of chemical processes and technologies.

Sulfide Precipitation

Sulfide precipitation is a common method for identifying the presence of metal ions in a solution. This involves passing hydrogen sulfide gas ( H_2S) through the solution. Many transition metals react with H_2S to form metal sulfides, which often precipitate out of the solution as solid particles. Based on the color and property of these precipitates, we can identify the metal ions.

• A black precipitate, like NiS, indicates the formation of nickel sulfide.
• The color and appearance of a precipitate vary for different metals due to their unique chemical properties.

These visual cues are essential in analytical chemistry to spot the presence of specific metal ions quickly. In our exercise, the formation of a black precipitate when H_2S was bubbled through the solution points towards the presence of nickel or iron due to its characteristic color. Understanding this concept assists students in visualizing chemical reactions and identifying ions through simple observation techniques.

Solubility in Acids

Solubility is a key property in chemistry that determines how substances dissolve in different solvents. When it comes to metal sulfides, their solubility in acids can help identify specific metal ions. Each metal sulfide possesses unique solubility characteristics that can act as indicators.

For instance, in our problem, the black precipitate dissolved readily in hydrochloric acid (HCl). This behavior is a distinctive characteristic of nickel sulfide ( NiS). Other metal sulfides, such as iron sulfide ( FeS), manganese sulfide ( MnS), and cobalt sulfide ( CoS), do not dissolve as easily in HCl.

• The solubility in HCl provides a contrast between different metal sulfides.
• This property aids in the confirmatory test for identifying metal ions in a mixture.

Recognizing these solubility patterns simplifies the process of determining the identity of metal ions, making laboratory practice more effective and insightful for chemistry students.