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 \(\mathrm{HCl}\). The metal ion present is (a) \(\mathrm{Fe}^{2+}\) (b) \(\mathrm{Mn}^{2+}\) (c) \(\mathrm{Ni}^{2+}\) (d) \(\mathrm{Co}^{2+}\)

Short answer

The metal ion present is Ni²⁺ (Nickel ion).

Step-by-step solution

Identify the Solubility Characteristics

The salt is described as being light greenish in color and soluble in water. This information aligns with certain characteristics of transition metal salts, notably those of nickel (Ni) as nickel salts often have a greenish color.

Analyze the Reaction with H₂S

On passing hydrogen sulfide (H₂S) into the solution, a black precipitate is formed. Both nickel and cobalt can form black sulfide precipitates, so observationally both NiS and CoS can be black. This characteristic eliminates FeS (greenish and soluble) and MnS (pink and not soluble) for these conditions.

Consider the Reaction with HCl

The problem states that the black precipitate dissolves in hydrochloric acid (HCl). Nickel sulfide (NiS) can dissolve in HCl producing nickel ions in solution, which is a key point in identifying nickel as it confirms the potential of the black sulfide precipitate dissolving to reform the greenish nickel ion solution.

Conclude Based on All Characteristics

Combining all the information: greenish color of nickel salts, the formation of a black precipitate with H₂S, and its solubility in HCl, the metal ion present is likely Ni²⁺.

Key concepts

Transition Metal Salts

Transition metal salts are fascinating because they often showcase a wide range of colors. This is due to the d-electrons found in transition metals, which can absorb light and lead to these colorful appearances. For example, many nickel salts, such as nickel(II) sulfate, have a characteristic light green color due to the presence of Ni²⁺ ions.

These compounds are also important in various chemical reactions, especially in analytical chemistry where they can help identify metal ions through colorimetric methods. When dissolved in water, transition metal salts release metal cations and anions into the solution, leading to these unique colorations. In summary, knowing the colors associated with different transition metal salts can often give a clue about the ions present in a solution.

Precipitation Reactions

Precipitation reactions occur when two soluble salts are mixed together to form an insoluble solid known as a precipitate. This solid forms because the ions in the solution can combine to form a compound that does not dissolve in water. For example, when hydrogen sulfide (H₂S) gas is bubbled into a solution containing metal ions, it can lead to the formation of metal sulfides.

In our case, mixing H₂S with a nickel-containing solution forms a black precipitate, nickel sulfide (NiS). Such reactions are useful in identifying and separating metal ions, as the color and solubility of the precipitate give information about the ions present. Knowing which metal sulfides are black and which dissolve in acids like HCl helps chemists pinpoint the specific metal ions in their samples.

Solubility of Metal Ions

The solubility of metal ions in water and other solvents can vary widely and is influenced by the nature of the metal ion itself and the temperature, pH, and other conditions of the solution. Solubility rules are an essential part of chemistry that can help predict whether a compound will dissolve or precipitate in a given situation. For example, nickel sulfide is only sparingly soluble in water, but it can dissolve in strong acids like hydrochloric acid.

During a precipitation reaction, the solubility of the resulting compound determines whether a precipitate will form. If a compound is insoluble in water but soluble in another solvent, such as HCl, this can be used as a diagnostic tool in analytical chemistry. By understanding solubility characteristics, chemists can efficiently separate and identify metal ions in complex mixtures.