Problem 8 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A solution containing a group-IV cation gives a precipitate on passing \(\mathrm{H}_{2} \mathrm{~S}\). A solution of this precipitate in dil.HCl produces a white precipitate with \(\mathrm{NaOH}\) solution and bluish-white precipitate with basic potassium ferrocyanide. The cation is : (a) \(\mathrm{Co}^{2+}\) (b) \(\mathrm{Ni}^{2+}\) (c) \(\mathrm{Mn}^{2+}\) (d) \(\mathrm{Zn}^{2+}\)

Step-by-Step Solution

Answer

The cation is \( \mathrm{Zn}^{2+} \).

Step 1: Identify the Group-IV Cations

Group-IV cations include elements such as \( \mathrm{Co}^{2+}, \mathrm{Ni}^{2+}, \mathrm{Mn}^{2+}, \) and \( \mathrm{Zn}^{2+} \). These ions typically form sulfide precipitates when \( \mathrm{H}_2\mathrm{S} \) is passed into their solutions under acidic conditions.

Step 2: Analyze the Precipitate Reactions

The problem states that the precipitate dissolves in \( \text{dil. HCl} \) and produces a white precipitate with \( \mathrm{NaOH} \) and a bluish-white precipitate with basic potassium ferrocyanide. These reactions are characteristic tests used to identify specific cations.

Step 3: Determine the Reaction with \( \mathrm{NaOH} \)

When reacted with \( \mathrm{NaOH} \), the precipitate forms a white precipitate, which is common for \( \mathrm{Zn}^{2+} \), as it forms \( \mathrm{Zn(OH)_2} \), a white insoluble hydroxide. Other cations may give different colored hydroxides.

Step 4: Reaction with Basic Potassium Ferrocyanide

A bluish-white precipitate formed with basic potassium ferrocyanide is indicative of \( \mathrm{Zn^{2+}} \). The compound \( \mathrm{K_2Zn_3[Fe(CN)_6]_2} \) is known for producing this characteristic color.

Step 5: Compare with Other Cation Reactions

\( \mathrm{Mn^{2+}} \), \( \mathrm{Ni^{2+}} \), and \( \mathrm{Co^{2+}} \) do not form a bluish-white precipitate with potassium ferrocyanide. For example, \( \mathrm{Ni^{2+}} \) typically forms a green precipitate, while \( \mathrm{Co^{2+}} \) precipitates are brownish or reddish.

Step 6: Conclusion

Based on the given reactions - white precipitate with \( \mathrm{NaOH} \) and a bluish-white precipitate with potassium ferrocyanide - the cation in the solution is \( \mathrm{Zn}^{2+} \).

Key Concepts

Cation Identification

Cation identification is crucial for analyzing the composition of unknown solutions. Group-IV cations such as \( \mathrm{Co}^{2+}, \mathrm{Ni}^{2+}, \mathrm{Mn}^{2+}, \) and \( \mathrm{Zn}^{2+} \) are commonly found in various chemical systems. When attempting to identify these cations, chemical tests are employed to observe specific reactions and formations of precipitates.
For example, when \( \mathrm{H}_2\mathrm{S} \) is passed through a solution containing these cations, they tend to form sulfide precipitates under acidic conditions. This initial step helps in classifying the cations into their respective groups and narrowing down the possibilities.
Recognizing the cation requires a detailed analysis of the reaction products such as color, solubility, and texture of precipitates.

Precipitation Reactions

Precipitation reactions are reactions that result in the formation of an insoluble solid from a solution. This solid is called a precipitate. In our context, the passage of \( \mathrm{H}_2\mathrm{S} \) through a solution with Group-IV cations leads to the formation of specific sulfide precipitates.
Understanding how these precipitates dissolve or react further provides insight into the identity of the cation in question. When a precipitate dissolves in diluted hydrochloric acid (dil. HCl) and forms a white precipitate upon adding \( \mathrm{NaOH} \), it suggests the presence of \( \mathrm{Zn}^{2+} \), which forms \( \mathrm{Zn(OH)_2} \).
Precise identification often necessitates reaction with other reagents based on color and other characteristics of the resulting compounds.

Potassium Ferrocyanide Test

The Potassium Ferrocyanide test is used for further confirmation in identifying cations by observing the color change when this reagent is introduced. For the Group-IV cations discussed, the test involves observing the color of the precipitate formed.
A bluish-white color, as observed in the problem's solution, indicates the presence of zinc ions (\( \mathrm{Zn}^{2+} \)). This specific reaction forms \( \mathrm{K_2Zn_3[Fe(CN)_6]_2} \), resulting in a distinctive bluish-white precipitate.
This test is quite distinct as other cations like \( \mathrm{Mn}^{2+}, \mathrm{Ni}^{2+}, \) and \( \mathrm{Co}^{2+} \) produce different colors when reacted with potassium ferrocyanide, helping isolate zinc ions from other possibilities in the cation identification process.