Problem 82 - A Complete Resource Book in Chemistry for JEE Main

Question

Three separate samples of a solution of single salt gave these results. One formed a white precipitate with excess ammonia solution, one formed a white precipitate with dil. \(\mathrm{NaCl}\) and one formed a black precipitate with \(\mathrm{H}_{2} \mathrm{~S} .\) The salt could be (a) \(\mathrm{AgNO}_{3}\) (b) \(\mathrm{MnSO}_{4}\) (c) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) (d) \(\mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

The salt could be (c) Pb(NO₃)₂.

Step 1: Understanding Precipitation with Ammonia

When a solution forms a white precipitate with excess ammonia, it typically indicates the presence of silver ions (Ag⁺). Silver chloride, AgCl, is a classic example that forms a white precipitate when silver nitrate reacts with sodium chloride and further with excess ammonia.

Step 2: Identifying Sodium Chloride Reaction

A white precipitate with diluted NaCl suggests the formation of an insoluble chloride like silver chloride (AgCl) or lead chloride (PbCl₂). Silver nitrate (AgNO₃) will form a white precipitate with NaCl, while lead nitrate forms a white precipitate in cold NaCl solution.

Step 3: Analyzing Precipitation with H₂S

A black precipitate forms with H₂S when metal ions like lead (Pb²⁺) react due to the formation of lead sulfide (PbS), which is black. In the case of mercury, H₂S can also form black precipitates due to the formation of mercury sulfide (HgS).

Key Concepts

Ammonia Solution Precipitation

Precipitation reactions occur when two solutions combine to form an insoluble solid, known as a precipitate. When it comes to ammonia solution precipitation, it is often used to identify metal ions like silver and copper. - Ammonia solutions can form complex ions with metals, often dissolving precipitates. - However, in some cases, a white precipitate may appear initially, especially with silver ions.In reactions involving silver ions (\( \text{Ag}^+ \)), adding ammonia can result in interesting changes. - Silver chloride (\( \text{AgCl} \)) forms a distinct white precipitate when ammonia is added to a solution containing silver ions. - This reaction is a classic test for \( \text{Ag}^+ \) presence, as \( \text{AgCl} \) is one of the few silver salts that is insoluble in water.However, further excess ammonia can dissolve the precipitate by forming the complex ion \( \text{[Ag(NH}_3)_2]^+ \), leaving the solution clear again.

Sodium Chloride Reaction

Sodium chloride (\( \text{NaCl} \)) commonly reacts with other solutions to form precipitates, mainly when heavy metal ions are present.- Upon addition of \( \text{NaCl} \), solutions that have silver ions will form silver chloride (\( \text{AgCl} \)), which is characterized by a white precipitate.- Lead ions (\( \text{Pb}^{2+} \)) can precipitate as lead chloride (\( \text{PbCl}_2 \)) when sodium chloride is introduced.These precipitation reactions highlight the formation of insoluble salts. - \( \text{AgCl} \) is a common example where silver nitrate (\( \text{AgNO}_3 \)) and sodium chloride form a white, cloudy precipitate.- In the case of lead, a similar white precipitate forms when lead nitrate solution is mixed with \( \text{NaCl} \), particularly at lower temperatures.

Hydrogen Sulfide Reaction

Hydrogen sulfide (\( \text{H}_2\text{S} \)) reactions are significant in identifying metal ions due to the formation of characteristic precipitates.- \( \text{H}_2\text{S} \) reacts with solutions containing lead ions to form lead sulfide (\( \text{PbS} \)), a black precipitate.- The black precipitate formation is a classic test for lead ions and confirms the presence of these ions in the solution.This reaction is specific to metals like lead and mercury which also form black sulfides. - Mercury salts react similarly, forming mercury sulfide (\( \text{HgS} \)), which appears black as well. These sulfide reactions are crucial in qualitative analysis, helping to identify and confirm the presence of certain metal ions efficiently.

Silver Nitrate

Silver nitrate (\( \text{AgNO}_3 \)) is a versatile reagent in chemistry, mainly used in precipitation reactions due to its ability to form precipitates with chloride ions.- When silver nitrate reacts with sodium chloride, silver chloride (\( \text{AgCl} \)) forms as a white precipitate.- This serves as a standard method to test for chloride ions in solutions.Silver nitrate offers a clear and observable reaction thanks to its limited solubility when forming \( \text{AgCl} \).- Being a strong oxidizing agent, it has other uses, but its application in chloride detection is prized due to the distinct white color of the precipitate. This ensures a straightforward method for identifying chloride in various chemical analyses.

Lead Sulfide Precipitation

The formation of lead sulfide (\( \text{PbS} \)) is another important aspect of precipitation reactions, particularly in testing for lead ions.- When lead ions (\( \text{Pb}^{2+} \)) in a solution come into contact with hydrogen sulfide (\( \text{H}_2\text{S} \)), lead sulfide forms, resulting in a black precipitate.- This is a key reaction for detecting the presence of lead in solutions.The formation of this black precipitate is one of the most identifiable signs of lead, which can also react in similar ways with other sulfide ions.- It highlights the insolubility of lead sulfide, which remains as a solid deposit.Due to these characteristics, lead sulfide precipitation is a fundamental concept in analytical chemistry, assisting in the qualitative analysis of samples containing lead ions.