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 \(\mathrm{Pb(NO_3)_2}\).

Step 1: Understand Precipitate Formation

Begin by identifying which reactions can form precipitates. Silver nitrate (\(\mathrm{AgNO_3}\)) can form a white precipitate with \(\mathrm{NaCl}\) to yield silver chloride (\(\mathrm{AgCl}\)). Similarly, lead nitrate (\(\mathrm{Pb(NO_3)_2}\)) reacts with \(\mathrm{NaCl}\) in a similar manner. Manganese sulfate (\(\mathrm{MnSO_4}\)) does not readily form a precipitate with \(\mathrm{NaCl}\), and mercury(II) nitrate (\(\mathrm{Hg(NO_3)_2}\)) can form a white precipitate with \(\mathrm{NaCl}\). Thus, \(\mathrm{AgNO_3}\), \(\mathrm{Pb(NO_3)_2}\), and \(\mathrm{Hg(NO_3)_2}\) can form white precipitates with \(\mathrm{NaCl}\).

Step 2: Evaluate Reactions with Ammonia

Next, consider the reaction of each salt with excess ammonia solution. \(\mathrm{AgNO_3}\) reacts with ammonia to form a white precipitate of \(\mathrm{Ag_2O}\). \(\mathrm{Pb(NO_3)_2}\) does not form a net iconic reaction to produce a significant white precipitate in excess ammonia; instead, it is mostly soluble. \(\mathrm{Hg(NO_3)_2}\), those, forms a white precipitate with excess ammonia because of complex ions. Thus, only \(\mathrm{AgNO_3}\) and \(\mathrm{Hg(NO_3)_2}\) are consistent at this step.

Step 3: Analyze Black Precipitate Formation

Consider the formation of a black precipitate with \(\mathrm{H_2S}\). \(\mathrm{Pb(NO_3)_2}\) can form \(\mathrm{PbS}\), a black precipitate, when reacting with \(\mathrm{H_2S}\). \(\mathrm{AgNO_3}\) and \(\mathrm{Hg(NO_3)_2}\) also form black precipitates \(\mathrm{Ag_2S}\) and \(\mathrm{HgS}\) respectively. However, in customary precipitate formation with \(\mathrm{H_2S}\), \(\mathrm{Pb(NO_3)_2}\) and \(\mathrm{Hg(NO_3)_2}\) are noted more significantly for black precipitate formation than \(\mathrm{AgNO_3}\).

Step 4: Final Analysis

After analyzing the steps thoroughly, we see that only \(\mathrm{Pb(NO_3)_2}\) can effectively be implicated in each described reaction across the different solutions: forming a white precipitate with \(\mathrm{NaCl}\), white precipitate with ammonia, and black precipitate with \(\mathrm{H_2S}\). Therefore, the salt is most likely \(\mathrm{Pb(NO_3)_2}\).

Key Concepts

Precipitate Formation

Precipitate formation is a key reaction in chemistry that occurs when two solutions react to form an insoluble solid, known as a precipitate. This process helps identify unknown compounds in a solution. Precipitates are usually formed when the product of a reaction is insoluble or only slightly soluble in water. For instance, when silver nitrate (\( \mathrm{AgNO_3} \)) reacts with sodium chloride (\( \mathrm{NaCl} \)) in solution, a white precipitate of silver chloride (\( \mathrm{AgCl} \)) forms. This reaction is very useful in analytical chemistry for qualitative analysis:

• Silver ions (\( \mathrm{Ag^+} \)) and chloride ions (\( \mathrm{Cl^-} \)) come together to form \( \mathrm{AgCl} \).
• The \( \mathrm{AgCl} \) is insoluble, hence it precipitates out as a white solid.

Recognizing when and why precipitates form is vital for identifying various ions in a solution.

Ammonia Reactions

Ammonia (\( \mathrm{NH_3} \)) is used in many chemical reactions, especially because it can react with various metals to form complex ions. When ammonia is added to solutions of metal ions, such as silver or copper, a change often occurs.

• Silver nitrate (\( \mathrm{AgNO_3} \)), upon reacting with ammonia, forms a complex ion. Excess \( \mathrm{NH_3} \) reacts with the silver to slowly dissolve \( \mathrm{Ag_2O} \) back into the solution, allowing for complex ions.
• Mercury(II) nitrate (\( \mathrm{Hg(NO_3)_2} \)) also forms a white precipitate when added to ammonia due to the formation of coordination complexes.

These reactions indicate the formation of precipitates with complex ions and showcase ammonia's role as a ligand in complexation reactions.

Sulfide Precipitation

Sulfide precipitation is a fascinating chemical reaction where sulfide ions (\( \mathrm{S^{2-}} \)) react with metal ions to form insoluble metal sulfides. This process is distinct due to the formation of colored precipitates, often black, which can help identify specific metals.For example, when hydrogen sulfide (\( \mathrm{H_2S} \)) is bubbled through a solution containing lead nitrate (\( \mathrm{Pb(NO_3)_2} \)), a black precipitate of lead sulfide (\( \mathrm{PbS} \)) forms. Similarly, silver nitrate (\( \mathrm{AgNO_3} \)) and mercury(II) nitrate (\( \mathrm{Hg(NO_3)_2} \)) will form black precipitates of silver sulfide (\( \mathrm{Ag_2S} \)) and mercury sulfide (\( \mathrm{HgS} \)), respectively.

• This reaction occurs because metal sulfides are typically highly insoluble in water.
• The formation of such precipitates is pivotal in qualitative analysis for detecting the presence of specific metal ions.

Understanding sulfide precipitation helps in effectively using these distinct reactions to identify metals.

Lead Nitrate Reactions

Lead nitrate (\( \mathrm{Pb(NO_3)_2} \)) is a versatile reagent in chemistry because of its ability to form various precipitates under different conditions.

• When reacting with sodium chloride (\( \mathrm{NaCl} \)), it forms a white precipitate of lead chloride (\( \mathrm{PbCl_2} \)), identifying the presence of lead ions.
• In reaction with hydrogen sulfide (\( \mathrm{H_2S} \)), it results in a black precipitate (\( \mathrm{PbS} \)), which is highly characteristic and useful for identification of lead.

These reactions demonstrate how lead nitrate can be a crucial tool in identifying unknown substances through their ionic interactions and formation of distinct precipitates.