Problem 79 - A Complete Resource Book in Chemistry for JEE Main

Question

A metal salt solution gives a yellow precipitate with silver nitrate. The precipitate dissolves in dilute nitric acid as well as in ammonium hydroxide. The solution contains (a) iodide (b) chromate (c) phosphate (d) bromide

Step-by-Step Solution

Answer

The solution contains chromate (option b).

Step 1: Precipitate Formation Description

A yellow precipitate forms when the metal salt solution is mixed with silver nitrate. This indicates the formation of a silver salt. Silver chromate (\(\text{Ag}_2\text{CrO}_4\)) is known to be a yellow colored precipitate, which forms when chromates react with silver nitrate.

Step 2: Solubility in Acid and Base

Consider the solubility of the formed yellow precipitate in dilute nitric acid and ammonium hydroxide. Silver chromate is known to dissolve in both dilute nitric acid and ammonium hydroxide. This aligns with the properties discussed in the exercise for the behavior of the precipitate.

Step 3: Identify the Solution Containing Ion

Based on the descriptions in the previous steps, the characteristics of the precipitate (forming a yellow precipitate, dissolving in dilute nitric acid and ammonium hydroxide) match those of silver chromate. Thus, the solution likely contains chromate ions.

Key Concepts

Understanding Chromate Ions

Chromate ions (\( ext{CrO}_4^{2-} \)) are a type of inorganic anion consisting of one chromium atom and four oxygen atoms. These ions are typically found in compounds with vibrant colors, often yellow, due to their ability to absorb specific wavelengths of light. This is why chromate salts are commonly used in indicators and pigments.
One fascinating characteristic of chromate ions is their transformation in solution. They can convert to dichromate ions (\( ext{Cr}_2 ext{O}_7^{2-} \)) in acidic environments. This transition is visible as a color change from yellow to orange. This property is linked to the equilibrium reaction:
\[2 ext{CrO}_4^{2-} + 2 ext{H}^+ ightleftharpoons ext{Cr}_2 ext{O}_7^{2-} + ext{H}_2 ext{O}\]
Chromate ions are not only colorful but also toxic. They are strong oxidizing agents, which makes handling them with care essential in laboratory settings.

Mechanism of Precipitate Formation

Precipitate formation occurs when ions in solution react to form an insoluble solid, often separated by a distinct color. For chromates, this happens with silver nitrate. Here is how it works:

• When a metal salt solution containing chromate ions is mixed with silver nitrate (\( ext{AgNO}_3 \)), a bright yellow precipitate forms, identified as silver chromate (\( ext{Ag}_2 ext{CrO}_4 \)).
• The reaction shows the combination of silver ions (\( ext{Ag}^+ \)) and chromate ions (\( ext{CrO}_4^{2-} \)).
• The process follows the reaction equation:
  \[2 ext{Ag}^+ + ext{CrO}_4^{2-} ightarrow ext{Ag}_2 ext{CrO}_4 (s)\]
  This equation indicates the stoichiometry of the reaction, showing how two silver ions react with one chromate ion to form the precipitate.

This visual transformation helps chemists identify the presence of particular ions in a solution, making it an effective diagnostic tool.

Exploring Solubility in Acids and Bases

Solubility is a key concept in understanding reactions in inorganic chemistry. It dictates why certain substances dissolve in some solvents but not others. For silver chromate, solubility behavior is particularly interesting.

• Silver chromate dissolves in dilute nitric acid due to the acidic environment pushing the equilibrium toward the formation of soluble dichromate ions, according to the equation:
  \[ ext{Ag}_2 ext{CrO}_4 (s) + 2 ext{H}^+ ightarrow 2 ext{Ag}^+ + ext{H}_2 ext{CrO}_4 ext{(aq)}\]
• In bases, like ammonium hydroxide, the solubility of silver chromate can also increase as the ammonia forms complex ions with silver ions, preventing reprecipitation.

The ability of silver chromate to dissolve in both acids and bases makes it quite unique. It showcases how changes in ionic environment influence solubility, allowing chemists to experiment and predict the states of substances under different conditions. Understanding these interactions is essential for applications in fields ranging from analytical chemistry to environmental science.