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.

Step 1: Analyze the Precipitate Color

When a solution of the metal salt is treated with silver nitrate, it produces a yellow precipitate. Silver chromate, Ag2CrO4, is known to form a yellow precipitate with silver nitrate. This information suggests the presence of the chromate ion, \( \text{CrO}_4^{2-} \), in the metal salt solution.

Step 2: Check Solubility in Dilute Nitric Acid

The precipitate is soluble in dilute nitric acid. Silver chromate, unlike silver iodide or silver bromide, can dissolve in dilute nitric acid. This aligns with the behavior of chromate ions, which dissolve in acidic conditions, forming dichromate ions.

Step 3: Versify Solubility in Ammonium Hydroxide

The precipitate also dissolves in ammonium hydroxide. Silver chromate can dissolve in ammonium hydroxide due to the formation of a complex ion. This further supports the presence of chromate ions.

Step 4: Eliminate Other Options

Silver iodide and silver bromide form precipitates that do not dissolve in dilute nitric acid or ammonium hydroxide. Silver phosphate can form a precipitate, but it is not typically yellow and does not match other given solubility properties. Thus, options iodide, phosphate, and bromide are eliminated.

Key Concepts

Precipitation Reaction

In inorganic chemistry, a precipitation reaction is a process where in an insoluble solid, called a precipitate, forms from the reaction of two aqueous solutions. This typically occurs when the ions in the solutions combine to form a compound that is not soluble in water. When silver nitrate (AgNO₃) is added to a metal salt solution, if a yellow precipitate forms, this indicates the formation of silver chromate (Ag₂CrO₄). This happens because the chromate ions (\[\text{CrO}_4^{2-}\]) react with silver ions to create the yellow precipitate.

• Precipitates form when the product of the ion concentration exceeds their solubility product.
• The color and solubility of the precipitate can help confirm the presence of certain ions.

Understanding precipitation reactions is key to identifying unknown ions in solutions based on the color and solubility of the resulting precipitate.

Chromate Ions

Chromate ions, represented as \[\text{CrO}_4^{2-}\], are a type of ion derived from chromic acid. They are often involved in reactions that produce vivid color changes, such as the yellow coloration when silver chromate forms. These ions are known for their distinct properties:

• Chromate ions are bright yellow, which helps in identifying them in mixtures.
• They can be converted to dichromate ions \[\text{(Cr}_2\text{O}_7^{2-)}\] under acidic conditions.
• Chromate ions are useful in analytical chemistry for indicating the presence of particular metals or salts.

Their ability to form colored precipitates and dissolve upon complexation is crucial in analytical chemistry, including tests for metals in aqueous solutions.

Solubility of Salts

The solubility of salts in water varies greatly and is influenced by the specific ions involved. Silver chromate, while forming an insoluble yellow precipitate in neutral conditions, becomes soluble under certain conditions:

• In dilute nitric acid, chromate ions can react to form more soluble compounds, generally with the help of acid-induced equilibrium shifts.
• In ammonium hydroxide, the formation of complex ions increases solubility.

This characteristic of varied solubility among different salts helps chemists separate and identify ions in mixtures.

Complex Ion Formation

Complex ion formation is an essential process where central metal ions bind with surrounding ligands to form a complex with distinct properties in terms of solubility and reactivity. When a metal ion forms a complex, it typically exhibits:

• Increased solubility due to the stabilized geometry of the complex.
• Altered color, which can indicate different coordination and spatial arrangements.

For instance, in the stepwise reaction of silver chromate in ammonium hydroxide, the silver ion can coordinate with ammonia ligands, making silver chromate dissolve, which doesn't happen with simple ionic dissolution. Complex formations are integral to understanding solubility, reactivity, and the conditions under which compounds will form stable species.