Question

A white sodium salt dissolves readily in water to give a solution which is neutral to litmus. When silver nitrate solution is added to the solution, a white precipitate is obtained which does not dissolve in dil \(\mathrm{HNO}_{3}\). The anion could be (a) \(\mathrm{Cl}^{-}\) (b) \(\mathrm{S}^{2-}\) (c) \(\mathrm{CO}_{3}^{2-}\) (d) \(\mathrm{SO}_{4}^{2-}\)

Short answer

The anion is \(\text{Cl}^-\).

Step-by-step solution

Understanding the chemical reaction

When sodium salt dissolves, it dissociates into ions in water. A solution that is neutral to litmus indicates the formation of neither acidic nor basic ions.

Identify possible reactions with silver nitrate

Silver nitrate (\(\text{AgNO}_3\)) reacts with certain anions to form a precipitate. The possible anions that can form a precipitate with silver nitrate are \(\text{Cl}^-\) and \(\text{CO}_3^{2-}\), as \(\text{AgCl}\) and \(\text{Ag}_2\text{CO}_3\) are known to be insoluble.

Elimination based on solubility in nitric acid

The precipitate formed is specified as being insoluble in dilute \(\text{HNO}_3\). \(\text{AgCl}\) is insoluble in dilute \(\text{HNO}_3\), while \(\text{Ag}_2\text{CO}_3\) would dissolve to form \(\text{CO}_2\) and water.

Conclude the identity of the anion

Based on the precipitate's insolubility in dilute \(\text{HNO}_3\), the anion is \(\text{Cl}^-\), as \(\text{AgCl}\) remains insoluble in these conditions.

Key concepts

Silver Nitrate Reaction

When you add silver nitrate (\( \text{AgNO}_3 \)) to a solution containing certain anions, a chemical reaction can occur that leads to the formation of a precipitate.
Silver nitrate is known for its ability to react with the anion chloride (\( \text{Cl}^- \)) in a solution, resulting in the formation of silver chloride (\( \text{AgCl} \)).
This compound forms as a white precipitate because it is insoluble in water.
The silver nitrate reaction is a classical method used to identify ions in qualitative analysis.
It's particularly helpful because reactions like these are easy to see with the naked eye due to the color change when the precipitate forms.
In this context, observing a white precipitate after the addition of \( \text{AgNO}_3 \) gives an essential clue about the presence of \( \text{Cl}^- \) anions in your solution.

Neutral Solution

A neutral solution is one where the concentration of hydrogen ions, \( \text{H}^+ \), equals the concentration of hydroxide ions, \( \text{OH}^- \).
This balance leads to a pH of 7, indicating neither acidic nor basic nature in the solution.
In practical terms, water is a neutral solution as it maintains this ion balance naturally.
When a sodium salt dissolves in water and results in a neutral solution, it means the ionic components neither increase the acidity nor the basicity of the solution.
The dissolution results in ions that do not interfere with water's neutrality, making the litmus paper remain unchanged, showing no acidic or basic reaction.
For students, it's important to note that not all dissolved salts will behave this way, as some may lead to a basic or acidic solution based on the resulting ions.

Insolubility of AgCl in HNO3

The compound silver chloride (\( \text{AgCl} \)) is a white precipitate that is known for its poor solubility, especially in dilute acids like nitric acid (\( \text{HNO}_3 \)).
This insolubility is an important characteristic because it helps to confirm the presence of chloride ions.
Understanding why \( \text{AgCl} \) does not dissolve in \( \text{HNO}_3 \) can be attributed to the chemical structure and interaction between the ions.
Even when diluted, \( \text{HNO}_3 \) does not provide enough interaction to disrupt the solid \( \text{AgCl} \) and bring it back into solution.
This is pivotal in salt analysis procedures as it allows chemists to confirm a specific anion identity without further experimental steps or complex equipment.
Thus, the addition of \( \text{HNO}_3 \) acts as a test to distinguish silver chloride from other silver salts that may dissolve.

Dissociation of Sodium Salt

When a sodium salt is dissolved in water, it undergoes a process called dissociation.
This refers to the breaking apart of the salt into its corresponding ions in the solution.
For instance, a sodium chloride (\( \text{NaCl} \)) salt would dissociate into sodium ions, \( \text{Na}^+ \), and chloride ions, \( \text{Cl}^- \).
The dissociation is critical as it determines the nature of the solution created, whether it maintains neutrality or shifts towards acidic or basic.
In our example, the resulting solution remains neutral because the ions do not react with water to alter its pH balance.
For students, understanding this concept is fundamental as it explains how salts behave in aqueous solutions and influences the subsequent reactions, such as with silver nitrate.
Thus, knowing the dissolution and dissociation behavior helps predict the outcome of various chemical reactions in salt analysis.