Problem 43 - A Complete Resource Book in Chemistry for JEE Main

Question

The compound whose \(0.1 \mathrm{M}\) solution is basic is (a) ammonium acetate (b) ammonium sulphate (c) sodium acetate (d) ammonium chloride

Step-by-Step Solution

Answer

The compound is (c) sodium acetate.

Step 1: Understanding the Nature of Each Compound

To determine which compound forms a basic solution, we first need to understand the nature of each compound listed. We know that a compound forms a basic solution if it contains a conjugate base of a weak acid or itself is a base.

Step 2: Dissociation and Ion Formation

Let's examine how each compound dissociates in water: - Ammonium acetate ( ext{NH}_4 ext{C}_2 ext{H}_3 ext{O}_2) dissociates into ext{NH}_4^+ and ext{C}_2 ext{H}_3 ext{O}_2^-. - Ammonium sulfate (( ext{NH}_4)_2 ext{SO}_4) dissociates into 2 ext{NH}_4^+ and ext{SO}_4^{2-}. - Sodium acetate ( ext{NaC}_2 ext{H}_3 ext{O}_2) dissociates into ext{Na}^+ and ext{C}_2 ext{H}_3 ext{O}_2^-. - Ammonium chloride ( ext{NH}_4 ext{Cl}) dissociates into ext{NH}_4^+ and ext{Cl}^-.

Step 3: Analyzing the Ions Formed

Among these ions: - ext{NH}_4^+ comes from a weak base ( ext{NH}_3), making it acidic. - ext{C}_2 ext{H}_3 ext{O}_2^- is the conjugate base of acetic acid (a weak acid), making it basic. - ext{Na}^+ and ext{Cl}^- are neutral as they come from strong bases and acids, respectively, so they don't affect the pH significantly.

Step 4: Identifying the Basic Solution

From the analysis, only sodium acetate contains a component ( ext{C}_2 ext{H}_3 ext{O}_2^-) that will result in a basic solution as it is the conjugate base of acetic acid. Therefore, the solution will be basic due to the presence of ext{C}_2 ext{H}_3 ext{O}_2^-.

Key Concepts

Conjugate Base

A conjugate base is formed when an acid loses a proton (H⁺). For acids, such as acetic acid, the conjugate base retains one fewer hydrogen ion. This forms the acetate ion (\(\text{C}_2\text{H}_3\text{O}_2^-\)). The key trait of a conjugate base is its ability to attract and accept a hydrogen ion. This interaction often results in an increase in pH when dissolved in water.

• The conjugate base can do this because it contains a negative charge that stabilizes the extra positive charge of a hydrogen ion.
• Not all conjugate bases increase the pH. Their effectiveness depends on the strength of the parent acid. Weak acids have strong conjugate bases.

In our original exercise, the acetate ion acts as the conjugate base, making sodium acetate a compound that forms a basic solution. This is in contrast to ammonium ions, which come from a weak base, rendering them acidic.

Dissociation in Water

When compounds dissolve in water, they typically dissociate into ions. This process plays a fundamental role in determining the nature, whether acidic or basic, of the resulting solution. Dissociation describes how compound molecules split as they react with water molecules.

• For example, sodium acetate (\(\text{NaC}_2\text{H}_3\text{O}_2\)) breaks down into \(\text{Na}^+\) and \(\text{C}_2\text{H}_3\text{O}_2^-\).
• In contrast, ammonium chloride (\(\text{NH}_4\text{Cl}\)) dissociates into the \(\text{NH}_4^+\) ion and \(\text{Cl}^-\).

The nature of the ions determines the characteristics of the solution:

• If the ions are derived from weak acids or bases, they tend to participate in reversible reactions in water, contributing to the pH.
• A strong acid or base dissociates completely, leaving neutral ions which don't alter the pH.

Sodium acetate dissociates to produce an acetate ion, the conjugate base of acetic acid, which makes the solution basic.

Acid-Base Equilibrium

The equilibrium between acids and bases in a solution is a delicate balance. It refers to the state where the rate of the forward reaction, forming ions, is equal to the rate of the backward reaction, where ions recombine to form the original compounds. This dynamic determines the solution's pH and is crucial in understanding the behavior of conjugate acids and bases.

• In an acid-base equilibrium, weak acids and their conjugate bases exist in a balance defined by their dissociation constant \(K_a\).
• A similar balance is present for bases and their conjugate acids, represented by the \(K_b\) value.

These constants help predict whether a solution will be acidic, neutral, or basic:

• If \(K_b\) for the base is larger than \(K_a\) for the acid, the solution tends to be basic.
• In our example, sodium acetate results in a basic solution as its acetate ion has more influence than any acidic component due to equilibrium favoring the ion form.

Understanding equilibrium is crucial for predicting the behavior of any acid-base system in water.