Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

The \(\mathrm{pH}\) values of \(1 \mathrm{M}\) solutions of \(\mathrm{CH}_{3} \mathrm{COOH}\) (I), \(\mathrm{CH}_{3}, \mathrm{COONa}(\mathrm{II}), \mathrm{CH}_{3} \mathrm{COONH}_{4}(\mathrm{III})\), and \(\mathrm{KOH}\) (IV) will be in the order (a) IV \(>\mathrm{III}>\mathrm{II}>\mathrm{I}\) (b) IV \(>\mathrm{II}>\mathrm{III}>\mathrm{I}\) (c) \(\mathrm{I}>\mathrm{III}>\mathrm{II}>\mathrm{IV}\) (d) \(\mathrm{II}>\mathrm{I}>\mathrm{III}>\mathrm{IV}\)

Step-by-Step Solution

Answer

Option (b) IV > II > III > I.

Step 1: Analyze the pH of each solution

First, identify the nature of each compound: - Acetic acid ( CH_3COOH ) is a weak acid. - Sodium acetate ( CH_3COONa ) will result in an alkaline solution due to the hydrolysis of acetate ions. - Ammonium acetate ( CH_3COONH_4 ) is a salt of a weak acid and weak base, resulting in a neutral pH. - Potassium hydroxide ( KOH ) is a strong base.

Step 2: Determine the strongest base

Among the given compounds, KOH is the strongest base. Therefore, it will have the highest pH value.

Step 3: Rank compounds based on basicity

Since KOH is the strongest base, it will have the highest pH . CH_3COONa will have a higher pH due to the hydrolysis of acetate, making the solution alkaline. CH_3COONH_4 will be neutral, lower than CH_3COONa . CH_3COOH will have the lowest pH as a weak acid.

Step 4: Determine the final order

The order of pH from highest to lowest is: KOH (IV) > Sodium acetate CH_3COONa (II) > Ammonium acetate CH_3COONH_4 (III) > Acetic acid CH_3COOH (I). This corresponds to option (b).

Key Concepts

Understanding Weak Acids

Weak acids are fascinating in the world of chemistry because they do not completely dissociate in water. Acetic acid, represented chemically as \( \text{CH}_3\text{COOH} \), is a common example of a weak acid. Unlike strong acids, which ionize completely, a weak acid only partially donates its hydrogen ions (\( \text{H}^+ \)) when dissolved in water.
This lack of full ionization results in a solution with only a small concentration of hydrogen ions, leading to a higher pH when compared to strong acids. It's important to recognize that the pH of a weak acid solution depends significantly on its concentration and the specific acid dissociation constant (\( K_a \)) of the acid. A higher \( K_a \) value indicates a stronger weak acid.
In practical terms, understanding weak acids can be crucial when predicting their behavior in various chemical reactions, including buffer solutions and in biological systems.

Insights into Strong Bases

Strong bases, such as potassium hydroxide (\( \text{KOH} \)), are known for their ability to dissociate completely in aqueous solutions. This complete dissociation means that strong bases generate a high concentration of hydroxide ions (\( \text{OH}^- \)), which results in a high pH value, typically above 7.
Unlike weak acids, which only partially dissociate, strong bases fully separate into ions, leaving no undissociated base molecules in the solution. This complete ionization explains why strong bases such as \( \text{KOH} \) have such a profound effect on pH.
When added to water, \( \text{KOH} \) increases the solution's alkalinity significantly, often used in laboratory settings and industries where strong alkaline conditions are necessary. Knowing how strong bases function is crucial for any application requiring precise pH adjustments.

The Role of Salt Hydrolysis in pH

Salt hydrolysis plays a significant role in determining the pH of a solution. This process involves the reaction of water with the ions from a dissolved salt that can affect the acidity or alkalinity of the solution. Sodium acetate (\( \text{CH}_3\text{COONa} \)) is an example of a salt that undergoes hydrolysis.
When \( \text{CH}_3\text{COONa} \) dissolves in water, the acetate ion (\( \text{CH}_3\text{COO}^- \)) undergoes hydrolysis with water, partially forming acetic acid and hydroxide ions. This reaction shifts the pH of the solution in the basic direction because of the additional \( \text{OH}^- \) generated.
Understanding the concept of salt hydrolysis is beneficial in predicting the pH of solutions formed by salts, particularly those derived from weak acids and strong bases. These types of salts often form slightly alkaline solutions.

Exploring Acid-Base Properties of Compounds

Every compound has unique acid-base properties. These properties dictate how they will interact in a solution and their effect on the pH. Different compounds like acetic acid, sodium acetate, ammonium acetate, and potassium hydroxide illustrate the diverse range of acid-base behaviors.
Acetic acid, as a weak acid, leads to lower pH due to limited dissociation. In contrast, strong bases like potassium hydroxide significantly increase the pH by fully dissociating into hydroxide ions. Sodium acetate represents an interesting case with its alkaline nature due to salt hydrolysis, while ammonium acetate tends to remain near neutral pH as it forms from a weak acid and weak base.
Grasping the acid-base properties helps us anticipate how these substances will behave if mixed or dissolved in different contexts, which is crucial for students aiming to excel in exams and practical chemistry applications.