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), \(\mathrm{CH}_{3}, \mathrm{COONa}\) (II), \(\mathrm{CH}_{3} \mathrm{COONH}_{4}(\mathrm{III})\), and \(\mathrm{KOH}\) (IV) will be in the order (a) IV>III > II > I (b) \(I V>I I>I I I>I\) (c) \(\mathrm{I}>\mathrm{III}>\mathrm{II}>\mathrm{IV}\) (d) \(\mathrm{II}>\mathrm{I}>\mathrm{III}>\mathrm{IV}\)

Step-by-Step Solution

Answer

The order of increasing pH is IV > II > III > I, answer (b).

Step 1: Understanding the Compounds

Each compound has different acidic or basic properties. \( \mathrm{CH_3COOH} \) (I) is a weak acid, \( \mathrm{CH_3COONa} \) (II) is a salt formed from a weak acid and a strong base, \( \mathrm{CH_3COONH_4} \) (III) is a salt of a weak acid and weak base, and \( \mathrm{KOH} \) (IV) is a strong base.

Step 2: Analyzing pH Values

The pH level of these compounds depends on their acidic or basic properties. \( \mathrm{CH_3COOH} \) will have a low pH due to being a weak acid. \( \mathrm{CH_3COONa} \) will have a higher pH due to being a salt of a weak acid and a strong base. \( \mathrm{CH_3COONH_4} \) will have an intermediate pH. \( \mathrm{KOH} \) will have a high pH since it's a strong base.

Step 3: Ordering the Compounds

Based on their pH values: \( \mathrm{KOH} \) will have the highest pH (strong base), followed by \( \mathrm{CH_3COONa} \) (salt of a strong base), then \( \mathrm{CH_3COONH_4} \) (intermediate salt) and finally \( \mathrm{CH_3COOH} \) (weak acid). Thus, the pH order is IV > II > III > I.

Key Concepts

weak acids and bases

Weak acids and bases are substances that partially ionize in water. This means that only a small fraction of the acid or base molecules donate or accept protons.For weak acids like acetic acid (\(\text{CH}_3\text{COOH}\)), the dissociation in water is limited, producing fewer hydrogen ions (\(\text{H}^+\)). This results in a higher pH, but still slightly below 7, indicating acidity.Conversely, weak bases do not fully accept protons in solution, leading to a lower concentration of hydroxide ions (\(\text{OH}^-\)) and a pH slightly above 7 but below that of strong bases.
Key characteristics of weak acids and bases:

• They do not completely dissociate in water.
• They have a pH closer to neutral (7) compared to their strong counterparts.
• The degree of ionization affects the pH level.

Understanding weak acids and bases is essential in predicting the behavior of many common substances in aqueous solutions.

salt hydrolysis

Salt hydrolysis is a process where a salt reacts with water to produce either an acidic or a basic solution. This phenomenon occurs when the ions from a dissolved salt affect the pH of the solution.Acetate salts such as sodium acetate (\(\text{CH}_3\text{COONa}\)) are formed from weak acids and strong bases. In this case, water molecules react with the acetate ion (\(\text{CH}_3\text{COO}^-\)), leading to a slight increase in pH as it results in the formation of hydroxide ions (\(\text{OH}^-\)).
On the other hand, salts like ammonium acetate (\(\text{CH}_3\text{COONH}_4\)) come from a weak acid and a weak base, resulting in a neutral pH as the hydrolysis reactions of the ions more or less cancel each other out.
Basic rules of salt hydrolysis include:

• Salts derived from strong bases and weak acids result in a basic solution.
• Salts from weak bases and strong acids result in an acidic solution.
• Salts from weak bases and weak acids generally give a neutral solution.

By understanding salt hydrolysis, you can predict whether a solution will become more acidic or basic based on the nature of the dissolved salt.

acidic and basic properties

Acids and bases display distinctive properties based on their ability to donate or accept protons.Acids are substances that can donate hydrogen ions (\(\text{H}^+\)) to a solution, leading to a lower pH.A typical example is acetic acid (\(\text{CH}_3\text{COOH}\)), which is a weak acid.Its partial dissociation in water results in fewer hydrogen ions, slightly acidifying the solution.
Bases, on the other hand, accept hydrogen ions and provide hydroxide ions (\(\text{OH}^-\)) to the solution.Strong bases like potassium hydroxide (\(\text{KOH}\)) fully dissociate to furnish a large number of hydroxide ions, resulting in a high pH solution.
Here are some basic properties of acids and bases:

• Acids generally taste sour, while bases tend to taste bitter.
• Acids turn blue litmus paper red, while bases turn red litmus paper blue.
• Acids can donate a proton (\(\text{H}^+\)), whereas bases can accept it.

Recognizing these properties aids in predicting the behavior of substances and their pH in aqueous environments.