Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

Which has highest \(\mathrm{pH}\) ? (a) \(\mathrm{CH}_{3} \mathrm{COOK}\) (b) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (c) \(\mathrm{NH}_{4} \mathrm{Cl}\) (d) \(\mathrm{NaNO}_{3}\)

Step-by-Step Solution

Answer

\( ext{Na}_2 ext{CO}_3\) has the highest pH.

Step 1: Understanding pH

To find which compound has the highest pH, we need to understand that pH measures how basic or acidic a solution is. A high pH means a solution is basic, while a low pH means it is acidic. Neutral solutions have a pH of 7.

Step 2: Analyze Each Compound

Evaluate each compound: - (a) \( ext{CH}_3 ext{COOK}\) is a salt of a weak acid and strong base, typically producing a basic solution.- (b) \( ext{Na}_2 ext{CO}_3\) is a salt of a weak acid and strong base, also typically basic.- (c) \( ext{NH}_4 ext{Cl}\) is a salt of a weak base and strong acid, typically acidic.- (d) \( ext{NaNO}_3\) is a salt of a strong acid and strong base, typically neutral.

Step 3: Determine the Most Basic

Compare the basicity of the compounds based on the hydrolysis of their ions:- (a) \( ext{CH}_3 ext{COOK}\) can result in acetate ions, leading to a slightly basic solution.- (b) \( ext{Na}_2 ext{CO}_3\) generates carbonate ions, which can further decompose to produce \( ext{OH}^-\), resulting in a more basic solution.- The \( ext{NH}_4 ext{Cl}\) and \( ext{NaNO}_3\) are acidic and neutral, respectively.

Step 4: Conclude the Highest pH

Based on the analyses, \( ext{Na}_2 ext{CO}_3\) produces a more basic environment than the others, as the carbonate ion is a more effective base in increasing pH compared to the acetate ion.

Key Concepts

Basic Solutions

Basic solutions, also known as alkaline solutions, have a pH greater than 7. These solutions occur when a substance releases hydroxide ions \( (\text{OH}^-) \) into the water, increasing the concentration of \( \text{OH}^- \) ions compared to hydrogen ions \( (\text{H}^+) \).
For example:

• Sodium carbonate \( (\text{Na}_2\text{CO}_3) \) dissociates in water to form carbonate ions \( (\text{CO}_3^{2-}) \), which can further react with water to form bicarbonate and hydroxide ions, increasing the pH substantially.
• The basic character is reflected in household items like baking soda, which is alkaline in nature.

Understanding basic solutions is crucial as they play significant roles in industries, everyday life, and natural environments.

Acidic Solutions

Acidic solutions have a pH less than 7, indicating a higher concentration of hydrogen ions \( (\text{H}^+) \). These solutions are usually created when an acid releases \( \text{H}^+ \) into water.For example:

• Ammonium chloride \( (\text{NH}_4\text{Cl}) \) is a salt formed from a weak base (ammonia) and a strong acid (hydrochloric acid). When dissolved, it produces ammonium ions \( (\text{NH}_4^+) \) which can increase the \( \text{H}^+ \) concentration, leading to a lower pH.
• Citric acid in lemons and acetic acid in vinegar are everyday examples, known for their sour taste and acidic nature.

Recognizing acidic solutions is essential in understanding various chemical processes, such as digestion and fermentation.

Neutral Solutions

Neutral solutions have a pH of exactly 7, which indicates equal concentrations of \( \text{H}^+ \) and \( \text{OH}^- \) ions. Pure water is the classic example of a neutral solution.

• Neutral solutions can also result when a strong acid and a strong base react completely, as in the case of sodium nitrate \( (\text{NaNO}_3) \), which originates from nitric acid and sodium hydroxide.

In daily life, neutrality is seen in substances such as pure water, which is used as a standard to compare acidity and basicity.
Comprehending neutral solutions is key for tasks like maintaining balanced pH levels in swimming pools and aquariums.

Hydrolysis of Salts

The hydrolysis of salts involves the reaction of a salt with water to form an acidic or basic solution. Not all salts hydrolyze; some maintain the neutrality of the solution.For example:

• Sodium carbonate \( (\text{Na}_2\text{CO}_3) \) undergoes hydrolysis to form hydroxide ions \( (\text{OH}^-) \), resulting in a basic pH.
• Conversely, ammonium chloride \( (\text{NH}_4\text{Cl}) \) hydrolyzes to form hydrogen ions \( (\text{H}^+) \), leading to an acidic solution.

Grasping the hydrolysis of salts is crucial for predicting the behavior of compounds in aqueous solutions, especially in industrial and laboratory settings.

Acid-Base Chemistry

Acid-base chemistry revolves around the concept of acids and bases reacting with each other, transferring protons \( (\text{H}^+) \) to achieve chemical balance. The key components include:

• Acids, which donate protons and increase \( \text{H}^+ \) concentration.
• Bases, which accept protons and increase \( \text{OH}^- \) concentration.

In the provided example, sodium carbonate acts as a base, contributing to a high pH through the release of carbonate ions which react with water to yield \( \text{OH}^- \) ions. Meanwhile, ammonium chloride, as an acid, increases \( \text{H}^+ \) ions, lowering the pH.
Mastering acid-base chemistry provides vital insights into reactions occurring naturally, industrially, and within living organisms.