Problem 17 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following would produce a buffer solution when mixed in equal volume? (a) \(1 \mathrm{M} \mathrm{CH}_{3} \mathrm{COOH}\) and \(0.5 \mathrm{M} \mathrm{NaOH}\) (b) \(1 \mathrm{M} \mathrm{CH}_{3} \mathrm{COOH}\) and \(0.5 \mathrm{M} \mathrm{HCl}\) (c) \(1 \mathrm{M} \mathrm{NH}_{4} \mathrm{OH}\) and \(0.5 \mathrm{M} \mathrm{NaOH}\) (d) \(1 \mathrm{M} \mathrm{NH}_{4} \mathrm{Cl}\) and \(0.5 \mathrm{M} \mathrm{HCl}\)

Step-by-Step Solution

Answer

Option (a) produces a buffer solution.

Step 1: Definition of a Buffer Solution

A buffer solution is a solution that resists a change in pH upon addition of small amounts of acid or base. It typically consists of a weak acid and its conjugate base, or a weak base and its conjugate acid.

Step 2: Analyze Option (a)

Mixing equal volumes of \(1 \, \text{M} \, \text{CH}_3\text{COOH}\) (acetic acid, a weak acid) and \(0.5 \, \text{M} \, \text{NaOH}\) will partially neutralize acetic acid, forming its conjugate base, \(\text{CH}_3\text{COO}^-\). After mixing, the solution will contain both the weak acid \(\text{CH}_3\text{COOH}\) and its conjugate base, therefore forming a buffer.

Step 3: Analyze Option (b)

Mixing equal volumes of \(1 \, \text{M} \, \text{CH}_3\text{COOH}\) with \(0.5 \, \text{M} \, \text{HCl}\) results in a solution of stronger acids and no formation of the weak acid's conjugate base. Thus, it does not form a buffer.

Step 4: Analyze Option (c)

Mixing equal volumes of \(1 \, \text{M} \, \text{NH}_4\text{OH}\) (ammonium hydroxide, a weak base) with \(0.5 \, \text{M} \, \text{NaOH}\) will not form a conjugate acid, hence, will not form a buffer. This solution consists of a combination of a weak base with a strong base, rather than with an acid to form its conjugate.

Step 5: Analyze Option (d)

When \(1 \, \text{M} \, \text{NH}_4\text{Cl}\) is mixed with \(0.5 \, \text{M} \, \text{HCl}\), the solution contains a salt (derived from \(\text{NH}_3\)) and strong acid. No conjugate acid of a weak base is formed, so no buffer results.

Key Concepts

Understanding Weak Acid and Conjugate Base

Buffer solutions are vital when aiming to maintain pH stability, and a common type is made from a weak acid and its conjugate base. Let's break this down:

• Weak Acid: Weak acids only partially ionize in solution, meaning they do not release all their acidic hydrogen ions. A classic example is acetic acid (\( ext{CH}_3 ext{COOH}\)), which only partially dissociates into hydrogen ions (\( ext{H}^+\)) and acetate ions (\( ext{CH}_3 ext{COO}^-\)).


• Conjugate Base: A conjugate base forms when a weak acid loses its proton (\( ext{H}^+\)). For acetic acid, its conjugate base is the acetate ion (\( ext{CH}_3 ext{COO}^-\)).

When combined, the weak acid and its conjugate base work together to resist changes in pH. They do this by absorbing excess hydrogen ions or hydroxide ions that are added to the solution.

Exploring Weak Base and Conjugate Acid

A buffer can also be created using a weak base and its conjugate acid, another reliable way to maintain pH.

• Weak Base: A weak base is less than fully ionized in water, such as ammonium hydroxide (\( ext{NH}_4 ext{OH}\)). It doesn't completely ionize into ammonium ions (\( ext{NH}_4^+\)) and hydroxide ions (\( ext{OH}^-\)).


• Conjugate Acid: The conjugate acid of a weak base forms when the base accepts a proton. For ammonium hydroxide, its conjugate acid is the ammonium ion (\( ext{NH}_4^+\)).

This pairing ensures the solution can neutralize added acids or bases, maintaining a consistent pH level. It's a balanced dance between the base soaking up protons and the conjugate acid releasing them to stabilize the environment.

The Science Behind pH Resistance in Buffers

A buffer's primary goal is to safeguard the pH of a solution, meaning it must resist alterations.

• How Buffers Resist pH Changes: Buffers work through equilibrium. When an acid is added, the conjugate base in the buffer absorbs it. Conversely, when a base is added, the weak acid of the buffer supplies hydrogen ions.


• Equilibrium Reaction: For a weak acid/conjugate base buffer, \[ ext{CH}_3 ext{COOH} \rightleftharpoons ext{CH}_3 ext{COO}^- + ext{H}^+\], both components react with added substances. Hydrogen ions bind to the conjugate base, \( ext{CH}_3 ext{COO}^-\), neutralizing the environment. This minimizes drastic changes in pH.


• The Importance of Balance: The buffer's capacity—its ability to resist pH change—is influenced by the initial concentrations of the acid and its conjugate base or the base and its conjugate acid. The closer these are in concentration, the more robust the buffer.

Buffers play a crucial role in biological systems and chemical applications where pH control is essential. They act as a stabilizing agent in many processes, making them invaluable in various scientific and industrial fields.