Problem 201 - A Complete Resource Book in Chemistry for JEE Main

Question

A buffer solution can be prepared from a mixture of (a) \(\mathrm{CH}_{3} \mathrm{COONa}\) and \(\mathrm{CH}_{3} \mathrm{COOH}\) in water (b) \(\mathrm{CH}_{3} \mathrm{COONa}\) and \(\mathrm{HCl}\) in water under certain conditions (c) \(\mathrm{NH}_{4} \mathrm{OH}\) and \(\mathrm{NH}_{4} \mathrm{Cl}\) in water (d) \(\mathrm{NaCl}\) and \(\mathrm{HCl}\) in water

Step-by-Step Solution

Answer

Options (a) and (c) can form buffer solutions.

Step 1: Understanding the buffer concept

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

Step 2: Identifying potential buffer systems

We need to identify which pairs provided can form a buffer. (a) \(\mathrm{CH}_{3} \mathrm{COONa}\) and \(\mathrm{CH}_{3} \mathrm{COOH}\): \(\mathrm{CH}_{3} \mathrm{COOH}\) is a weak acid and \(\mathrm{CH}_{3} \mathrm{COONa}\) is its conjugate base.(b) \(\mathrm{CH}_{3} \mathrm{COONa}\) and \(\mathrm{HCl}\): \(\mathrm{HCl}\) is a strong acid, and cannot pair with a conjugate base of a weak acid to form a buffer.(c) \(\mathrm{NH}_{4} \mathrm{OH}\) and \(\mathrm{NH}_{4} \mathrm{Cl}\): \(\mathrm{NH}_{4} \mathrm{OH}\) is a weak base, and \(\mathrm{NH}_{4} \mathrm{Cl}\) is its conjugate acid.(d) \(\mathrm{NaCl}\) and \(\mathrm{HCl}\): No weak acid or base is present, so no buffer can be formed.

Step 3: Selecting buffer-compatible pairs

From the identified pairs, options (a) and (c) meet the criteria for buffer formation.(a) \(\mathrm{CH}_{3} \mathrm{COONa}\) with \(\mathrm{CH}_{3} \mathrm{COOH}\), and (c) \(\mathrm{NH}_{4} \mathrm{OH}\) with \(\mathrm{NH}_{4} \mathrm{Cl}\), both consist of a weak acid/base and its conjugate.

Key Concepts

Weak Acid and Conjugate Base

Buffer solutions are unique in their ability to maintain a stable pH. One of the primary methods to achieve this is through the combination of a weak acid and its conjugate base. A weak acid is a substance that partially ionizes in solution, providing a buffer against pH changes. The conjugate base is the species formed when the weak acid donates a proton (H⁺) to the solution.
Consider the combination of acetic acid (\(\mathrm{CH}_{3}\mathrm{COOH}\)) and sodium acetate (\(\mathrm{CH}_{3}\mathrm{COONa}\)). Acetic acid acts as a weak acid while sodium acetate provides the conjugate base, acetate ion (\(\mathrm{CH}_{3}\mathrm{COO}^-\)). Together, they form a buffer solution that resists changes in pH:

• The acetic acid can neutralize added bases by donating hydrogen ions.
• Conversely, the acetate ions can neutralize added acids by accepting hydrogen ions.

This dynamic equilibrium between the weak acid and its conjugate base ensures the solution's pH remains relatively stable, even when subjected to external forces such as the addition of more acid or base.

Weak Base and Conjugate Acid

Buffer systems can also consist of a weak base and its conjugate acid. For instance, ammonium hydroxide (\(\mathrm{NH}_{4}\mathrm{OH}\)) is a common weak base, and ammonium chloride (\(\mathrm{NH}_{4}\mathrm{Cl}\)) provides the conjugate acid, ammonium ion (\(\mathrm{NH}_{4}^+\)). Here's how they work:

• The weak base, ammonium hydroxide, can accept protons from added acids, partially neutralizing them.
• The ammonium ions can donate protons to counteract added bases, restoring balance in pH fluctuations.

This interaction ensures that despite external changes, such as the introduction of additional acids or bases, the pH of the solution is maintained with minimal change.When you combine the weak base with its conjugate acid, they function similarly to the weak acid-conjugate base buffers, emphasizing the versatile roles different chemical compositions can play in buffering solutions.

pH Resistance

One of the defining characteristics of buffer solutions is their remarkable ability to resist changes in pH. This property is crucial in many chemical and biological systems where even minor fluctuations in pH could lead to significant impacts.
Buffers work by neutralizing added acids or bases:

• If an acid is added to a buffer, the conjugate base present in the buffer will react with the extra hydrogen ions and prevent the pH from dropping drastically.
• If a base is added, the weak acid in the buffer will donate hydrogen ions, preventing the pH from rising sharply.

This balance results in a stable environment that allows chemical processes to proceed as required without interruption or alteration due to pH changes. Understanding the concept of pH resistance also provides insight into the choices of buffer components in various applications, from laboratory experiments to maintaining homeostasis in living organisms.