Problem 21 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following can act like a buffer? (a) \(\mathrm{CH}_{3} \mathrm{COOH}+\mathrm{CH}_{3} \mathrm{COONa}\) (b) \(\mathrm{NH}_{4} \mathrm{ND}_{3}+\mathrm{NH}_{4} \mathrm{OH}\) (c) \(50 \mathrm{ml} 0.1 \mathrm{~m} \mathrm{NaCN}+30 \mathrm{ml} 0.1 \mathrm{~m} \mathrm{HCl}\) (d) All of these

Step-by-Step Solution

Answer

The correct answer is (d) all of these.

Step 1: Understanding a Buffer

A buffer solution is a solution that can resist pH change upon the addition of an acidic or basic components. It usually consists of a weak acid and its conjugate base or a weak base and its conjugate acid.

Step 2: Analyzing Option (a)

Option (a) contains acetic acid (3CH_{3}COOH
64) and sodium acetate (
63CH_{3}COONa
64). Acetic acid is a weak acid, and sodium acetate is its conjugate base. Thus, this combination can act as a buffer.

Step 3: Analyzing Option (b)

Option (b) consists of an incorrect notation
63NH_{4}ND_{3}
64 which is likely a typo, but more importantly, the correct buffer pair would be a weak base (like
63NH_{3}
64) and its conjugate acid (
63NH_{4}^{+}
64). Assuming a combination of
63NH_{3}
64 and
63NH_{4}OH
64, it forms a buffer solution.

Step 4: Analyzing Option (c)

Option (c) comprises sodium cyanide (
63NaCN
64) and hydrochloric acid (
63HCl
64). When
63HCl
64 is added to
63NaCN
64, it converts it to
63HCN
64, creating
63HCN
64 and
63CN^{-}
64, which can act as a buffer.

Step 5: Conclusion

Each option (a, b, c) has the components needed to form a buffer solution. Therefore, all options can potentially work as buffers.

Key Concepts

Understanding Weak Acids

Weak acids play a pivotal role in buffer solutions because they partially dissociate in water. This means they can release hydrogen ions (H\(^+\)) under certain conditions, which is key for maintaining pH levels. Unlike strong acids, weak acids do not fully dissociate, providing a balance in the solution that allows for pH buffering.
For instance, acetic acid (CH\(_3\)COOH) is a common weak acid used in buffers. It only partially donates H\(^+\) ions to the solution, making it ideal for reacting with added bases. This property of partial dissociation ensures that the solution can neutralize added acids and bases without a significant change in pH.
Key characteristics of weak acids include:

• Partial dissociation in water.
• Ability to donate hydrogen ions.
• Maintaining equilibrium in solution.

These attributes make weak acids fundamental in the formation of buffer solutions, alongside their conjugate bases.

Role of Conjugate Bases

Conjugate bases are crucial in buffer solutions as they partner with weak acids to resist changes in pH. When a weak acid partially dissociates, it leaves behind a conjugate base. This conjugate base can then react with added acid to prevent pH shift.
For example, when acetic acid (CH\(_3\)COOH) dissociates, it produces acetate ions (CH\(_3\)COO\(^-\)), its conjugate base. These acetate ions can react with any additional hydronium ions (H\(_3\)O\(^+\)) in the solution, reducing their impact on pH.
Conjugate bases have these important roles:

• Neutralizing added acids by forming a weak acid.
• Participating in equilibrium that stabilizes pH.
• Ensuring the buffer's capacity to maintain pH.

This intricate dance between a weak acid and its conjugate base is what allows buffer solutions to regulate pH effectively.

Understanding pH Resistance

pH resistance is the core characteristic of buffer solutions. It refers to the ability of buffers to withstand changes in pH when small amounts of acids or bases are added. This resistance is vital for many biological and chemical processes that require a stable pH environment.
The mechanism that underscores pH resistance involves the equilibrium between weak acids and their conjugate bases. When an acid is added to the buffer, the conjugate base captures the additional H\(^+\) ions, forming the weak acid again, and thus minimizing the pH change. Conversely, when a base is added, the weak acid donates H\(^+\) ions to neutralize the base.
Key points about pH resistance include:

• It allows for stable environmental conditions.
• Enables biological functions to proceed without disruption.
• Prevents drastic pH fluctuations in the system.

This equilibrium-driven resistance is what makes buffers indispensable in both natural systems and industrial applications.