Problem 47 - A Complete Resource Book in Chemistry for JEE Main

Question

Which among the following acts as a basic buffer solution? (a) \(\mathrm{NH}_{4} \mathrm{Cl}+\mathrm{NH}_{4} \mathrm{OH}\) (b) \(\mathrm{NaCl}+\mathrm{NaOH}\) (c) \(\mathrm{NH}_{4} \mathrm{Cl}+\mathrm{CH}_{3} \mathrm{COOH}\) (d) \(\mathrm{CH}_{3} \mathrm{COONa}+\mathrm{CH}_{3} \mathrm{COOH}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{NH}_{4} \mathrm{Cl}+\mathrm{NH}_{4} \mathrm{OH}\) is the basic buffer solution.

Step 1: Understand Buffer Solutions

Buffer solutions resist changes in pH when small amounts of acid or base are added. A basic buffer is typically made from a weak base and its conjugate acid.

Step 2: Analyze Option (a)

Option (a) consists of \(\mathrm{NH}_{4}\mathrm{Cl}+\mathrm{NH}_{4}\mathrm{OH}\). Ammonium chloride (\(\mathrm{NH}_4\mathrm{Cl}\)) is the conjugate acid of the weak base ammonium hydroxide (\(\mathrm{NH}_4\mathrm{OH}\)). Thus, this combination acts as a basic buffer.

Step 3: Analyze Option (b)

Option (b) consists of \(\mathrm{NaCl}+\mathrm{NaOH}\). Sodium chloride (\(\mathrm{NaCl}\)) is a neutral salt, and combined with a strong base like sodium hydroxide (\(\mathrm{NaOH}\)), it does not act as a buffer.

Step 4: Analyze Option (c)

Option (c) consists of \(\mathrm{NH}_{4}\mathrm{Cl}+\mathrm{CH}_{3}\mathrm{COOH}\). Ammonium chloride (\(\mathrm{NH}_4\mathrm{Cl}\)) is a salt of a weak base, but acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)) is a weak acid, so this does not form a basic buffer.

Step 5: Analyze Option (d)

Option (d) consists of \(\mathrm{CH}_{3}\mathrm{COONa}+\mathrm{CH}_{3}\mathrm{COOH}\). This combination represents an acidic buffer, not a basic buffer, because sodium acetate (\(\mathrm{CH}_3\mathrm{COONa}\)) is a salt of a weak acid.

Step 6: Conclusion: Identify the Basic Buffer

Based on the analysis, option (a) \(\mathrm{NH}_{4}\mathrm{Cl}+\mathrm{NH}_{4}\mathrm{OH}\) forms the basic buffer solution as it contains a weak base and its conjugate acid.

Key Concepts

Basic Buffer

A basic buffer is an interesting mixture that helps maintain a stable pH level even when acids or bases are added to it. This kind of buffer is essential in many biological and chemical systems where pH stability is crucial. A basic buffer is commonly prepared by combining a weak base with its conjugate acid. This pair works together to neutralize changes in the pH. When we talk about a basic buffer, we should remember:

• It contains a weak base and its conjugate acid.
• It effectively resists changes in the pH level, particularly against the addition of strong acids.
• The pH of the solution stays basic, meaning it is above 7.

For example, in the case of ammonium hydroxide and ammonium chloride, the conjugate acid-base pair maintains the pH of the solution, thereby creating a basic buffer.

Weak Base

Weak bases are substances that do not completely dissociate into ions in a solution. This means they only partially ionize, resulting in a dynamic equilibrium between the ionized and unionized forms. Due to this partial dissociation, weak bases are excellent components for forming buffer solutions, particularly basic buffers. Key characteristics of weak bases include:

• They have a higher pH compared to strong acids but are lower than strong bases.
• They are only partially ionized in a solution, which means they exist as both ionized and unionized molecules.
• They are less likely to completely neutralize acids due to their incomplete ionization capability.

A common example of a weak base is ammonium hydroxide ( NH _4OH). When paired with its conjugate acid, like ammonium chloride, it can form a basic buffer, balancing the pH by reacting with any added acids.

Conjugate Acid

A conjugate acid is what you get when a base accepts a proton (H⁺). In a buffer solution, the conjugate acid plays a crucial role. It can neutralize small amounts of added base, helping maintain stability in the pH level of the solution. Some important attributes of conjugate acids include:

• A conjugate acid forms from a base after it gains a hydrogen ion.
• It helps resist changes in pH when a strong base is added to the buffer system.
• In buffer solutions, the conjugate acid stabilizes the pH, working alongside the weak base.

For instance, when ammonium hydroxide is in solution with ammonium chloride, the ammonium ion ( NH_4^+ ) is the conjugate acid, supporting the buffering action by counteracting excess bases.