Problem 202 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following solution in water act as buffer? (a) \(0.5\) mol of pyridine \(+0.5\) mol of Pyridinium chloride(b) \(0.1 \mathrm{~mol}\) of \(\mathrm{NaOH}+0.15 \mathrm{~mol}\) of \(\mathrm{CH}_{3} \mathrm{COOH}\) (c) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\) (d) \(0.25 \mathrm{~mol}\) of \(\mathrm{NH}_{4} \mathrm{Cl}+0.5 \mathrm{~mol}\) of \(\mathrm{NaOH}\).

Step-by-Step Solution

Answer

The buffer solution is option (a) pyridine + pyridinium chloride.

Step 1: Understanding Buffers

Buffers are solutions that resist changes in pH when small amounts of acid or base are added. They are typically made from a weak acid and its conjugate base or a weak base and its conjugate acid.

Step 2: Examine Option (a)

Option (a) contains pyridine (a weak base) and pyridinium chloride (its conjugate acid). This sets up an ideal buffer solution because it includes both a weak base and its conjugate acid.

Step 3: Examine Option (b)

Option (b) involves NaOH (a strong base) and CH₃COOH (a weak acid). While there could be partial neutralization, the presence of a strong base disrupts the buffer, leading to a non-buffer solution.

Step 4: Examine Option (c)

Option (c) involves CH₃COONH₄, which is a salt derived from a weak acid (acetic acid) and a weak base (ammonia). However, since there is no presence of both a weak acid and its conjugate base or weak base and its conjugate acid, it does not act as a buffer.

Step 5: Examine Option (d)

Option (d) contains NH₄Cl (ammonium chloride) and NaOH (a strong base). NH₄Cl provides NH₄⁺, a weak acid. When mixed with an excess of strong base like NaOH, it doesn't form a proper buffer.

Step 6: Conclude

The only solution that meets the buffer criteria, containing both a weak base and its conjugate acid, is option (a).

Key Concepts

Weak Acid and Conjugate Base

Buffer solutions are fascinating as they maintain a relatively stable pH despite the addition of small amounts of acids or bases. One way to create these solutions is by mixing a weak acid with its conjugate base.

• A **weak acid** is one that partially ionizes in a solution, meaning only a small amount of its molecules donate protons (H\(^+\)).
• The **conjugate base** is what remains after the weak acid donates its proton. It can then accept a proton back, facilitating a balance within the solution.

This balance between the weak acid and its conjugate base is critical for creating a buffering action. The solution can neutralize either additional acids or bases, preventing significant pH changes. For example, acetic acid and sodium acetate combine to form such a buffer.

Weak Base and Conjugate Acid

The other type of buffer solution involves a weak base paired with its conjugate acid. These solutions play a vital role in maintaining pH stability.

• A **weak base** partially accepts protons in the solution, contrasting with strong bases that would fully accept available protons.
• The **conjugate acid** results when the weak base takes a proton, yet can release it back under the right conditions.

One classic example of such a buffer is a solution containing pyridine (a weak base) and pyridinium chloride (its conjugate acid). Together, they create an environment where the solution can effectively counterbalance the addition of added acids or bases.

pH Resistance

One of the most impressive features of buffer solutions is their resistance to pH changes, a phenomenon that serves numerous chemical and biological systems.
The ability of a buffer to resist pH changes stems from the presence of both a weak acid and its conjugate base, or a weak base and its conjugate acid. When additional H\(^+\) (hydrogen ions) or OH\(^-\) (hydroxide ions) are introduced, the buffer components act:

• Conjugate bases will neutralize added hydrogen ions.
• Conjugate acids will neutralize added hydroxide ions.

This functionality ensures that the pH remains relatively stable within its buffer capacity, making these solutions invaluable in settings where pH control is essential, such as in biochemical reactions or industrial processes.

Buffer Criteria

Understanding the criteria for a buffer solution is crucial in identifying which combinations of compounds will effectively stabilize pH. A buffer requires:

• A mixture of a weak acid with its conjugate base or a weak base with its conjugate acid.
• Relatively equal concentrations of these components to maximize pH stability.

It's important to note that adding a strong acid or base overly disrupts this delicate balance. Therefore, options, where the mixture involves strong acids or bases, generally do not act effectively as buffers. For example, a solution of pyridine and pyridinium chloride satisfies these conditions, unlike others that might involve strong acids or bases causing pH fluctuations. These criteria help evaluate potential buffer solutions, guiding the selection of compounds to best maintain a desired pH range.