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

Question

Which of the following solution in water act as buffer? (a) \(0.5 \mathrm{~mol}\) of pyridine \(+0.5 \mathrm{~mol}\) of Pyridinium chloride (b) \(0 . \mathrm{I} \mathrm{mol}\) of \(\mathrm{NaOH}+0.15 \mathrm{~mol}\) of \(\mathrm{CH}_{3} \mathrm{COOH}\) (c) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\) (d) \(0.25\) mol of \(\mathrm{NH}_{4} \mathrm{Cl}+0.5 \mathrm{~mol}\) of \(\mathrm{NaOH}\).

Step-by-Step Solution

Answer

Option (a) acts as a buffer.

Step 1: What is a Buffer Solution?

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

Step 2: Analyze Option (a)

In option (a), the solution contains 0.5 mol of pyridine, which is a weak base, and 0.5 mol of pyridinium chloride, which is the conjugate acid of pyridine. Thus, this combination forms a buffer solution.

Step 3: Analyze Option (b)

In option (b), adding NaOH (a strong base) to CH₃COOH (a weak acid) creates a mixture where NaOH will neutralize some of the acetic acid, but does not form a buffer since the mole ratio is not adequate for buffering action.

Step 4: Analyze Option (c)

Option (c) presents CH₃COONH₄, which is a salt derived from a weak acid (acetic acid) and a weak base (ammonia), potentially forming a buffer if dissolved in water and maintaining some of the weak acid and its conjugate base in solution.

Step 5: Analyze Option (d)

Option (d), which adds 0.25 mol of NH₄Cl (ammonium chloride) to 0.5 mol of NaOH. The strong base NaOH will fully react with the NH₄Cl, leaving no components to act as a buffer.

Step 6: Determine the Correct Buffer Solution

Reviewing each option, the solution in (a), which contains both a weak base and its conjugate acid (pyridine and pyridinium chloride), fulfills the condition needed for a buffer solution.

Key Concepts

Weak Acid and Conjugate Base

Buffer solutions often include a weak acid and its conjugate base. This pairing is crucial for the solution’s ability to resist changes in pH. A weak acid, unlike a strong acid, only partially dissociates in water.
The conjugate base is what remains when the acid gives up a proton (H+). Together, they work to neutralize added acids or bases within the solution. For instance, if a strong acid is added to the buffer, the conjugate base can combine with the additional protons to form the weak acid, thus resisting a significant drop in pH. Some key aspects of this process are:

• Partial dissociation of the weak acid allows the conjugate base to react with added hydrogen ions.
• Equilibrium shifts are minimal due to the presence of both components.

This balance between a weak acid and its conjugate base maintains the buffer's capacity to "absorb" pH changes, making it an essential aspect of chemical stability in solutions.

Weak Base and Conjugate Acid

Another type of buffer solution involves a weak base and its conjugate acid. A weak base can react with water to form its conjugate acid, which in turn can react with added hydroxide ions (OH-) to maintain a stable pH level.
An example of such a system is the mixture of pyridine (a weak base) and pyridinium chloride (the conjugate acid of pyridine). When an acid is added to the solution, the weak base (pyridine) can react with the additional hydrogen ions to form more of the conjugate acid. Important points to remember include:

• The weak base partially ionizes, producing its conjugate acid.
• The presence of both components allows the solution to neutralize additional acids or bases.

This interaction between a weak base and its conjugate acid illustrates how buffer solutions can withstand acidic or basic disruptions, thanks to their unique chemistry.

pH Change Resistance

The ability of buffer solutions to resist changes in pH is fundamental to their utility. This resistance to pH fluctuation means that when small amounts of strong acids or bases are added, the pH remains relatively stable.
Buffer solutions can absorb the excess hydrogen or hydroxide ions introduced, ensuring that the solution maintains its pH within a narrow range. Key features of this resistance include:

• The presence of both an acid and its conjugate base, or a base and its conjugate acid, neutralizes added substances.
• The equilibrium between the buffer components allows for minimal change in pH.

Understanding these qualities helps explain why buffer solutions are pivotal in many biological and chemical processes, where maintaining a stable pH is crucial for proper functioning.