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 \(+0.5 \mathrm{~mol}\) 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

Solutions (a) and (c) act as buffers.

Step 1: Identify Buffer Components

Buffers are solutions that contain a weak acid and its conjugate base or a weak base and its conjugate acid. They are able to resist changes in pH upon the addition of small amounts of acid or base.

Step 2: Analyze Option (a)

Option (a) contains pyridine and pyridinium chloride. Pyridine, a weak base, and pyridinium chloride, its conjugate acid, form a buffer solution. This meets the criteria for a buffer as it includes both a weak base and its conjugate acid.

Step 3: Analyze Option (b)

Option (b) includes NaOH, a strong base, and acetic acid. When a strong base and a weak acid are present, the weak acid can be partially neutralized to form its conjugate base (acetate ion), potentially forming a buffer if the proportions are correct.

Step 4: Analyze Option (c)

Option (c) suggests ammonium acetate (CH₃COONH₄), which is the salt of a weak acid (acetic acid) and a weak base (ammonia), thus it can also act as a buffer solution because it contains the conjugate base CH₃COO⁻ and the conjugate acid NH₄⁺.

Step 5: Analyze Option (d)

Option (d) consists of NH₄Cl (ammonium chloride) and NaOH. Here, NH₄Cl is a weak acid's salt, and NaOH is a strong base. This will not form a buffer because the strong base could fully neutralize the weak acid and significantly alter the pH.

Step 6: Select the Buffer Solution

Considering the analysis, options (a) and (c) form buffer solutions due to their compositions of weak acids and their conjugate bases, or vice versa.

Key Concepts

Weak Acid

A weak acid is a type of acid that partially ionizes in a solution. This means that only a small fraction of the acid molecules donate protons (H⁺ ions) to the solution, while the majority remains un-ionized. When dissolved in water, weak acids create an equilibrium between the un-ionized acid and its ionized form. This characteristic equilibrium is essential in forming a buffer solution.
The ability to only partially ionize allows weak acids to gradually adjust their concentration of H⁺ ions in response to changes in the environment. This property is crucial when paired with a conjugate base, creating a buffer that helps stabilize the pH of a solution.
Common examples of weak acids include:

• Acetic acid (CH₃COOH)
• Citric acid (C₆H₈O₇)
• Formic acid (HCOOH)

Conjugate Base

In the context of acids and bases, a conjugate base is formed when a weak acid donates a proton (H⁺). After the proton is released, the remaining part of the acid is called its conjugate base. This behavior is crucial for the buffer mechanism, as it allows the solution to absorb excess H⁺ ions without a significant change in pH.
Together with a weak acid, the conjugate base forms an equilibrium that can neutralize added acids or bases, minimizing pH changes. This equilibrium is a key aspect of a buffer solution. If additional acid is added to the solution, the conjugate base will react with the H⁺ ions. Conversely, if a base is added, the weak acid can donate H⁺ ions to counteract the increase in pH.
For acetic acid, CH₃COOH, its conjugate base is the acetate ion, CH₃COO⁻. These pairs ensure the resilience of buffer solutions against large pH swings.

pH Changes

pH is a measure of the acidity or basicity of a solution. It is logarithmically related to the concentration of hydrogen ions in the solution. A buffer solution is specifically designed to resist major shifts in pH when small amounts of acid or base are introduced.
This is possible because of the dynamic equilibrium between the weak acid and its conjugate base (or weak base and its conjugate acid) in the buffer. When an acid is introduced, the conjugate base present in the buffer neutralizes it by forming more of the weak acid. Similarly, when a base is added, the weak acid counteracts it by producing more conjugate base.
The ability of a solution to resist pH changes is defined by its buffer capacity, which depends on the concentrations of the buffer components. High buffer capacity implies better durability against pH fluctuations, which is vital in various chemical, biological, and physiological processes.

Weak Base

A weak base is a substance that partially accepts protons or donates hydroxide ions (OH⁻) in a solution. Like weak acids, weak bases do not fully ionize, establishing an equilibrium in the solution. This characteristic is essential for creating a buffer solution alongside its conjugate acid.
In a buffer system containing a weak base and its conjugate acid, both components work together to maintain pH stability. When an acid is added, the weak base can neutralize it by accepting protons. Conversely, when a base is added, the conjugate acid can donate protons back into the solution to counterbalance the increase in pH.
Examples of weak bases include:

• Ammonia (NH₃)
• Pyridine (C₅H₅N)

Buffers that contain weak bases are as effective as those with weak acids in preventing abrupt pH changes, making them instrumental in chemical reactions and processes where pH stability is crucial.