Problem 201 - A Complete Resource Book in Chemistry for JEE Main

Question

A buffer solution can be prepared from a mixture of (a) \(\mathrm{CH}_{3} \mathrm{COONa}\) and \(\mathrm{CH}_{3} \mathrm{COOH}\) in water (b) \(\mathrm{CH}_{3} \mathrm{COONa}\) and \(\mathrm{HCl}\) in water under certain conditions (c) \(\mathrm{NH}_{4} \mathrm{OH}\) and \(\mathrm{NH}_{4} \mathrm{Cl}\) in water (d) \(\mathrm{NaCl}\) and \(\mathrm{HCl}\) in water

Step-by-Step Solution

Answer

Options (a) and (c) are correct, forming buffer solutions.

Step 1: Understanding Buffers

A buffer solution is made from weak acid and its conjugate base, or a weak base and its conjugate acid. It resists changes in pH when small amounts of strong acid or base are added.

Step 2: Analyzing Option A

Option (a) involves a mixture of sodium acetate (\(\mathrm{CH}_{3} \mathrm{COONa}\), the conjugate base of acetic acid) and acetic acid (\(\mathrm{CH}_{3} \mathrm{COOH}\), a weak acid). This forms an acetate buffer, fulfilling the criteria for a buffer solution.

Step 3: Analyzing Option B

Option (b) contains sodium acetate (\(\mathrm{CH}_{3} \mathrm{COONa}\)), a conjugate base, and hydrochloric acid (\(\mathrm{HCl}\), a strong acid). This does not make a buffer solution because a strong acid is involved instead of a weak acid.

Step 4: Analyzing Option C

Option (c) involves ammonium hydroxide (\(\mathrm{NH}_{4} \mathrm{OH}\), a weak base) and ammonium chloride (\(\mathrm{NH}_{4} \mathrm{Cl}\), the conjugate acid). This creates a buffer solution as it consists of a weak base and its conjugate acid.

Step 5: Analyzing Option D

Option (d) contains sodium chloride (\(\mathrm{NaCl}\), a salt that neither acts as an acid nor a base) and hydrochloric acid (\(\mathrm{HCl}\), a strong acid). This does not form a buffer solution, as it includes no weak acid or base.

Key Concepts

Acid-Base Equilibrium

Acid-base equilibrium is a fundamental concept in chemistry that refers to the balance between acid and base concentrations in a solution. This equilibrium is crucial in many biological and chemical processes.
In a solution, acids donate protons ( H^+ ), while bases accept protons. The point at which the rate of proton donation equals the rate of proton acceptance defines the state of equilibrium.

• Weak acids and bases do not completely dissociate in water. This property allows them to establish an equilibrium state, making them essential for buffer solutions.
• By maintaining this equilibrium, solutions can resist changes in pH to a certain extent.
• For example, the mixture in option (a) from the original exercise uses acetic acid and sodium acetate, establishing a dynamic equilibrium between the weak acid and its conjugate base.

This equilibrium ensures that, when a bit of a strong acid or base is added, the overall change in pH remains minimal.

pH Stability

pH stability refers to the ability of a solution to maintain a constant pH even when small amounts of an acid or a base are added.
This stability is essential in various applications, from biological systems where a stable pH is necessary for enzyme activities to industrial processes.
A buffer solution achieves pH stability by employing:

• Weak acids and their conjugate bases or weak bases and their conjugate acids.
• These pairs work together to neutralize added acids or bases, thus minimizing changes in pH.

For instance, in option (c), ammonium hydroxide and ammonium chloride make a solution capable of maintaining pH stability because:
Ammonium hydroxide neutralizes added acid by providing OH^- ions, while ammonium chloride reacts with added bases by releasing H^+ ions.
This balance keeps the pH fluctuations in check, ensuring a stable environment.

Conjugate Acid-Base Pairs

Conjugate acid-base pairs play a vital role in buffer solutions, forming the basis for resisting pH changes. When an acid donates a proton, it becomes a conjugate base; conversely, when a base accepts a proton, it becomes a conjugate acid.
These pairs make buffer solutions effective by working in tandem to neutralize any added strong acids or bases.

• In option (a) and (c), the pairs involved are acetic acid and sodium acetate, and ammonium hydroxide and ammonium chloride respectively.
• The ability of these pairs to react with additional acids or bases helps them in maintaining equilibrium.

This mutual reaction ensures that neither of the new additions can drastically alter the pH .
By providing both a donor (acid) and an acceptor (base), conjugate acid-base pairs maintain the delicate balance within a solution, thus forming a stable environment.