Problem 17 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following would produce a buffer solution when mixed in equal volume? (a) \(1 \mathrm{M} \mathrm{CH}_{3} \mathrm{COOH}\) and \(0.5 \mathrm{M} \mathrm{NaOH}\) (b) \(1 \mathrm{M} \mathrm{CH}_{3} \mathrm{COOH}\) and \(0.5 \mathrm{M} \mathrm{HCl}\) (c) \(1 \mathrm{M} \mathrm{NH}_{4} \mathrm{OH}\) and \(0.5 \mathrm{M} \mathrm{NaOH}\) (d) \(1 \mathrm{M} \mathrm{NH}_{4} \mathrm{Cl}\) and \(0.5 \mathrm{M} \mathrm{HCl}\)

Step-by-Step Solution

Answer

Option (a) produces a buffer solution.

Step 1: Understanding Buffer Solutions

A buffer solution consists of a weak acid and its conjugate base, or a weak base and its conjugate acid. When such pairs are present, the solution resists significant changes in pH when small amounts of strong acids or bases are added.

Step 2: Analyzing Option (a)

In option (a), we mix equal volumes of 1 M CH₃COOH (a weak acid) and 0.5 M NaOH (a strong base). The NaOH will neutralize only half of the CH₃COOH, forming CH₃COONa (sodium acetate), which is the conjugate base of CH₃COOH. Thus, it creates a buffer solution composed of CH₃COOH and CH₃COO⁻.

Step 3: Analyzing Option (b)

In option (b), we mix equal volumes of 1 M CH₃COOH and 0.5 M HCl (a strong acid). Since both CH₃COOH and HCl are acids, no buffer solution can be formed because there is no conjugate base present.

Step 4: Analyzing Option (c)

In option (c), the mixture combines equal volumes of 1 M NH₄OH (a weak base) and 0.5 M NaOH (a strong base). The presence of two bases does not form a buffer, as a buffer requires a weak acid/base pair.

Step 5: Analyzing Option (d)

In option (d), we have equal volumes of 1 M NH₄Cl (an acidic salt) and 0.5 M HCl (a strong acid). The addition of a strong acid leaves no room for the formation of a basic component, hence not forming a buffer solution.

Step 6: Conclusion and Final Answer

Only option (a) produces a buffer solution as it creates a mixture of a weak acid (CH₃COOH) and its conjugate base (CH₃COO⁻) after partial neutralization with NaOH.

Key Concepts

Weak Acid

A weak acid is one that does not completely disassociate into its ions in a solution. Instead, it only partially ionizes, meaning only a small fraction of its molecules release hydrogen ions ( H^+ ). This characteristic is what makes weak acids distinct from strong acids, which fully disassociate.

Examples of weak acids include:

• Acetic acid ( CH_3COOH )
• Formic acid
• Citric acid

Knowing whether an acid is weak or strong is crucial when creating buffer solutions because weak acids have the property of maintaining equilibrium between the acid and its conjugate base.

This equilibrium allows the solution to resist drastic changes in pH when small amounts of acid or base are added.

Conjugate Base

The concept of a conjugate base is essential when we discuss weak acids. When a weak acid disassociates, it releases a hydrogen ion and transforms into its conjugate base. This conjugate base is basically the parent acid minus a proton.

In the case of acetic acid ( CH_3COOH ), when it loses a H^+ , it becomes acetate ( CH_3COO^- ), which is its conjugate base.

• Conjugate bases are not merely byproducts, but active participants that help buffer solutions function.
• They stabilize pH changes by reacting with added acid or base in a buffer system.

By understanding conjugate bases, we can infer how buffer solutions maintain their pH levels, as you'll see in our exploration of pH resistance below.

Neutralization

Neutralization is a chemical reaction in which an acid and a base react to form water and a salt. This reaction is key in creating buffer solutions, particularly when a weak acid and its conjugate base are involved.

In the problem's context, when acetic acid ( CH_3COOH ) is mixed with a strong base like NaOH , the base partly neutralizes the acid:

• NaOH converts some CH_3COOH into CH_3COO^- .
• This leaves a mixture of CH_3COOH and CH_3COO^- .

• The presence of both a weak acid and its conjugate base creates a buffering pair.

This pairing is key to a buffer's functionality, enabling it to resist rapid pH changes upon the addition of more acid or base.

pH Resistance

One of the standout qualities of a buffer solution is its ability to resist changes in pH. When you mix a weak acid with its conjugate base (or a weak base with its conjugate acid), they work together to neutralize small amounts of added acids or bases.

This is how:

• If more acid is added, the conjugate base ( CH_3COO^- ) reacts with the added H^+ ions, limiting the change in pH.
• Conversely, if a base is added, the weak acid ( CH_3COOH ) donates H^+ ions to neutralize the base.

Thus, buffer solutions are essential in processes where stable pH conditions are necessary, such as biological systems or chemical reactions needing precise conditions.

This characteristic is pivotal in many scientific and industrial applications that require a controlled environment.