Problem 34 - A Complete Resource Book in Chemistry for JEE Main

Question

A acidic buffer solution can be prepared by mixing the solutions of (a) sodium chloride and sodium hydroxide (b) sulphuric acid and sodium sulphate (c) ammonium chloride and ammonium hydroxide (d) ammonium acetate and acetic acid

Step-by-Step Solution

Answer

The correct answer is (d) ammonium acetate and acetic acid.

Step 1: Understand the Nature of a Buffer

A buffer solution resists changes in pH when small amounts of acid or base are added. An acidic buffer typically consists of a weak acid and its conjugate base.

Step 2: Examine Each Option for Buffer Components

Evaluate each option given in the problem: (a) Sodium chloride and sodium hydroxide consist of strong base and neutral salt, not forming an acidic buffer. (b) Sulphuric acid and sodium sulphate consist of strong acid and neutral salt, not forming an acidic buffer. (c) Ammonium chloride and ammonium hydroxide consist of a weak base and its conjugate acid, forming a basic buffer. (d) Ammonium acetate and acetic acid consist of a weak acid (acetic acid) and its salt (ammonium acetate), forming an acidic buffer.

Step 3: Choose the Correct Option

Since a weak acid and its salt is required to form an acidic buffer, option (d), which consists of ammonium acetate and acetic acid, fits this criterion.

Key Concepts

Weak Acids

Weak acids are an essential component when creating buffer solutions, especially acidic buffers. These acids only partially dissociate in water. This means they do not release all of their hydrogen ions into the solution. As a result, the concentration of hydrogen ions remains relatively low, which in turn keeps the solution from being overly acidic.

When thinking about weak acids, imagine substances like acetic acid (found in vinegar) which partially disassociate into acetate ions and hydrogen ions in water. This partial dissociation is crucial in maintaining a steady pH even when small amounts of a strong acid or base are added. In these systems:

• The presence of undissociated weak acid molecules provides a reservoir of protons (H⁺ ions).
• These protons can be released again if necessary, helping to counteract any sudden influx of hydroxide ions (OH⁻) or additional acids.

Thus, weak acids play a pivotal role by staying mostly undissociated and ensuring just the right amount of hydrogen ions for stable pH conditions.

Conjugate Base

A conjugate base is the counterpart of a weak acid in an acidic buffer solution. It forms when the weak acid donates a proton (H⁺). The conjugate base is essential in maintaining the balance of the buffer solution. For instance, when discussing acetic acid, its conjugate base is acetate ion \(\text{CH}_3\text{COO}^{−}\).

The presence of conjugate bases allows buffers to neutralize added acids. They react with the extra H⁺ ions to limit any drastic changes in pH.

• For example, when more acid is added to the solution, the conjugate base will react with the additional H⁺ ions, forming more of the weak acid.
• This reaction reduces the number of free hydrogen ions in the solution, which helps to maintain a constant pH.

The constant interplay between weak acids and their conjugate bases allows buffers to perform their function of resisting pH changes effectively.

pH Stability

The main purpose of a buffer solution is pH stability. It helps maintain a relatively constant pH even when small amounts of a strong acid or base are introduced. Buffer solutions achieve this through their unique composition of weak acid and conjugate base pairs or weak base and conjugate acid pairs.

When discussing pH stability, it's important to note the following:

• The weak acid and its conjugate base work together to neutralize additions to the system.
• If a strong acid is added, it is neutralized by the conjugate base. This action prevents the system's pH from falling too drastically.
• Similarly, if a base is added, the weak acid donates a proton to form water, neutralizing the base's effect.

This dynamic balance between the acid and base components allows the buffer solution to resist sudden pH changes, thus maintaining pH stability within a narrow range. This property is crucial for many biological and chemical processes where a stable pH is necessary for optimal functioning.