Problem 261 - A Complete Resource Book in Chemistry for JEE Main

Question

An aqueous solution of \(1 \mathrm{M} \mathrm{NaCl}\) and \(\mathrm{IM} \mathrm{HCl}\) is [2002] (a) not a buffer but \(\mathrm{pH}<7\) (b) not a buffer but \(\mathrm{pH}>7\) (c) a buffer with \(\mathrm{pH}<7\) (d) a buffer with \(\mathrm{pH}>7\)

Step-by-Step Solution

Answer

(a) not a buffer but \( \mathrm{pH}<7 \).

Step 1: Understanding the Components

Identify the components in the solution: NaCl and HCl. NaCl is a neutral salt and does not affect the pH of the solution. HCl is a strong acid, which dissociates completely, contributing H⁺ ions.

Step 2: Identifying Buffer Conditions

A buffer solution contains a weak acid and its conjugate base, or a weak base and its conjugate acid. Here, HCl is a strong acid and there is no conjugate base present in significant amounts from a weak acid to form a buffer.

Step 3: Predicting the pH of the Solution

Because HCl is a strong acid, the solution will have an excess of H⁺ ions. Therefore, the solution is acidic, and the pH will be less than 7.

Step 4: Final Analysis

Since the solution is neither a mixture of a weak acid and its conjugate base nor a weak base and its conjugate acid, it is not a buffer. Given that it contains a strong acid, the pH is less than 7.

Key Concepts

Buffer Solutions

A buffer solution is a special type of solution that resists changes in its pH. It's a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid.
These solutions are crucial, especially in maintaining the pH necessary for various biological and chemical processes.

• A buffer solution can maintain its pH even when a small amount of strong acid or strong base is added.
• In contrast, solutions without these buffering components can undergo significant pH changes.
• Buffer solutions are typically made by partially neutralizing a weak acid with a strong base, or vice versa.

When thinking about a solution’s ability to act as a buffer, it’s key to identify if both a weak acid/base and its conjugate are present in sufficient amounts.
This is because they work together to neutralize additional acids or bases.

Strong Acids

Strong acids are chemicals that dissociate completely in water, releasing protons (H⁺ ions) and increasing the hydrogen ion concentration of the solution.
This complete dissociation makes them very effective at lowering the pH of a solution.

• Common examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃).
• Unlike weak acids, which only partially dissociate, strong acids fully release their protons when dissolved.

In the context of the original exercise, the presence of strong acid like HCl means the solution's pH will be significantly below 7, indicating that the solution is acidic.
This is because there is an abundance of H⁺ ions, which are responsible for the acidic property.

Conjugate Base

The conjugate base of an acid is what remains after the acid has donated a proton. In other words, it's the substance formed when an acid loses an H⁺ ion.
An essential aspect of buffer solutions is the presence of sufficient quantities of both the weak acid and its conjugate base.

• For example, the conjugate base of acetic acid (CH₃COOH) is acetate (CH₃COO⁻).
• A conjugate base is typically a weaker base than the strong bases found in solution.

In the context of the question discussed, the lack of a conjugate base from a weak acid, alongside the strong acid HCl, indicates the solution can’t function as a buffer.
This is because there is no equilibrium established between a weak acid and its conjugate base to adjust pH changes.