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

Question

An aqueous solution of \(\mathrm{lM} \mathrm{NaCl}\) and \(\mathrm{IM} \mathrm{HCl}\) is 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

The solution is not a buffer and the \\(\text{pH} < 7\\).

Step 1: Understanding Buffers

A buffer solution is a solution that resists changes in pH when small amounts of an acid or a base are added. Buffers typically consist of a weak acid and its conjugate base or a weak base and its conjugate acid.

Step 2: Composition of the Solution

The solution contains \( ext{IM NaCl}\) and \( ext{IM HCl}\). NaCl is a salt that dissociates into Na⁺ and Cl⁻ in solution, neither of which affect the pH significantly. HCl is a strong acid that dissociates completely into H⁺ and Cl⁻.

Step 3: Effect of HCl on the Solution

HCl, being a strong acid, releases a high concentration of H⁺ ions into the solution, which lowers the pH of the solution significantly below 7, as strong acids lower the pH.

Step 4: Buffer Characteristics Check

Since the solution contains a strong acid (HCl) and a neutral salt (NaCl), there is no weak acid or base along with its conjugate pair to serve as a buffer. Thus, it cannot be a buffer solution.

Key Concepts

Buffer Characteristics

A buffer solution is like a superhero for maintaining stable pH levels. It doesn't let the pH of a solution change much, even if you add a little bit of acid or base.
It achieves this stability using a powerful duo: usually a weak acid and its conjugate base, or a weak base and its conjugate acid. These components balance each other out, absorbing excess hydrogen ions (\(\text{H}^+\)) or hydroxide ions (\(\text{OH}^-\)) that are added.

Imagine you have a weak acid, like acetic acid, paired with its conjugate base, acetate. When you add an acid to the mixture, the excess \(\text{H}^+\) ions are neutralized by the acetate ions. Similarly, if a base is added, the acetic acid gives off hydrogen ions to neutralize the \(\text{OH}^-\) ions.
This dual-action enables the buffer to resist drastic pH changes. But remember, for a solution to exhibit buffer characteristics, it must contain these special weak components.

Strong Acid

When you think about strong acids, imagine them as the powerhouse of protons.
A strong acid, like hydrochloric acid (HCl), fully dissociates in water. This means it releases a large number of \(\text{H}^+\) ions into the solution in one solid swoop.

This overwhelming release leads to a significant lowering of pH, pushing it below 7.
Thus, solutions containing strong acids are known for their decisively acidic nature. In the case of strong acids, the concept of buffers does not easily apply.
Why? Because the acid doesn't form a significantly reversible reaction with its conjugate base to neutralize added substances.
Instead, the sheer number of \(\text{H}^+\) ions keeps the solution strongly acidic, without the balancing act characteristics needed for a buffer.
In short, strong acids lead the charge in lowering pH while quite defiantly opposing buffer properties.

Solution Composition

Understanding the composition of a solution gives us insight into its behavior and characteristics.
In the context of the solution with 1M \(\text{NaCl}\) and 1M \(\text{HCl}\), the focus is to pinpoint contributors to its pH.
NaCl dissociates into \(\text{Na}^+\) and \(\text{Cl}^-\) ions. Since these ions do not affect the pH directly, NaCl doesn't play a role in buffering.

HCl, however, is a game-changer.
Being a strong acid, it enhances the acidic nature of the solution by churning out copious \(\text{H}^+\) ions. The presence of a strong acid like HCl and a neutral component like NaCl form a solution with a prominent acidic character without a buffer mechanism in place.
For a solution to function as a buffer, you would typically need a combination of a weak acid with its conjugate base or a weak base with its conjugate acid.
Thus, the solution in question fails to act as a buffer, but it distinctly lowers the pH, making it more acidic.