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

Question

An aqueous solution of IM \(\mathrm{NaCl}\) and \(1 \mathrm{M} \mathrm{HCl}\) 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 correct answer is (a) not a buffer but \(\text{pH} < 7\).

Step 1: Define Buffer Solution

A buffer solution is composed of a weak acid and its conjugate base or a weak base and its conjugate acid. This composition allows the solution to resist significant changes in pH when small amounts of acid or alkali are added.

Step 2: Analyze the Given Solution Composition

The given solution contains 1M NaCl and 1M HCl. Here, NaCl is a salt that completely dissociates in water and does not affect the pH. HCl is a strong acid that also completely dissociates, contributing significantly to the pH of the solution.

Step 3: Determine Buffer Characteristics

Buffers require the presence of a weak acid/base pair. Since HCl is a strong acid and NaCl does not contribute a weak base, the solution cannot act as a buffer. Therefore, the system is not a buffer solution.

Step 4: Assess the pH Level

Since the solution has 1M HCl, which is a strong acid, it will have a pH less than 7. Strong acids dissociate fully to provide higher concentrations of hydrogen ions \((H^+)\), thus resulting in a lower pH.

Step 5: Evaluate and Choose the Correct Option

Based on the analysis, the solution is not a buffer as it lacks the components required for buffering. Additionally, due to the presence of a strong acid (HCl), the pH of the solution is less than 7. Therefore, the correct answer is (a) not a buffer but \(\text{pH} < 7\).

Key Concepts

Strong Acids

Strong acids are a fascinating subject in the world of chemistry. They are known for their complete dissociation in water, meaning they release all their hydrogen ions \((H^+)\) into the solution. This process results in a significant increase in the concentration of hydrogen ions, which directly influences the pH of the solution. Well-known examples of strong acids include Hydrochloric Acid (HCl), Sulfuric Acid (H₂SO₄), and Nitric Acid (HNO₃).

• Complete Ionization: Strong acids ionize entirely in aqueous solutions, unlike weak acids which only partially ionize.
• Influence on pH: The complete dissociation of strong acids leads to high concentrations of hydrogen ions, making the solution very acidic.
• Reactive Nature: Due to their high concentration of hydrogen ions, strong acids are very reactive and can participate in numerous chemical reactions.

In the context of the exercise, our strong acid is HCl, which ensures the solution maintains a low pH.

pH Level

The pH level is a measure of how acidic or basic a solution is. It ranges from 0 to 14, with 7 being neutral. Any solution with a pH less than 7 is considered acidic, while one with a pH greater than 7 is basic. The pH scale is logarithmic, meaning each whole number change represents a tenfold change in hydrogen ion concentration.

• Understanding the Scale: Solutions with more \((H^+)\) ions have a lower pH, indicating increased acidity.
• Logarithmic Nature: A solution with a pH of 4 is ten times more acidic than one with a pH of 5.
• Neutral Point: Pure water is considered neutral with a pH of 7 because its \((H^+)\) and hydroxide (\((OH^-\)) concentrations are equal.

In the exercise, the presence of strong hydrochloric acid (HCl) ensures that the pH level of the solution is below 7, making it acidic.

Characteristics of Buffers

Buffers play a crucial role in maintaining stable pH levels in various solutions, which is especially important in biological systems and chemical applications. A buffer solution is typically composed of a pair of weak acid and its conjugate base or weak base and its conjugate acid. These solutions are specifically designed to resist changes in pH when small amounts of acid or base are introduced.

• Constituents: Buffers contain either a weak acid and its salt or a weak base and its salt.
• pH Stability: They absorb excess \((H^+)\) or \((OH^-\)) ions, preventing drastic changes in pH.
• Applications: Buffers are critical in biological systems, such as maintaining blood pH, and in industrial processes where controlled pH is crucial.

In the provided exercise, the solution does not qualify as a buffer because it contains a strong acid (HCl) without the necessary weak acid or base components.