Problem 8 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following pairs constitutes buffer solutions? (a) \(\mathrm{HNO}_{3}\) and \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) (b) \(\mathrm{HCl}\) and \(\mathrm{KCl}\) (c) \(\mathrm{HNO}_{2}\) and \(\mathrm{NaNO}_{2}\) (d) \(\mathrm{NaOH}\) and \(\mathrm{NaCl}\)

Step-by-Step Solution

Answer

Pair (c) \(\mathrm{HNO}_{2}\) and \(\mathrm{NaNO}_{2}\) is the buffer solution.

Step 1: Understand what a buffer solution is

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

Step 2: Analyze Pair (a)

Pair (a) consists of \(\mathrm{HNO}_{3}\) (a strong acid) and \(\mathrm{NH}_{4}\mathrm{NO}_{3}\) (a salt of a weak base). Since there is a strong acid involved, this pair does not constitute a buffer solution.

Step 3: Analyze Pair (b)

Pair (b) consists of \(\mathrm{HCl}\) (a strong acid) and \(\mathrm{KCl}\) (a neutral salt). Since both components are derived from strong species, this pair cannot form a buffer solution.

Step 4: Analyze Pair (c)

Pair (c) consists of \(\mathrm{HNO}_{2}\) (a weak acid) and \(\mathrm{NaNO}_{2}\) (the salt of its conjugate base). This pair is able to constitute a buffer solution because it meets the condition of having a weak acid and its conjugate base.

Step 5: Analyze Pair (d)

Pair (d) consists of \(\mathrm{NaOH}\) (a strong base) and \(\mathrm{NaCl}\) (a neutral salt). This pair does not constitute a buffer solution as it lacks the presence of a weak acid or base and its conjugate counterpart.

Key Concepts

Weak Acid

A weak acid is a type of acid that only partially dissociates in water. This means it does not completely break down into ions in a solution. Knowing this is crucial for understanding buffer solutions. Weak acids are less aggressive in releasing hydrogen ions compared to strong acids.
Some examples of weak acids include acetic acid (\( ext{CH}_3 ext{COOH}\)) and nitrous acid (\( ext{HNO}_2\)). These acids do not have a high tendency to lose the hydrogen ion, making them critical in forming buffer systems.
When a weak acid is mixed with its conjugate base, it can help maintain pH balance by neutralizing added acids or bases. This ability to mitigate pH changes lies at the core of what makes buffer solutions extremely useful, especially in biological systems. The weak acid component of a buffer absorbs excess hydroxide ions.

Conjugate Base

A conjugate base is the species that remains after a weak acid donates a hydrogen ion (H\(^+\)). It plays a crucial role in forming a buffer solution.
For example, with nitrous acid (\( ext{HNO}_2\)), its conjugate base is nitrite (\( ext{NO}_2^-\)). When a buffer solution is formed, the weak acid and its conjugate base stabilize pH by adjusting to changes.
When an acid is added to the solution, the conjugate base will neutralize the excess hydrogen ions. Conversely, if a base is added, the weak acid in the buffer will react to neutralize the excess. By working together, the weak acid and its conjugate maintain the solution's pH within a narrow range.

pH Resistance

One of the most important features of a buffer solution is its ability to resist changes in pH. This resistance is due to the interplay between the weak acid and its conjugate base. In practical terms, when you add a small amount of acid to a buffer solution, the conjugate base will "absorb" these additional protons, minimizing any change in pH. Similarly, if a small amount of base is added, the weak acid will donate protons to counter the increase in pH.
The concept of pH resistance is essential in many fields, such as biochemical processes and industrial applications. For instance, human blood has a buffering system to maintain a stable pH, which is vital for physiological functions. The effectiveness of a buffer solution depends on the concentrations of the weak acid and its conjugate base, alongside the inherent strength of the weak acid. Thus, a well-prepared buffer solution can keep the pH of a solution relatively stable, ensuring optimal conditions for various chemical reactions.