Problem 68 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Aqueous solutions of \(\mathrm{HNO}_{3}, \mathrm{KOH}, \mathrm{CH}_{3} \mathrm{COOH}\) and \(\mathrm{CH}_{3} \mathrm{COONa}\) of identical concentrations are provided. The pair(s) of solutions which form a buffer upon mixing is(are) (a) \(\mathrm{HNO}_{3}\) and \(\mathrm{CH}_{3} \mathrm{COOH}\) (b) \(\mathrm{KOH}\) and \(\mathrm{CH}_{3} \mathrm{COONa}\) (c) \(\mathrm{HNO}_{3}\) and \(\mathrm{CH}_{3} \mathrm{COONa}\) (d) \(\mathrm{CH}_{3} \mathrm{COOH}\) and \(\mathrm{CH}_{3} \mathrm{COONa}\)

Step-by-Step Solution

Answer

(d) \(\mathrm{CH}_3 \mathrm{COOH}\) and \(\mathrm{CH}_3 \mathrm{COONa}\) form a buffer.

Step 1: Understanding Buffers

Buffers are solutions that resist changes in pH upon the addition of small amounts of acid or base. Typically, a buffer solution consists of a weak acid and its conjugate base, or a weak base and its conjugate acid.

Step 2: Identify the Components

We need to identify which of the given solutions can form a weak acid and its conjugate base pair. The solutions provided are: \( \mathrm{HNO}_3 \), \( \mathrm{KOH} \), \( \mathrm{CH}_3 \mathrm{COOH} \), and \( \mathrm{CH}_3 \mathrm{COONa} \).

Step 3: Analyze Each Pair

- Pair (a): \( \mathrm{HNO}_3 \) (strong acid) and \( \mathrm{CH}_3 \mathrm{COOH} \) (weak acid) cannot form a buffer.- Pair (b): \( \mathrm{KOH} \) (strong base) and \( \mathrm{CH}_3 \mathrm{COONa} \) (salt containing the conjugate base of a weak acid).- Pair (c): \( \mathrm{HNO}_3 \) and \( \mathrm{CH}_3 \mathrm{COONa} \) cannot form a buffer.- Pair (d): \( \mathrm{CH}_3 \mathrm{COOH} \) (weak acid) and \( \mathrm{CH}_3 \mathrm{COONa} \) (conjugate base of the weak acid) can form a buffer.

Step 4: Verify the Suitable Pair

In a buffer solution like pair (d), the weak acid \( \mathrm{CH}_3 \mathrm{COOH} \) and its conjugate base \( \mathrm{CH}_3 \mathrm{COONa} \) combine to resist changes in pH when an acid or base is added. They meet the criteria needed for buffer formation.

Step 5: Finalize the Answer

After considering all pairs, the only correct pair that forms a buffer is (d) \(\mathrm{CH}_3 \mathrm{COOH}\) and \(\mathrm{CH}_3 \mathrm{COONa}\).

Key Concepts

Weak Acid

A weak acid is an acid that does not completely dissociate in solution. This means only a small fraction of the weak acid's molecules will donate a hydrogen ion (H⁺) into the solution. Unlike strong acids, which dissociate completely, weak acids only partially ionize. This partial ionization is key when forming a buffer solution.
For example:- Acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)) is a classic example of a weak acid.- In water, it partially dissociates into \(\mathrm{CH}_3\mathrm{COO}⁻\) (acetate ion) and \(\mathrm{H}⁺\).- The weak bond holding the hydrogen ion allows it to easily re-associate with the acetate ion.UlWeak acids are essential in the formation of buffer solutions as they provide a reserve of \(\mathrm{H}⁺\) ions that can react with added bases, thereby helping to keep the pH stable.

Conjugate Base

The conjugate base forms when a weak acid donates a hydrogen ion (\(\mathrm{H}⁺\)). It is what remains after the acid loses its proton and can behave as a base by accepting a \(\mathrm{H}⁺\) ion. When combined with its weak acid counterpart, it forms a buffer solution capable of maintaining a stable pH.
Consider the example of acetic acid and its conjugate base:- The conjugate base of acetic acid is the acetate ion (\(\mathrm{CH}_3\mathrm{COO}⁻\)).- When \(\mathrm{H}⁺\) ions are added to a solution containing acetate ions, these ions can bond to form acetic acid, minimizing changes in pH.

This capacity to neutralize additional hydrogen ions is why the conjugate base is crucial in buffer systems. It allows the system to resist acidic swings by re-forming the weak acid, maintaining equilibrium.

pH Resistance

pH resistance is the ability of a solution to resist drastic changes in pH when small amounts of acids or bases are added. This characteristic is what defines a buffer. Buffers are made from mixtures of weak acids and their conjugate bases (or weak bases and their conjugate acids). The concept revolves around the equation of equilibrium: \[ \mathrm{HA} \rightleftharpoons \mathrm{H}^+ + \mathrm{A}^- \] where \(\mathrm{HA}\) is the weak acid and \(\mathrm{A}^-\) is its conjugate base.
The balance between the weak acid and conjugate base ensures that:

• Any added \(\mathrm{H}⁺\) ions will be "buffered" by the conjugate base, \(\mathrm{A}^-\), reforming \(\mathrm{HA}\).
• Any added \(\mathrm{OH}⁻\) ions will react with the \(\mathrm{H}⁺\) ions to form water, pulling the equilibrium to release more \(\mathrm{H}⁺\) from \(\mathrm{HA}\).

This equilibrating action prevents significant shifts in pH, which vital in many chemical and biological processes that require stable conditions.