Question

Which one of the pairs will form a buffer solution?

• A. $\text{CH}_3\text{COONa}$ & $\text{NaOH}$
• B. $\text{CH}_3\text{COONH}_4$ & $\text{NH}_4\text{Cl}$
• C. $\text{NH}_4\text{Cl}$ & $\text{NH}_4\text{OH}$
• D. $\text{CH}_3\text{COONa}$ & $\text{HCl}$

Hint:

To quickly identify a buffer solution:
- Look for a Weak Acid + its conjugate salt (Acidic Buffer).
- Look for a Weak Base + its conjugate salt (Basic Buffer).
Pairs containing strong acids or bases alone (like $\text{NaOH}$) do not form buffers unless they are part of a partial neutralization reaction where the weak component remains in excess.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
A buffer solution is a chemical system designed to maintain a nearly constant pH even when small amounts of a strong acid or base are added to it.
The mechanics of a buffer rely on the presence of a "conjugate acid-base pair".
This pair consists of a weak species (either a weak acid or a weak base) and its corresponding salt.
There are two primary types of buffers:
1. Acidic Buffer: Composed of a weak acid and its salt with a strong base (e.g., Acetic acid and Sodium acetate).
2. Basic Buffer: Composed of a weak base and its salt with a strong acid (e.g., Ammonium hydroxide and Ammonium chloride).
The buffer works because the weak base neutralizes added \( H^+ \) ions, while the conjugate acid (from the salt) neutralizes added \( OH^- \) ions.
Step 2: Detailed Explanation:
Let's evaluate each option provided in the question:
- Option (A): \( CH_3COONa \) & \( NaOH \)
\( CH_3COONa \) is a salt of a weak acid. \( NaOH \) is a strong base. Since there is no weak acid or weak base component here (only a salt and a strong base), this solution cannot neutralize added base effectively and will result in a very high pH. It is not a buffer.
- Option (B): \( CH_3COONH_4 \) & \( NH_4Cl \)
This is a mixture of two different salts. While both contain the ammonium ion, there is no neutral weak base like \( NH_3 \) or \( NH_4OH \) present in sufficient quantities to act as a buffer according to standard definitions.
- Option (C): \( NH_4Cl \) & \( NH_4OH \)
\( NH_4OH \) (Ammonium hydroxide) is a well-known weak base. \( NH_4Cl \) (Ammonium chloride) is its salt formed with a strong acid (\( HCl \)). This mixture contains a weak base and its conjugate acid (\( NH_4^+ \)). This is the perfect example of a Basic Buffer. This pair will effectively resist pH changes.
- Option (D): \( CH_3COONa \) & \( HCl \)
This involves a salt and a strong acid. While they might react to form some weak acid (\( CH_3COOH \)), they only form a buffer if the salt is in significant excess such that some salt remains after all the \( HCl \) is consumed. Without these specific concentration details, this pair is not classified as a standard buffer mixture.
Based on the direct chemical definitions, option (C) is the correct choice.
Step 3: Final Answer:
The pair that will form a buffer solution is \( NH_4Cl \) & \( NH_4OH \).