Problem 21 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following can act like a buffer? (a) \(\mathrm{CH}_{3} \mathrm{COOH}+\mathrm{CH}_{3} \mathrm{COONa}\) (b) \(\mathrm{NH}_{4} \mathrm{ND}_{3}+\mathrm{NH}_{4} \mathrm{OH}\) (c) \(50 \mathrm{ml} 0.1 \mathrm{~m} \mathrm{NaCN}+30 \mathrm{ml} 0.1 \mathrm{~m} \mathrm{HCl}\) (d) All of these

Step-by-Step Solution

Answer

Option (a) \(\mathrm{CH}_{3} \mathrm{COOH}+\mathrm{CH}_{3} \mathrm{COONa}\) can act as a buffer.

Step 1: Understanding Buffers

A buffer is a solution that can resist pH changes upon the addition of an acid or a base. Typically, it consists of a weak acid and its conjugate base or a weak base and its conjugate acid.

Step 2: Evaluating Option (a)

Option (a) consists of acetic acid (\(\mathrm{CH}_{3} \mathrm{COOH}\)), which is a weak acid, and sodium acetate (\(\mathrm{CH}_{3} \mathrm{COONa}\)), its conjugate base. This combination forms a buffer solution.

Step 3: Evaluating Option (b)

Option (b) consists of ammonium deuteride (\(\mathrm{NH}_{4} \mathrm{ND}_{3}\)) and ammonium hydroxide (\(\mathrm{NH}_{4} \mathrm{OH}\)), which are both basic solutions but do not form a conventional buffer pair as they lack a suitable weak acid-conjugate base pair.

Step 4: Evaluating Option (c)

Option (c) combines sodium cyanide (\(\mathrm{NaCN}\)) and hydrochloric acid (\(\mathrm{HCl}\)). While \(\mathrm{NaCN}\) can form a weak acid, \(\mathrm{HCN}\), the presence of a strong acid \(\mathrm{HCl}\) overwhelms the buffer characteristics. Therefore, this does not act as a buffer.

Step 5: Making a Conclusion

Based on the evaluations, only option (a) contains a suitable weak acid and its conjugate base, fulfilling the criteria for a buffer solution.

Key Concepts

Weak Acid and Conjugate Base

Buffer solutions are fascinating because they can maintain a consistent pH even when small amounts of acid or base are added. These solutions usually consist of a weak acid and its conjugate base. A weak acid, unlike strong acids, doesn't fully dissociate in water. This means it only partially breaks into ions, making it ideal for creating a buffer solution. The conjugate base, formed when the weak acid loses a proton, pairs with the weak acid to help "catch" added acids or bases. This buffering action occurs due to the equilibrium established between the weak acid and its conjugate base. When more acid is added, the conjugate base can neutralize some of the excess protons. Likewise, when a base is added, the weak acid can donate extra protons to neutralize it. This dance between the weak acid and its conjugate base is key in maintaining the solution's pH stability.

pH Resistance

One of the key characteristics of buffer solutions is their ability to resist changes in pH. This resistance is crucial in many biological and chemical systems where a stable pH is needed. When a small amount of acid or base is introduced into the solution, the buffer acts by either absorbing or releasing protons, counteracting major changes in pH. This buffering capability is dependent on the presence of both a weak acid and its conjugate base. The combination creates an equilibrium that can shift to accommodate added acids or bases. Essentially, the weak acid will neutralize added bases and the conjugate base will neutralize added acids, maintaining the pH within a narrow range. This feature is especially important in biological systems where a stable pH is necessary for enzymes and other biochemical processes to function properly.

Acetic Acid and Sodium Acetate

Acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)) and sodium acetate (\(\mathrm{CH}_3\mathrm{COONa}\)) are a classic example of a buffer solution. Acetic acid acts as the weak acid, while sodium acetate serves as the conjugate base. When dissolved in water, acetic acid partially dissociates to form acetate ions and hydrogen ions. Similarly, sodium acetate dissociates to provide more acetate ions in the solution.

• When an acid is added to the solution, the extra hydrogen ions are neutralized by the acetate ions, minimizing the overall change in pH.
• Conversely, when a base is added, the additional hydroxide ions are neutralized by the acetic acid, again minimizing pH changes.

This delicate balance allows the system to maintain its pH stability, demonstrating why the acetic acid-sodium acetate pair is often used in laboratory buffers. It effectively protects solutions from drastic pH shifts, making it a reliable and useful example of how buffer solutions work.