Question

A acidic buffer solution can be prepared by mixing the solutions of: (a) Sodium chloride and sodium hydroxide (b) Sulphuric acid and sodium sulphate (c) Ammonium chloride and ammonium hydroxide (d) Ammonium acetate and acetic acid

Short answer

Option (d): Ammonium acetate and acetic acid form an acidic buffer solution.

Step-by-step solution

Understand Buffer Solution Components

An acidic buffer solution is typically made by mixing a weak acid with its salt containing a strong base. The weak acid and its salt maintain a relative pH near the weak acid's pKa.

Evaluate Option (a)

Sodium chloride and sodium hydroxide are neither a weak acid nor a salt derived of a weak acid. Sodium hydroxide is a strong base, not suitable for an acidic buffer. Therefore, this option cannot form an acidic buffer.

Evaluate Option (b)

Sulphuric acid is a strong acid, and sodium sulphate is a salt from a strong acid. An acidic buffer typically requires a weak acid, so this combination does not suit the criteria for an acidic buffer solution.

Evaluate Option (c)

Ammonium chloride is a salt of a weak base (NH4OH) and ammonium hydroxide is a weak base, thus not fitting the typical composition of an acidic buffer where a weak acid is needed.

Evaluate Option (d)

Ammonium acetate and acetic acid are suitable for forming an acidic buffer. Acetic acid is a weak acid, and ammonium acetate acts as a salt derived from it, allowing for the maintenance of an acidic pH.

Key concepts

Weak Acid

A weak acid is an acid that partially dissociates in solution, meaning it does not completely release all of its hydrogen ions. This partial dissociation is what gives a weak acid its unique properties. Unlike strong acids, which dissociate fully and produce a high concentration of hydrogen ions, weak acids maintain a balance between the undissociated acid and the hydrogen ions. This balance is crucial for forming buffer solutions.
A weak acid is characterized by a higher pKa value compared to strong acids. The pKa is a measure of the acidity of the substance, indicating how easily the acid donates its protons to the solution. The higher the pKa, the weaker the acid. Examples of weak acids include acetic acid, citric acid, and formic acid.
Understanding weak acids is important when studying buffer solutions because these acids are the main component that, along with their salt counterpart, helps resist changes in pH when small amounts of strong acids or bases are added.

Salt of Weak Acid

The salt of a weak acid is typically formed by the reaction between the weak acid and a strong base. This reaction results in a salt that plays an essential role in buffer solutions. A salt, in this context, means an ionic compound composed of the cation from the base and the anion from the acid.
For example, in the case of acetic acid, when it reacts with sodium hydroxide, it forms sodium acetate. Sodium acetate is the salt of the weak acid acetic acid, and it completely dissociates in solution, providing the acetate ions that are necessary for the buffer action.

• The salt pairs with the weak acid to create what we call a 'buffer pair', which works together to neutralize added acids or bases.
• Unlike the weak acid, the salt exists in almost completely dissociated form, making the pH adjusting process more efficient.

Therefore, in a buffer solution, both the weak acid and its salt are needed. The salt maintains the equilibrium by adjusting the pH and minimizing the effect of external changes.

Acetic Acid

Acetic acid is a common example of a weak acid used in buffer solutions. It is characterized by its formula, CH₃COOH, and is known for its vinegar-like smell. Acetic acid only partially dissociates in aqueous solutions, making it an essential buffer component.
The dissociation of acetic acid in water is represented by the equation: \[ ext{CH}_3 ext{COOH} ightleftharpoons ext{CH}_3 ext{COO}^- + ext{H}^+ \] The acetate ion \(( ext{CH}_3 ext{COO}^-)\) produced, plays a critical role in buffering capacity. In buffer solutions, acetic acid absorbs the added hydroxide ions, while acetate ions neutralize added hydrogen ions.

• Acetic acid is a perfect choice for preparing acidic buffer solutions, given its reliable pKa value, which is about 4.76. This makes it suitable for maintaining a consistent pH in many biological and chemical processes.
• In industrial and laboratory environments, acetic acid is often mixed with its sodium salt, sodium acetate, to form a buffer that can uphold the pH around its pKa value, showcasing the fundamental properties of buffer systems.

Buffer Components

Buffer solutions are special mixtures that resist changes in pH, even with the addition of acids or bases. The main components of an acidic buffer solution include a weak acid and its corresponding salt derived from the acid.
Here are the key components and characteristics of an effective acidic buffer solution:

• **Weak Acid**: This component partially dissociates in the solution, providing hydrogen ions. Its presence helps in absorbing excess hydroxide ions when bases are added to the solution.
• **Salt of the Weak Acid**: This component is fully dissociated in the solution, offering cations and anions that counteract the addition of acids or bases. It contributes to maintaining a consistent pH by reacting with added acids or bases.

In a typical buffer scenario like acetic acid and its salt (sodium acetate),

• The acetic acid provides the necessary protons for neutralizing extra bases.
• Sodium acetate supplies acetate ions that react with added hydrogen ions, neutralizing the impact.

A buffer's ability to maintain pH makes it highly valuable in biological systems and chemical applications where pH stability is essential.