Question

A buffer solution can be prepared from a mixture of: 1\. Sodium acetate and acetic acid in water. 2\. Sodium acetate and hydrochloric acid in water. 3\. Ammonia and ammonium chloride in water. 4\. Ammonia and sodium hydroxide in water. (a) \(1,3,4\) (b) \(2,3,4\) (c) \(1,2,4\) (d) \(1,2,3\)

Short answer

Correct pairs that form buffers are 1 and 3, but none provided answers correctly match this.

Step-by-step solution

Understanding Buffer Solutions

A buffer solution is a system that minimizes pH changes when small amounts of acid or base are added. It usually consists of a weak acid and its conjugate base, or a weak base and its conjugate acid.

Analyze First Option - Sodium Acetate and Acetic Acid

Sodium acetate (a salt) and acetic acid (a weak acid) make a buffer solution because acetic acid provides the weak acid, while acetate ions provide the conjugate base. So, option 1 is a valid buffer system.

Analyze Second Option - Sodium Acetate and Hydrochloric Acid

Sodium acetate and hydrochloric acid do not form a buffer since hydrochloric acid is a strong acid, not a weak acid. Therefore, this pair cannot resist pH changes effectively.

Analyze Third Option - Ammonia and Ammonium Chloride

Ammonia (a weak base) and ammonium chloride (provides the conjugate acid, ammonium ions) together form a buffer solution. So, option 3 is also a valid buffer system.

Analyze Fourth Option - Ammonia and Sodium Hydroxide

Ammonia and sodium hydroxide cannot form a buffer since sodium hydroxide is a strong base, rather than a weak acid or conjugate acid. This system cannot equilibrate the pH.

Determine Correct Answer Choice

From the analysis, the valid buffer systems are in options 1 and 3. Therefore, the correct answer is (a) \(1,3,4\). However, we found option 4 is incorrect, indicating a need to reconsider the sets of answers provided in the problem.

Key concepts

Weak Acid

A weak acid is a chemical substance that partially ionizes in water. Unlike strong acids, weak acids do not fully dissociate into ions in solutions, meaning they hold onto their hydrogen ions more tightly.
One classic example of a weak acid is acetic acid, commonly found in vinegar. In a solution, acetic acid does not fully break apart into hydrogen ions and acetate ions, making it stable and able to moderate changes in pH.
Weak acids are integral to forming buffer solutions. They work best in combination with their conjugate bases, as this duo can effectively neutralize small amounts of added acids or bases. By doing so, the solution's pH remains relatively stable and steady. Key Points about Weak Acids:

• Only partly ionize in solutions.
• Includes examples like acetic acid and citric acid.
• Fundamental to buffer solutions when paired with conjugate bases.

Conjugate Base

A conjugate base is what remains after a weak acid has donated its proton (H⁺ ion). When a weak acid dissociates, it leaves behind a conjugate base, a particle that can react with hydrogen ions.
A familiar example of a conjugate base is acetate, formed from acetic acid's ionization process. When acetic acid gives off a proton, acetate ions are left, ready to counter further changes in the solution’s pH.
In buffer solutions, the presence of a conjugate base alongside a weak acid is pivotal. Together, they can neutralize incoming strong bases or acids without significant changes to the solution's pH.
Essential Highlights about Conjugate Bases:

• Produced after weak acids lose a hydrogen ion.
• Examples include acetate from acetic acid, and bicarbonate from carbonic acid.
• Work in tandem with weak acids to maintain buffer solutions.

Weak Base

A weak base is a compound that only partially accepts hydrogen ions in a solution. These bases do not completely ionize or dissolve, making them weaker proton acceptors compared to strong bases.
Ammonia is a prime example of a weak base. In water, it partially gains hydrogen ions to form ammonium, but not all ammonia molecules are involved in this process, preserving its status as a weak base.
For buffer solutions, pairing a weak base with its conjugate acid is key. This combination is adept at regulating a solution's pH by neutralizing added acids or bases, providing a stable pH environment. Key Takeaways about Weak Bases:

• Partial ionization in solutions.
• Common examples include ammonia and methylamine.
• Serve as critical components in buffer solutions with their conjugate acids.

Conjugate Acid

A conjugate acid results when a weak base gains a proton. It is essential in processes that require maintaining equilibrium, especially in buffer solutions.
An excellent case is the ammonium ion, derived from ammonia when it uptakes a hydrogen ion. Conjugate acids in solutions can donate protons, making them able to counteract incoming bases that might otherwise shift the pH.
In buffer systems, a weak base and its conjugate acid work together. This duo allows the solution to resist drastic pH changes by neutralizing excess acids or bases and thereby sustaining a more constant pH environment. Key Insights into Conjugate Acids:

• Formed when weak bases acquire a hydrogen ion.
• Examples include ammonium from ammonia.
• Integral to the stability of buffer solutions alongside weak bases.