Problem 198 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following solutions will have no effect on pH on dilution? (a) \(0.1 \mathrm{M} \mathrm{CH}_{3} \mathrm{COONa}\) (b) \(1 \mathrm{M} \mathrm{CH}_{3} \mathrm{COONH}_{4}\) (c) \(0.1 \mathrm{M} \mathrm{NH}_{4} \mathrm{OH}+0.1 \mathrm{M} \mathrm{NH}_{4} \mathrm{Cl}\) (d) \(0.5 \mathrm{M} \mathrm{H}_{2} \mathrm{CO}_{3}+0.5 \mathrm{M} \mathrm{NaHCO}_{3}\)

Step-by-Step Solution

Answer

Option (c) (0.1 M NH₄OH + 0.1 M NH₄Cl) resists pH changes upon dilution.

Step 1: Identify Buffer Solutions

A buffer solution is a mixture that resists changes in pH when an acid or base is added. It usually consists of a weak acid and its conjugate base or a weak base and its conjugate acid. Upon dilution, the ratio of the concentrations of the acid and base components in a buffer remains constant, so the pH of the solution does not change.

Step 2: Analyze Option (a)

Option (a) is a solution of sodium acetate, which is a salt of a weak acid (acetic acid) and a strong base (sodium hydroxide). This is not a buffer solution on its own, so its pH may change upon dilution.

Step 3: Analyze Option (b)

Option (b) is a solution of ammonium acetate. This compound consists of a salt formed from a weak acid (acetic acid) and a weak base (ammonia). Hence, it acts as a buffer because it can moderate changes in pH upon dilution.

Step 4: Analyze Option (c)

Option (c) is a mixture of ammonium hydroxide (a weak base) and ammonium chloride (the salt of ammonia). This forms a buffer solution that resists changes in pH upon dilution.

Step 5: Analyze Option (d)

Option (d) consists of carbonic acid and sodium bicarbonate, which together form a buffer system. This mixture can also resist changes in pH upon dilution.

Step 6: Determine Which Solution Has No Effect on pH Upon Dilution

Based on the analysis, both options (b), (c), and (d) are buffers. However, option (c) is the typical example of a buffer containing a weak base and its salt, providing a more straightforward buffer example without additional complexities, showcasing no effect on pH upon dilution.

Key Concepts

Weak acid and conjugate base

A buffer solution often consists of a weak acid and its conjugate base. Imagine a weak acid, such as acetic acid (\[ \text{CH}_3\text{COOH} \],which only partially dissociates in solution. Its conjugate base, acetate ion (\[ \text{CH}_3\text{COO}^- \],is present in the solution and able to accept hydrogen ions. When an external acid or base is added to this solution, the dynamic equilibrium between the weak acid and its conjugate base shifts, minimizing changes in pH.

• Weak Acid Side: Due to the limited ionization, weak acids contribute fewer hydrogen ions causing the equilibrium to maintain the pH.
• Conjugate Base Role: The presence of the conjugate base counters any added hydrogen ions by reacting with them to form the original weak acid.

This combination is the basic building block of many buffer systems, which helps maintain a stable pH upon dilution or when small amounts of strong acids or bases are introduced.

Weak base and conjugate acid

Buffers can also be created using a weak base and its conjugate acid. Let's take ammonium hydroxide (\[ \text{NH}_4\text{OH} \],a weak base that partially dissociates in water. The solution also contains its conjugate acid, the ammonium ion (\[ \text{NH}_4^+ \].

• Weak Base Side: The weak base does not fully ionize, providing a limited source of hydroxide ions and maintaining equilibrium.
• Conjugate Acid Role: If additional acid is added, the ammonium ion helps absorb excess hydrogen ions, reducing potential pH changes.

In this system, when you dilute the solution, the ratio between the weak base and its conjugate acid stays consistent. This aspect of buffers allows them to effectively stabilize the pH, a critical property for biological systems and various chemical processes.

pH change upon dilution

Typically, when a solution is diluted, the concentrations of all solutes decrease, which can lead to a pH shift. However, buffer solutions are an exception. The stability in the pH of buffer solutions, even upon dilution, is due to the constant ratio of the concentrations of the acid and base components. Let's explore why this is the case: 1. **Ratio Consistency**: Upon adding water, the concentrations of the weak acid/base and its conjugate in a buffer both decrease equally. Hence, their ratio remains the same. 2. **Le Chatelier's Principle**: This principle explains that equilibrium systems will adjust concentrations to counteract changes. Thus, even as dilution occurs, the buffer minimizes the impact on pH. A classic example is the ammonium hydroxide and ammonium chloride buffer system, which effectively neutralizes added acids or bases, ensuring the pH remains stable. Understanding this property is crucial for predicting how real-world systems will react to changes, an essential concept in chemistry.