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

Options (c) and (d) have no effect on pH upon dilution as they are buffer solutions.

Step 1: Identify the Concept

Buffers are solutions that resist changes in pH upon dilution. They usually consist of a weak acid and its conjugate base or a weak base and its conjugate acid.

Step 2: Analyze Each Option

Examine the four given options to determine which ones form buffer solutions.\\(0.1 \, M \, \mathrm{CH}_3\mathrm{COONa}\) is an example of a salt solution.\\(1 \, M \, \mathrm{CH}_3\mathrm{COONH}_4\) is a solution of an ammonium salt of a weak acid; however, it does not act as a buffer.\\(0.1 \, M \, \mathrm{NH}_4\mathrm{OH}+0.1 \, M \, \mathrm{NH}_4 \mathrm{Cl}\) forms a buffer system because it contains a weak base \(\mathrm{NH}_4\mathrm{OH}\) and its conjugate acid \(\mathrm{NH}_4\mathrm{Cl}\).\\(0.5 \, M \, \mathrm{H}_2\mathrm{CO}_3+0.5 \, M \, \mathrm{NaHCO}_3\) is also a buffer as it is made up of \(\mathrm{H}_2\mathrm{CO}_3\) (a weak acid) and \(\mathrm{NaHCO}_3\) (its conjugate base).

Step 3: Determine Buffer Resistance

A buffer solution, such as \(0.1 \, M \, \mathrm{NH}_4\mathrm{OH}+0.1 \, M \, \mathrm{NH}_4\mathrm{Cl}\) or \(0.5 \, M \, \mathrm{H}_2\mathrm{CO}_3+0.5 \, M \, \mathrm{NaHCO}_3\), will resist changes in pH upon dilution due to the presence of both components of the buffer system.

Key Concepts

pH and Dilution

When dealing with solutions, understanding how dilution affects pH is crucial. If we consider a buffer solution, it is designed to maintain a stable pH. When you dilute such a solution, the pH remains largely unchanged. This is because the buffer contains both components needed to counteract any small pH changes, namely a weak acid and its conjugate base or a weak base and its conjugate acid.
This resistance to pH change upon dilution is due to the buffer's ability to neutralize added H⁺ or OH⁻ ions. Imagine you have a buffer containing \(\mathrm{NH}_4\mathrm{OH}\) and \(\mathrm{NH}_4Cl\). When you add water and dilute it, the balance between the weak base and its conjugate acid helps maintain the original pH level.

• A key property of a buffer is its ability to control pH when small amounts of acid or base are added.
• The pH of a buffer is given by the Henderson-Hasselbalch equation, which incorporates the concentrations of the components.

When choosing buffer solutions like \(0.1 \, M \, \mathrm{NH}_4\mathrm{OH}+0.1 \, M \, \mathrm{NH}_4\mathrm{Cl}\) and \(0.5 \, M \, \mathrm{H}_2\mathrm{CO}_3+0.5 \, M \, \mathrm{NaHCO}_3\), you are selecting systems that resist dilution-induced pH changes effectively.

Weak Acid and Conjugate Base

A weak acid, paired with its conjugate base, forms a buffer solution that can stabilize pH. Take \(\mathrm{H}_2\mathrm{CO}_3\) (carbonic acid) and \(\mathrm{NaHCO}_3\) (sodium bicarbonate) as an example. These two create a classic buffer solution.
In this system, the weak acid \(\mathrm{H}_2\mathrm{CO}_3\) can donate protons (H⁺) when the pH rises due to added base. Conversely, the conjugate base \(\mathrm{HCO}_3^-\) can absorb excess protons when the pH drops, maintaining the solution's stability.

• Weak acids partially dissociate in water, establishing an equilibrium.
• The conjugate base is what remains when the weak acid gives up a proton.

The balance between a weak acid and its conjugate base is essential for the buffer's effectiveness. If you disrupt one part of this balance, the buffer's ability to resist pH changes will decrease.

Weak Base and Conjugate Acid

Buffers can also consist of a weak base and its conjugate acid, such as \(\mathrm{NH}_4\mathrm{OH}\) (ammonium hydroxide) and \(\mathrm{NH}_4\mathrm{Cl}\). \(\mathrm{NH}_4\mathrm{OH}\) acts as the weak base, while \(\mathrm{NH}_4^+\) from \(\mathrm{NH}_4\mathrm{Cl}\) serves as the conjugate acid.
In this system, the weak base can accept protons when the pH drops, and the conjugate acid can release protons when the pH rises. This dual action helps the buffer maintain a consistent pH even when you dilute it or add small amounts of acid or base.

• Weak bases only partially ionize in water, establishing an equilibrium.
• The conjugate acid is the species formed when the weak base accepts a proton.

This equilibrium between a weak base and its conjugate acid is key to the buffer's resilience. It ensures that pH fluctuations are minimized, even under conditions that would normally alter the pH significantly.