Problem 13 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is not a buffer solution? (a) \(0.8 \mathrm{M} \mathrm{H}_{2} \mathrm{~S}+0.8 \mathrm{M} \mathrm{KHS}\) (b) \(2 \mathrm{M} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}+2 \mathrm{M} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3}^{+} \mathrm{Br}^{-}\) (c) \(3 \mathrm{M} \mathrm{H}_{2} \mathrm{CO}_{3}+3 \mathrm{M} \mathrm{KHCO}_{3}\) (d) \(0.05 \mathrm{M} \mathrm{KCIO}_{4}+0.05 \mathrm{M} \mathrm{HClO}_{4}\)

Step-by-Step Solution

Answer

Option (d) is not a buffer solution because it involves a strong acid. It doesn't meet the criteria for a buffer solution.

Step 1: Buffer Solution Definition

A buffer solution is a system that maintains a relatively constant pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid.

Step 2: Examine Each Option

Examine each of the given options to identify whether they consist of a weak acid and its conjugate base or a weak base and its conjugate acid.

Step 3: Analyze Option (a)

Option (a) consists of 0.8 M H2S and 0.8 M KHS. H2S is a weak acid, and its conjugate base is HS^-. This is a buffer solution because it consists of a weak acid and its conjugate base.

Step 4: Analyze Option (b)

Option (b) consists of 2 M C6H5NH2 and 2 M C6H5NH3^+ Br^-. C6H5NH2 is a weak base (aniline), and C6H5NH3^+ is its conjugate acid. This is a buffer solution.

Step 5: Analyze Option (c)

Option (c) consists of 3 M H2CO3 and 3 M KHCO3. H2CO3 is a weak acid and HCO3^- is its conjugate base. This is a buffer solution.

Step 6: Analyze Option (d)

Option (d) consists of 0.05 M KClO4 and 0.05 M HClO4. HClO4 is a strong acid and it does not have a weak conjugate base in the solution. This is not a buffer solution.

Step 7: Conclusion

Based on the definitions and analysis, option (d) is not a buffer solution because it includes a strong acid and its corresponding salt, which doesn't form a buffer system.

Key Concepts

Weak Acid

A weak acid is a type of acid that only partially dissociates in solution. This means that not all of the acid molecules give up their hydrogen ions ( H^+ ) to become ions in the solution. The partial dissociation occurs because the acid and its ions exist in equilibrium. As a result, there are always some molecules of the original acid present, along with its ions.

Here are a few key points about weak acids:

• They have a higher pH compared to strong acids because they do not fully release their hydrogen ions.
• Examples include acetic acid ( CH_3COOH ) and hydrogen sulfide ( H_2S ).
• Weak acids can form buffer solutions when paired with their conjugate base in equal amounts.

Weak acids play an essential role in buffer solutions, as their presence ensures that small additions of acids or bases will not significantly change the system's pH.

Conjugate Base

The conjugate base of an acid is what remains after the acid has donated a hydrogen ion. For example, when a weak acid like CH_3COOH loses a H^+ , it forms the acetate ion ( CH_3COO^- ), which is its conjugate base.

These are important characteristics of conjugate bases:

• They help build buffer solutions when combined with their corresponding weak acid.
• The strength of a conjugate base is typically inversely related to the strength of its acid; weaker acids have stronger conjugate bases.
• They will accept H^+ ions if the solution becomes too acidic, helping to stabilize the pH level.

Conjugate bases are crucial in forming buffers. They can neutralize added acids, keeping the pH of the solution relatively stable.

Weak Base

Weak bases only partially accept hydrogen ions in a solution. They do not fully ionize or dissociate, which makes them behave differently compared to strong bases like sodium hydroxide ( NaOH ). Aniline ( C_6H_5NH_2 ) is an example of a weak base, where only a small fraction of the aniline molecules accept H^+ ions to form its conjugate acid.

Some characteristics of weak bases include:

• They typically have a lower pH than strong bases due to incomplete ionization.
• They are essential for buffer solutions when paired with their conjugate acid.
• They are less reactive, making them stable in solutions over a range of pH values.

Understanding weak bases helps explain their role in maintaining the pH of a buffer solution balanced, as they can bind to H^+ ions when needed.

Conjugate Acid

The conjugate acid is formed when a base accepts a hydrogen ion (H^+). If a weak base like aniline ( C_6H_5NH_2 ) accepts a hydrogen ion, it becomes C_6H_5NH_3^+ , which is the conjugate acid.

Conjugate acids are integral to buffer solutions for the following reasons:

• They provide stability to the solution by donating H^+ ions when the solution becomes too basic.
• They are derived from weak bases, sharing the partial ionization characteristics.
• When paired with a weak base, they can resist drastic pH changes.

In buffer systems, conjugate acids are essential to balance sudden shifts in hydrogen ion concentration, aiding in maintaining a consistent pH.