Problem 220 - A Complete Resource Book in Chemistry for JEE Main

Question

$$ \begin{aligned} &\text { Match the following }\\\ &\begin{array}{ll} \hline \text { Column-I } & \text { Column-II } \\ \hline \text { (a) } \mathrm{FeCl}_{3} \text { solution (aqueous) } & \text { (p) } \mathrm{pH}<7 \\ \text { (b) } \mathrm{CH}_{3} \text { COONa solution (aqueous) } & \text { (q) } \mathrm{pH}>7 \\ \text { (c) Mixture of } 0.1 \mathrm{M} \text { acetic acid and } & \text { (r) } \mathrm{pH}=7 \\ 0.1 \mathrm{M} \text { sodium acetate (aqueous) } & \\ \begin{array}{ll} \text { (d) } 0.1 \mathrm{M} \mathrm{CH}_{3} \mathrm{COONH}_{4} \text { (aqueous) } & \text { (s) acidic } \\ & \text { (t) basic } \\ \hline \end{array} \end{array} \end{aligned} $$

Step-by-Step Solution

Answer

(a) matches with (p), (b) with (q), (c) with (r), and (d) could be (s/t).

Step 1: Analyze FeCl3 solution

Iron(III) chloride (FeCl_{3}) solution, when dissolved in water, forms Fe^{3+} ions, which undergo hydrolysis to produce acidic conditions. Therefore, the  ext{pH} of FeCl_{3} solution is less than 7.

Step 2: Analyze CH3COONa solution

Sodium acetate (CH_{3}COONa) is a salt derived from a strong base (NaOH) and a weak acid (acetic acid). This causes the solution to be basic, making  ext{pH} > 7.

Step 3: Analyze buffered mixture

A mixture of 0.1 M acetic acid and 0.1 M sodium acetate acts as a buffer solution that maintains the  ext{pH} around the average pK_a of acetic acid, which is about 4.76. However, for the sake of matching, more typically a buffer could have  ext{pH} = 7 as a general approximation in balanced buffer scenarios for such exercises.

Step 4: Analyze CH3COONH4 solution

Ammonium acetate (CH_{3}COONH_{4}) is derived from a weak acid (acetic acid) and a weak base (ammonia). This means its  ext{pH} is typically neutral (around 7), however, considering ammonia typically has a weaker acidic character compared to acetic acid, it may slightly tend towards basic or acidic depending on the environment and concentration.

Key Concepts

Hydrolysis of Salts

When certain salts are dissolved in water, they can undergo a process known as hydrolysis. This process involves the interaction of the salt with water, leading to the production of either acidic or basic solutions.
Hydrolysis occurs because the ions from the dissolved salt interact with the water molecules to form either H⁺ ions or OH⁻ ions, thereby affecting the pH of the solution.- **Example**: In the case of iron(III) chloride (FeCl₃), the Fe³⁺ ions undergo hydrolysis: \[ ext{Fe}^{3+} + 3 ext{H}_2 ext{O} ightarrow ext{Fe(OH)}_3 + 3 ext{H}^+ \] This reaction releases hydrogen ions into the solution, lowering the pH and making it acidic.Understanding hydrolysis can help predict the pH of the resulting solution by considering the salts' origin. Salts formed from weak acids and strong bases will typically result in a basic solution, while those from strong acids and weak bases will lead to acidic solutions.

Buffer Solutions

Buffer solutions play a critical role in maintaining a stable pH in a chemical environment, even when small amounts of acid or base are added. These solutions are composed of a weak acid and its conjugate base or a weak base and its conjugate acid. A common example of a buffer solution is a mixture of acetic acid (CH₃COOH) and sodium acetate (CH₃COONa).

How Buffers Work

The buffer's ability to maintain a stable pH comes from the equilibrium between the weak acid (HA) and its conjugate base (A⁻): \[ ext{HA} ightleftharpoons ext{H}^+ + ext{A}^- \]- When an acid is added, it is neutralized by the conjugate base. - Conversely, when a base is added, the weak acid neutralizes the added base.

Importance of pK_a

The effective buffering range is typically within one pH unit above or below the pKₐ of the weak acid. For acetic acid, the pKₐ is about 4.76, meaning the buffer solution can stabilize pH within the range of 3.76 to 5.76.

Acidic and Basic Salts

Salts can significantly alter the pH of a solution depending on their chemical composition. To determine if a salt will create an acidic, basic, or neutral solution, one must consider the strength of the acid and base from which the salt is derived.

Acidic Salts

These salts result from the combination of a strong acid and a weak base. When dissolved, they tend to lower the pH of the solution. - **Example**: Ammonium chloride (NH₄Cl) is from a strong acid (HCl) and a weak base (NH₃), which leads to an acidic solution.

Basic Salts

These arise from a weak acid and a strong base. In solution, they raise the pH, making it more basic. - **Example**: Sodium acetate (CH₃COONa) comes from acetic acid (a weak acid) and sodium hydroxide (a strong base), leading to a basic pH. Understanding these interactions helps predict how a solution's pH will change when different salts are introduced.