Highest pH will be recorded for which of the following solutions if they are equimolar

The correct option is BaCl₂.

To understand why a solution of barium chloride (BaCl₂) has the highest pH among the given equimolar salt solutions, let us analyze the nature of each salt and the behavior of their constituent ions in aqueous solution.

Each of the given salts is composed of a metal cation (Al³⁺, Ba²⁺, Be²⁺, or Li⁺) and a chloride anion (Cl^-). When these salts dissolve in water, they dissociate completely into their respective ions. The chloride ion (Cl^-) is the conjugate base of hydrochloric acid (HCl), which is a strong acid. Because Cl^- is an extremely weak conjugate base, it does not undergo hydrolysis in water and does not affect the pH of the solution. Therefore, the pH of each solution is determined solely by the interaction of the metal cation with water (cationic hydrolysis).

Cations can undergo hydrolysis in water according to the general equation:
Mⁿ⁺ + H₂O ⇌ M(OH)^(n-1)+ + H⁺
This hydrolysis reaction releases H⁺ ions into the solution, making the solution acidic (lowering the pH). The extent of this hydrolysis depends on the charge density of the cation. Charge density is directly proportional to the charge of the ion and inversely proportional to its ionic size. Cations with high charge and small size (high charge density) polarize the surrounding water molecules strongly, facilitating the release of H⁺ ions and leading to significant hydrolysis (lower pH). Conversely, cations with low charge density undergo negligible hydrolysis, leaving the solution closer to neutral (higher pH).

Let us compare the charge density of the cations in the given options:
1. Al³⁺ (from AlCl₃): This ion has a high positive charge (+3) and a relatively small ionic size. Thus, it has a very high charge density, undergoes extensive hydrolysis, and produces a highly acidic solution with a very low pH.
2. Be²⁺ (from BeCl₂): Beryllium is a small alkaline earth metal ion with a +2 charge. Due to its exceptionally small size, Be²⁺ has a very high charge density, undergoes significant hydrolysis, and forms an acidic solution with a low pH.
3. Li⁺ (from LiCl): Lithium is a small alkali metal ion with a +1 charge. It has a moderate charge density, but it is much less polarizing than Al³⁺ and Be²⁺, resulting in very weak hydrolysis.
4. Ba²⁺ (from BaCl₂): Barium is located near the bottom of Group 2 (alkaline earth metals). Although it carries a +2 charge, its ionic radius is very large compared to the other cations. This large size results in a very low charge density. As a result, the Ba²⁺ ion does not undergo hydrolysis in aqueous solution.

Because Ba²⁺ does not hydrolyze, and Cl^- does not hydrolyze, a solution of BaCl₂ remains practically neutral (pH is closest to 7). Since all the other salts yield acidic solutions due to varying degrees of cation hydrolysis, the equimolar solution of BaCl₂ will record the highest pH.