Equimolar solutions of the following were prepared in water separately, which one of the solutions will record the highest pH?

The pH of a solution is a measure of its acidity or alkalinity. It is determined by the concentration of hydrogen ions (H+) in the solution. A higher concentration of H+ ions leads to a lower pH, indicating a more acidic solution, while a lower concentration of H+ ions leads to a higher pH, indicating a more alkaline or basic solution.

To determine which solution will record the highest pH, we need to understand the behavior of the cations present in each solution. The cations in the solutions mentioned are Sr2+, Ba2+, Mg2+, and Ca2+.

Behavior of Cations:
- Sr2+ and Ba2+ are from Group 2 elements, known as alkaline earth metals. These metals have large atomic radii and low charge densities. As a result, they have a lower tendency to attract water molecules and form hydrated ions.
- Mg2+ and Ca2+ are also from Group 2 elements. However, they have smaller atomic radii and higher charge densities compared to Sr2+ and Ba2+. This higher charge density allows them to attract water molecules more strongly and form hydrated ions more easily.

Hydration of Cations:
- When these cations dissolve in water, they become hydrated ions by attracting water molecules around them. The hydration process releases H+ ions, leading to an increase in the concentration of H+ ions in the solution.
- The larger atomic radii and lower charge densities of Sr2+ and Ba2+ make it more difficult for them to attract water molecules and form hydrated ions. Therefore, they release fewer H+ ions into the solution, resulting in a higher pH.
- On the other hand, the smaller atomic radii and higher charge densities of Mg2+ and Ca2+ allow them to attract water molecules more effectively and form hydrated ions more readily. This leads to a higher concentration of H+ ions in the solution and a lower pH.

Conclusion:
Considering the above explanations, the solution of BaCl2 will record the highest pH among the given options. The Ba2+ cation has a larger atomic radius and lower charge density, making it less likely to attract water molecules and release H+ ions. As a result, the concentration of H+ ions in the solution will be lower, leading to a higher pH.