Magnesium chloride (MgCl2) is a salt that, when dissolved in water, dissociates into magnesium (Mg2+) and chloride (Cl-) ions. Mg2+ ions undergo hydrolysis in water, which means they react with water molecules to produce hydrogen ions (H+) and hydroxide ions (OH-). This hydrolysis reaction can be represented as follows:

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Mg2+ + H2O ⇌ Mg(OH)+ + H+

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The hydrogen ions produced in this reaction contribute to the acidic nature of the solution, lowering its pH. The extent of hydrolysis depends on the concentration of magnesium chloride in the solution. At higher concentrations, the hydrolysis reaction is more pronounced, resulting in a lower pH.

Sodium chloride solution (pH 7):

Sodium chloride (NaCl), on the other hand, is a salt that, when dissolved in water, dissociates into sodium (Na+) and chloride (Cl-) ions. Unlike magnesium ions, sodium ions do not undergo significant hydrolysis in water. This means that they do not produce hydrogen ions or hydroxide ions, and the pH of the solution remains close to neutral (pH 7).

In summary, the difference in pH between aqueous solutions of magnesium chloride and sodium chloride is primarily due to the hydrolysis of magnesium ions, which contributes to the acidity of the solution. Since magnesium ions undergo more hydrolysis compared to sodium ions, the magnesium chloride solution has a lower pH (5.5) compared to the sodium chloride solution (pH 7).