Mg(CLO4)2 dissolved in water acidic, basic, or neutral? and why?
The Correct Answer and Explanation is:
When magnesium perchlorate (Mg(ClO₄)₂) dissolves in water, the resulting solution is slightly acidic.
Explanation
Magnesium perchlorate (Mg(ClO₄)₂) is a salt formed from the reaction of magnesium hydroxide (Mg(OH)₂), a weak base, and perchloric acid (HClO₄), a strong acid. To determine the nature of its aqueous solution—acidic, basic, or neutral—we analyze the behavior of its constituent ions when dissolved in water.
Upon dissolution, Mg(ClO₄)₂ dissociates into its ions: Mg(ClO₄)₂ (s)→Mg2+(aq)+2ClO₄−(aq)\text{Mg(ClO₄)₂ (s)} \rightarrow \text{Mg}^{2+} (aq) + 2 \text{ClO₄}^- (aq)
- ClO₄⁻ (perchlorate ion): This is the conjugate base of a strong acid (HClO₄), so it is extremely weak and does not hydrolyze in water. It has no significant effect on the pH.
- Mg²⁺ (magnesium ion): This is a small, highly charged cation (a 2+ charge) and can undergo hydrolysis. It interacts with water molecules to form a hydrated ion like [Mg(H₂O)₆]²⁺. The positive charge on Mg²⁺ polarizes the water molecules in its hydration shell, making them more likely to donate protons (H⁺). This results in the release of hydrogen ions (H⁺), causing a slight acidification of the solution:
[Mg(H₂O)₆]2+⇌[Mg(H₂O)₅OH]++H+\text{[Mg(H₂O)₆]}^{2+} \rightleftharpoons \text{[Mg(H₂O)₅OH]}^{+} + \text{H}^{+}
Thus, the presence of Mg²⁺ contributes to a slight increase in hydrogen ion concentration, lowering the pH and making the solution slightly acidic.
In summary, Mg(ClO₄)₂ yields a slightly acidic solution because the magnesium ion hydrolyzes, while the perchlorate ion remains inert. The overall result is a solution with a pH just below 7, typically around 5.5–6.5, depending on concentration.
