Solubility trend of Group 2 hydroxides determines pH of saturated solutions

Steps:

• Group 2 oxides (A–C) react with water to form basic hydroxides: MO + H2O → M(OH)2.
• SO (likely SO2) forms acidic sulfurous acid, yielding low pH solution.
• Solubility of M(OH)2 increases down Group 2: Mg(OH)2 < Ca(OH)2 < Ba(OH)2.
• Highest solubility means highest [OH⁻] in saturated solution, thus highest pH for Ba(OH)2.

Why A is correct:

• BaO produces Ba(OH)2, the most soluble Group 2 hydroxide (Ksp ≈ 5 × 10⁻³), maximizing [OH⁻] and pH per the solubility product principle.

Why the others are wrong:

• B. CaO forms less soluble Ca(OH)2 (Ksp ≈ 5 × 10⁻⁶), lower [OH⁻] than Ba(OH)2.
• C. MgO forms least soluble Mg(OH)2 (Ksp ≈ 6 × 10⁻¹²), minimal [OH⁻] among basics.
• D. SO forms H2SO3, an acidic solution with pH < 7.

Final answer: A