pH of Period 3 Oxide Solutions

The pH of the solution formed when each Period 3 oxide is added to water:• Na₂O: pH 14 (strongly alkaline)
• MgO: pH about 9 to 10 (weakly alkaline)
• Al₂O₃ and SiO₂: pH 7 (insoluble, so the water is unchanged)
• P₄O₁₀: pH about 0 to 1
• SO₂: pH about 2 to 3
• SO₃: pH about 0 to 1Why the metal oxides give alkaline solutions. Ionic oxides contain the oxide ion, a strong base, which takes a proton from water:O²⁻ + H₂O → 2OH⁻NaOH is very soluble, so [OH⁻] is high and the pH is high. Mg(OH)₂ is only slightly soluble, so fewer hydroxide ions are released and the pH is lower.Why the non-metal oxides give acidic solutions. They form acids that release H⁺ ions. H₂SO₄ is a strong acid, so SO₃ gives a very low pH. H₂SO₃ is a weak acid (only partly dissociated), so SO₂ gives a higher pH than SO₃. H₃PO₄ is also a weak acid, but it forms at high concentration, so the pH is still low.Worked example: Na₂O gives a solution with [OH⁻] = 0.100 mol dm⁻³ at 298 K. [H⁺] = K_w ÷ [OH⁻] = 1.00 × 10⁻¹⁴ ÷ 0.100 = 1.00 × 10⁻¹³ mol dm⁻³, so pH = −log(1.00 × 10⁻¹³) = 13.00.