Metal identity from oxygen moles and oxide formula Steps:

• Convert 300 cm³ oxygen to 0.3 dm³; moles O₂ = 0.3 / 24 = 0.0125 mol.
• Moles O atoms = 2 × 0.0125 = 0.025 mol (from O₂ → 2O²⁻).
• For 1.00 g metal forming oxide with O²⁻, moles metal = 1 / atomic mass (M).
• Match stoichiometry: MO oxides need moles metal = 0.025 (M = 40 g/mol); M₂O need 0.05 mol (M = 20 g/mol).

Why A is correct:

• Calcium (40 g/mol) fits MO oxide (CaO) exactly, as 0.025 mol Ca × 40 = 1 g balances 0.025 mol O²⁻.

Why the others are wrong:

• B. Magnesium (24 g/mol) in MgO requires 1.67 g for 0.025 mol, exceeding 1 g.
• C. Potassium (39 g/mol) in K₂O requires 1.95 g for 0.05 mol, exceeding 1 g.
• D. Sodium (23 g/mol) in Na₂O requires 1.15 g for 0.05 mol, exceeding 1 g.

Final answer: A