Calculate oxygen mass by moles of O atoms

Steps:

• Identify O atoms per formula unit: Al(NO₃)₃ has 9, K₂SO₄ has 4, NaNO₃ has 3, MgCO₃ has 3.
• Compute moles of O: multiply compound moles by O atoms (A: 0.2×9=1.8; B: 0.3×4=1.2; C: 0.4×3=1.2; D: 0.5×3=1.5).
• Find mass of O: multiply moles of O by 16 g/mol (A: 28.8 g; B: 19.2 g; C: 19.2 g; D: 24 g).
• Compare: A has the greatest mass.

Why A is correct:

• Al(NO₃)₃ contains 9 O atoms per unit (three NO₃ groups), yielding 28.8 g O for 0.2 mol, exceeding others per molar ratio calculation.

Why the others are wrong:

• B: Fewer O atoms (4 per unit) result in only 19.2 g O for 0.3 mol.
• C: 3 O atoms per unit give 19.2 g O for 0.4 mol, tied with B but less than A.
• D: 3 O atoms per unit produce 24 g O for 0.5 mol, still below A's value.

Final answer: A