Calculate mass of oxygen in each compound Steps:

• Identify oxygen 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).
• Calculate O mass: moles of O × 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 (3 NO₃ groups × 3 O each), yielding 28.8 g O for 0.2 mol, per molar mass formula.

Why the others are wrong:

• B: Only 4 O atoms, gives 19.2 g, less than A.
• C: 3 O atoms, gives 19.2 g, less than A.
• D: 3 O atoms, gives 24 g, less than A.

Final answer: A