Mass percentage of oxygen in Na₂CO₃·10H₂O

Steps:

• Molar mass of Na₂CO₃·10H₂O = 2(23) + 12 + 3(16) + 10(2(1) + 16) = 286 g/mol.
• Mass of oxygen = 3(16) from CO₃ + 10(16) from H₂O = 48 + 160 = 208 g/mol.
• Percentage = (208 / 286) × 100 = 72.7%.

Why D is correct:

• Percentage composition formula (mass of oxygen / molar mass of compound) × 100 yields exactly 72.7%.

Why the others are wrong:

• A: Uses only oxygen from CO₃ (48 g), ignoring H₂O oxygen.
• B: Arises from misassigned atomic masses or unrelated compound calculation.
• C: Counts only oxygen from 10H₂O (160 g), excluding CO₃ oxygen.

Final answer: D