Chemistry 1LD Practice Test

Mass percent shows how much of a compound’s mass comes from a given element. For CH2O, add up the masses in one mole: C ≈ 12.01 g, H2 ≈ 2.016 g, O ≈ 16.00 g, giving a molar mass of about 30.03 g/mol. The oxygen portion is 16.00 g per mole. So the mass percent of oxygen is (16.00 / 30.03) × 100 ≈ 53.3%. This matches the chosen answer.