Empirical Formula from Mass Percentages

Steps:

• Assume 100 g sample: 40 g C, 6.7 g H, 53 g O.
• Calculate moles: C = 40/12 = 3.33 mol; H = 6.7/1 = 6.7 mol; O = 53/16 = 3.31 mol.
• Divide by smallest (3.31): C ≈ 1, H ≈ 2, O ≈ 1.
• Empirical formula is CH₂O.

Why A is correct:

• CH₂O matches the simplest whole-number mole ratio (1:2:1) from mass percentages, per the definition of empirical formula as lowest-ratio molecular composition.

Why the others are wrong:

• B: C₂H₅O has higher C:H ratio (2:5), not matching 1:2.
• C: CH₃O has H:O ratio of 3:1, exceeding calculated 2:1.
• D: CHO has H ratio of 1, below calculated 2.

Final answer: A