Chemistry

Question Analysis
This question tests the student's ability to determine both the empirical and molecular formulas from the percent composition and the molar mass of a compound.

Key Concept Explanation
First, determine the empirical formula from the percent composition, then use the molar mass to find the molecular formula.

Step-by-step Solution
1. Assume a 100 g sample: C: 40.0 g, H: 6.7 g, O: 53.3 g.
2. Convert grams to moles: C: 40.0 g / 12.01 g/mol ≈ 3.33 mol, H: 6.7 g / 1.008 g/mol ≈ 6.75 mol, O: 53.3 g / 16.00 g/mol ≈ 3.33 mol.
3. Divide each mole value by the smallest mole value (3.33): C: 3.33 / 3.33 = 1, H: 6.65 / 3.33 ≈ 2, O: 3.33 / 3.33 = 1.
4. The empirical formula is CH₂O.
5. Determine the molar mass of the empirical formula: (1 × 12.01) + (2 × 1.008) + 16.00 ≈ 30.03 g/mol.
6. Calculate n: n = 90 g/mol / 30.03 g/mol ≈ 3.
7. Multiply the subscripts in the empirical formula by n:

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