Questions: Percent Composition and Empirical Formulas

Question 1

A compound is found to be 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass, with a measured molar mass of 60 g/mol. What is its molecular formula?

• A. CH₂O — the empirical formula matches the molar mass
• B. C₂H₄O₂ — the empirical formula mass (30 g/mol) multiplies by 2 to reach 60 g/mol
• C. C₂H₄O — the hydrogen and oxygen counts are adjusted to reach 60 g/mol
• D. C₃H₆O₃ — the empirical formula repeats three times

Answer: (B)

First find the empirical formula: 40.0g C / 12.01 = 3.33 mol, 6.7g H / 1.008 = 6.65 mol, 53.3g O / 16.00 = 3.33 mol. Dividing by the smallest (3.33) gives CH₂O, with an empirical formula mass of 30 g/mol. To get the molecular formula, divide the measured molar mass by the empirical formula mass: 60 / 30 = 2. Multiply each subscript by 2 to get C₂H₄O₂. The common misconception (option A) confuses empirical and molecular formulas — they are only the same when the multiplier is 1.

Question 2

A compound is 75.0% carbon and 25.0% hydrogen by mass. What is its empirical formula?

• A. CH₃ — a 1:3 ratio of C to H
• B. CH₄ — a 1:4 ratio of C to H
• C. C₂H₆ — a 2:6 ratio of C to H
• D. C₃H₁₂ — a 3:12 ratio of C to H

Answer: (B)

Assume 100g: 75.0g C / 12.01 g/mol = 6.25 mol C; 25.0g H / 1.008 g/mol = 24.8 mol H. Divide by the smaller (6.25): C = 1, H = 24.8/6.25 ≈ 3.97 ≈ 4. The empirical formula is CH₄. Note that C₂H₈ (option D simplified) is the same ratio and is equivalent, but CH₄ is the simplest whole-number form. C₃H₁₂ and C₂H₆ represent different ratios and are incorrect.

Question 3

Glucose (C₆H₁₂O₆) and acetic acid (C₂H₄O₂) have different empirical formulas.

• T. True
• F. False

Answer: False

Both glucose and acetic acid share the same empirical formula: CH₂O. Glucose has a C:H:O ratio of 6:12:6, which simplifies to 1:2:1. Acetic acid has 2:4:2, which also simplifies to 1:2:1. This is exactly why percent composition alone cannot distinguish between them — you also need the molar mass to determine the molecular formula.

Question 4

To determine a compound's molecular formula from experimental data, percent composition alone is sufficient.

• T. True
• F. False

Answer: False

Percent composition gives you the empirical formula — the simplest whole-number ratio of atoms. But many different molecular formulas can share the same empirical formula (e.g., CH₂O, C₂H₄O₂, C₆H₁₂O₆ all reduce to CH₂O). To find the molecular formula, you additionally need the compound's actual molar mass, typically from mass spectrometry. The molecular formula multiplier is: (measured molar mass) / (empirical formula mass).

Question 5

Why can't percent composition data alone determine a compound's molecular formula? What additional information is needed, and how is it used?

Model answer: Percent composition gives only the simplest atom ratio (the empirical formula), not the actual count. Multiple different compounds can have identical percent compositions if their molecular formulas are integer multiples of the same empirical formula. To determine the molecular formula, you need the compound's measured molar mass (from an experiment like mass spectrometry). Divide the measured molar mass by the empirical formula mass to get a whole-number multiplier, then multiply each subscript in the empirical formula by that number.

The key distinction is ratio vs. count. Formaldehyde (CH₂O, 30 g/mol), acetic acid (C₂H₄O₂, 60 g/mol), and glucose (C₆H₁₂O₆, 180 g/mol) all yield identical percent compositions — they are chemically distinct but share a 1:2:1 C:H:O ratio. Only by combining percent composition (to get the empirical formula) with molar mass (to get the multiplier) can you uniquely identify the molecular formula.