A compound has mass percentages 40.0% C, 6.7% H, and 53.3% O. What is its empirical formula?

• A. CHO
• B. CH2O
• C. C2HO
• D. CH2

Answer: (B)

Finding the empirical formula from mass percentages means turning those percentages into a mole ratio and then simplifying to the smallest whole numbers.

Convert each element’s mass to moles using their approximate molar masses:

• Carbon: 40.0 g / 12.01 g/mol ≈ 3.33 mol

• Hydrogen: 6.7 g / 1.008 g/mol ≈ 6.65 mol

• Oxygen: 53.3 g / 16.00 g/mol ≈ 3.33 mol

Now divide by the smallest number of moles (about 3.33):

• Carbon: 3.33 / 3.33 ≈ 1

• Hydrogen: 6.65 / 3.33 ≈ 2

• Oxygen: 3.33 / 3.33 ≈ 1

This gives the simplest whole-number ratio: C1H2O1, i.e., CH2O. Hence the empirical formula is CH2O.