An organic compound is known to contain only carbon, hydrogen and oxygen. The compound contains, by mass, 39.1% of carbon and 8.7% of hydrogen. The number of carbon atoms in the empirical formula is

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The question gives the composition of the compound by mass: 39.1% carbon (C), 8.7% hydrogen (H), and the remainder as oxygen (O). 
First, determine the percentage of oxygen: 100% − 39.1% − 8.7% = 52.2% O.
Next, convert these mass percentages to moles per 100 g of compound:
- C: 39.1 g / 12.01 g/mol ≈ 3.26 mol
- H: 8.7 g / 1.008 g/mol ≈ 8.63 mol
- O: 52.2 g / 16.00 g/mol ≈ 3.26 mol
Now compare the mole ratios by dividing each by the smallest number of moles among them, which is about 3.26:
- C: 3.26 / 3.26 ≈ 1.00
- H: 8.63 / 3.26 ≈ 2.65
- O: 3.26 / 3.26 ≈ 1.00
These ratios suggest the empirical formula is close to C1H2.65O1. To convert these into whole numbers, multiply all by an appropriate factor to clear the decimal. 
- Multiplying by 2 gives C2H5.30O2, still not integers for H.
- Multiplying by 3 yields C3H7.95O3, which rounds to C3H8O3 for practical formula representation.
Thus, the empirical formula is C3H8O3, which contains 3 carbon atoms per empirical unit.
In the absence of provided answer choices, this reasoning leads to the conclusion that the empirical formula contains three carbon atoms.
If you were comparing multiple-choice options, you would select the option stating 3 carbon atoms in the empirical formula, and the accompanying empirical formula would be C3H8O3.

MUF0041 Chemistry Unit 1 - Semester 1, 2025 2.12 Quiz: Percentage Composition and Empirical Formula

An organic compound is known to contain only carbon, hydrogen and oxygen. The compound contains, by mass, 39.1% of carbon and 8.7% of hydrogen. The number of carbon atoms in the empirical formula is

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