Empirical Formula from Mass Percent Calculator
Find the empirical formula of a compound from the mass percentages of carbon, hydrogen and oxygen — the classic combustion-analysis calculation.
How it works
moles of each element = mass % ÷ atomic mass · divide all by the smallest · scale to whole numbers
Assuming a 100 g sample turns each mass percent directly into grams, which you convert to moles by dividing by the atomic mass. Dividing every mole value by the smallest gives the simplest whole-number ratio of atoms — the empirical formula. When the ratios come out near halves or thirds, multiply through by a common factor to reach integers.
Worked example
A compound is 40.0% C, 6.7% H, 53.3% O. Moles: C = 40.0/12.011 = 3.33, H = 6.7/1.008 = 6.65, O = 53.3/15.999 = 3.33. Dividing by 3.33 gives 1 : 2 : 1, so the empirical formula is CH₂O.
Sources: Empirical formula — Wikipedia
Frequently asked questions
What is the difference between empirical and molecular formula?
The empirical formula is the simplest whole-number ratio of atoms (e.g. CH₂O). The molecular formula is the actual count (e.g. C₆H₁₂O₆ for glucose) and is a whole-number multiple of the empirical formula, found by comparing molar masses.
Do the percentages have to add up to 100?
Ideally yes. If a compound also contains an element you have not entered here, the remaining percent belongs to it; this tool handles carbon, hydrogen and oxygen — the elements found by combustion analysis.
Why divide by the smallest number of moles?
Dividing by the smallest value forces that element to a ratio of 1 and expresses the others relative to it, which is the quickest route to the simplest whole-number ratio.
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This calculator is for educational purposes. Double-check important results before acting on them.