Empirical Formula Calculator

How the Empirical Formula Calculator works

An empirical formula gives the simplest ratio of atoms in a compound, while the molecular formula gives the actual number in one molecule. Glucose, C₆H₁₂O₆, and formaldehyde, CH₂O, share the same empirical formula because their atoms are in the same 1 : 2 : 1 ratio. Combustion analysis and other lab methods give percentages, and this is how chemists get back to a formula.

How the empirical formula calculator works

Treat percentages as grams in a 100 g sample. Divide each element's mass by its atomic mass to get moles, then divide every mole value by the smallest. If the ratios are close to halves, thirds or quarters, multiply all of them by 2, 3 or 4 to clear the fraction. The molecular formula is the empirical formula times n, where n = molar mass ÷ empirical formula mass.

First example. C 40.0%, H 6.71%, O 53.3% gives 3.33 mol C, 6.66 mol H and 3.33 mol O, a 1 : 2 : 1 ratio, so the empirical formula is CH₂O at 30.03 g/mol. With a molar mass of 180.16, n = 6 and the molecular formula is C₆H₁₂O₆.

Second example. Fe 69.94%, O 30.06% gives a ratio of 1 : 1.5, which doubles to Fe₂O₃.

How many decimal places do I need in the ratio? A ratio within about 0.1 of a whole number can be rounded; 1.5, 1.33 or 1.25 need multiplying first.

Empirical and molecular formulas of common compounds

Tips and pitfalls

• Grams work too. You do not need percentages; any masses of each element from one sample give the same ratio.
• Find oxygen by difference. Combustion analysis often reports only C and H, so subtract their percentages from 100 to get oxygen.
• Do not round too early. Rounding 1.5 to 2 turns Fe₂O₃ into FeO, a different compound.
• Molar mass must be a multiple. If the molar mass is not close to a whole multiple of the empirical mass, check the data.

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