🧪Empirical Formula Calculator

About this tool

Add each element and its percentage by mass or its mass in grams. The calculator converts to moles, divides by the smallest, multiplies to whole numbers and shows the empirical formula. Enter the compound's molar mass to get the molecular formula too. A table shows each step.

How to work out an empirical formula

The empirical formula is the simplest whole-number ratio of atoms in a compound, while the molecular formula is the actual number of atoms in one molecule. To find the empirical formula from percentages, imagine a 100 g sample so each percentage becomes grams. Divide each mass by the element's atomic mass to get moles, then divide every mole value by the smallest one. If the results aren't close to whole numbers, multiply them all by a small integer: ratios ending in .5 need doubling, and .33 or .67 need tripling. For a compound that is 40.0% carbon, 6.7% hydrogen and 53.3% oxygen, the moles are about 3.33, 6.65 and 3.33, a ratio of 1 : 2 : 1, so the empirical formula is CH₂O. Its formula mass is about 30 g/mol, so if the molar mass is 180 g/mol, the molecule is six times larger: C₆H₁₂O₆, glucose. A compound that is 69.9% iron and 30.1% oxygen works out as Fe₂O₃. Honest limits: experimental percentages carry measurement error, so ratios are rounded when they're within 0.1 of a whole number after multiplying; check that your percentages add up to about 100. The molar mass calculator, the chemical equation balancer and the percent error calculator are related.

Frequently asked questions