Interactive: empirical and molecular formulae. The top half of the picture turns the percentage composition of a compound into grams in a 100 gram sample, then moles, then the simplest whole-number ratio of carbon to hydrogen to oxygen, which gives the empirical formula CH2O. The bottom half shows the atoms in one molecule. Choosing a molar mass from 30 to 180 grams per mole sets the whole-number multiplier n, and the molecule grows from methanal to ethanoic acid, lactic acid, erythrose, ribose and glucose, while the percentage composition stays exactly the same. Stats show the molecular formula and the number of molecules in 100 grams. Buttons switch the worked explanation between the empirical formula, the molecular formula and counting particles.

Six different molecules. One identical recipe.

Methanal, the acid in vinegar and glucose are all 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass, so percentages alone give only the simplest ratio of atoms. The molar mass is what tells them apart. Raise the molar mass and watch what changes in one molecule, and what never changes at all.