Empirical Formula Calculator

In short An empirical formula is the simplest whole-number ratio of atoms in a compound, and percent composition is enough to find it. Divide each element's percentage by its atomic weight to get relative moles, divide all of those by the smallest, and multiply up until every ratio is a whole number. Glucose analyses as 40.00% carbon, 6.71% hydrogen and 53.29% oxygen, which gives 3.330 : 6.657 : 3.331 relative moles, or 1 : 2 : 1 — the empirical formula CH2O. It does not matter whether you enter percentages or actual masses, because only the ratios are used; assuming a 100 g sample is what makes percentages work directly. The important limit is that composition alone can never give the molecular formula. CH2O is also the empirical formula of formaldehyde, of acetic acid and of glucose, and all three have identical percentages by mass. Only the measured molar mass separates them: 30.03 g/mol is formaldehyde, 60.05 is acetic acid, 180.16 is glucose at six times the empirical unit. Benzene and acetylene make the same point more sharply — both are 92.26% carbon, both are CH by composition, and 78.11 g/mol makes it C6H6 while 26.04 makes it C2H2. Two practical warnings. Oxygen is usually determined by difference rather than measured, so if the percentages do not total about 100 it is normally the oxygen figure that is missing or wrong. And if the ratios do not land close to whole numbers, the answer is a best fit rather than a formula — real experimental error rarely pushes a ratio more than a few hundredths away from an integer.