Empirical and molecular formulae

A molecular formula shows the actual number of atoms of each element in one molecule. The empirical formula shows only the simplest whole-number ratio. To go from molecular to empirical, divide all the numbers of atoms by their highest common factor. Glucose, C₆H₁₂O₆, divides by 6 to give CH₂O. Ethane, C₂H₆, divides by 2 to give CH₃. Hydrogen peroxide, H₂O₂, divides by 2 to give HO, and butane, C₄H₁₀, gives C₂H₅.

To go from empirical to molecular, you need the relative formula mass of the compound. Work out the empirical formula mass by adding up the A_r values in the empirical formula. Then divide the relative formula mass by the empirical formula mass to get a whole number, n. Multiply every number in the empirical formula by n.

Example: a compound has the empirical formula CH₂ (empirical formula mass 14) and a relative formula mass of 56. Then n = 56 ÷ 14 = 4, so the molecular formula is C₄H₈. The empirical formula mass of HO is 17, and hydrogen peroxide has a relative formula mass of 34, so n = 2 and its formula is H₂O₂.