13.1.6—The molecular and/or empirical formula

The molecular formula counts every atom in the displayed or skeletal structure. The empirical formula is the simplest whole-number ratio of those atoms.

Count carbons and heteroatoms directly; infer hydrogens from the bonds and carbon valency. To obtain the empirical formula, divide all subscripts by their greatest common factor.

Butan-2-ol has molecular formula C₄H₁₀O. Because the subscripts have no common factor, its empirical formula is also C₄H₁₀O; C₆H₁₂ would reduce to CH₂.

Do not simplify a molecular formula when the question asks for it, and do not count the line segments in a skeletal formula as atoms.