Determining Molecular Formula from Empirical Formula and Molar Mass

Determining a molecular formula from an empirical formula means dividing the compound's known molar mass by its empirical formula's mass, then multiplying every empirical subscript by that whole-number result to expand the ratio back out.

Nicotine has the empirical formula C5H7N, with an empirical formula mass near 81.13 g/mol, and a separately measured molar mass near 162.26 g/mol. Dividing 162.26 by 81.13 gives almost exactly 2, so every subscript in C5H7N is doubled: five carbons become ten, seven hydrogens become fourteen, and one nitrogen becomes two, giving the molecular formula C10H14N2, twice the empirical formula's mass and twice its atom count.