Steps for empirical formula: (1) Get mass% of each element (or mass from combustion data). (2) Convert to moles: divide mass% by atomic mass. (3) Divide each by the smallest mole value. (4) Round to nearest whole numbers (multiply all by 2, 3, etc. if needed). For molecular formula: n = $M_{molecular}$ / $M_{empirical}$. Molecular formula = n x empirical formula. Combustion analysis: burn organic compound, collect CO2 and H2O. Mass C = $\frac{12}{44}$ x mass CO2. Mass H = $\frac{2}{18}$ x mass H2O. Mass O = original mass - mass C - mass H. Vapour density (VD) = M/2 (relative to H2). M = 2 x VD.