$word_{count}$: 210

Empirical formula = simplest whole number ratio of atoms. Steps: (1) Get mass% of each element (or mass from combustion analysis). (2) Divide each by its atomic mass to get moles. (3) Divide all by the smallest mole value. (4) If not whole numbers, multiply all by 2, 3, etc. Molecular formula = n x empirical formula, where n = molecular mass / empirical formula mass. Molecular mass from: vapour density (M = 2 x VD), mass spectrometry, colligative properties, or PV = nRT. Combustion analysis for CxHyOz: burn in excess O2, collect CO2 and H2O. Mass C = $\frac{12}{44}$ x mass CO2. Mass H = $\frac{2}{18}$ x mass H2O. Mass O = sample mass - C - H. Common traps: (a) forgetting to account for oxygen in the original compound, (b) not multiplying by appropriate factor when ratios are 1:1.5 (multiply by 2 to get 2:3), (c) using molecular mass as empirical formula mass. Percent composition: mass% of element = $\frac{atoms x atomic mass}{molecular mass}$ x 100.