For CxHyOz + excess O2 -> x CO2 + $\frac{y}{2}$ H2O. Given masses of CO2 and H2O collected: mass C = $\frac{12}{44}$ x mass CO2 = $\frac{3}{11}$ x mass CO2. mass H = $\frac{2}{18}$ x mass H2O = $\frac{1}{9}$ x mass H2O. mass O = mass of sample - mass C - mass H. If N or S present: N -> converted to N2 (measure volume) or absorbed as specific product. S -> SO2 (absorbed in peroxide, titrated). From masses of elements, calculate empirical formula. With molar mass, find molecular formula. Common trap: not accounting for oxygen already present in the compound.