Composition Stoichiometry

Calculating Empirical Formulas from Combustion AnalysisAP ChemistryVista Ridge High School

What is combustion analysis?Combustion analysis is often used to determine the empirical formula of an organic compound.

A sample of an unknown substance is burned and the gases produced are trapped and measured. The data is then used to calculate the empirical formula of the substance.

The data is then used to calculate the empirical formula of the substance.What are some important points to make about combustion analysis?The elements making up the unknown substance almost always include carbon and hydrogen. Oxygen is often involved and nitrogen is involved sometimes. Other elements can be involved, but problems with C, H, O, and N predominate.The mass of the unknown substance before burning it must be known.The unknown will be burnt in pure oxygen, present in large excess.The combustion products will be trapped separately from each other and the mass of each combustion product will be determined. The typical products includeCO2 (used to calculate the amount of carbon in the unknown)H2O (used to calculate the amount of hydrogen in the unknown)NO2, N2 or even NH3 from a different analysis (used to determine the amount of nitrogen in the unknown)Note: The amount of oxygen is usually determined by subtraction.

How can we calculate the empirical formula of a compound from its combustion analysis?CH3CH2Ob. If the compound has a molar mass of 90.0 g mol-1, what is the molecular formula of this compound?1.Determine the molar mass of the empirical formula.12.01 g mol-1 + 2(1.008 g mol-1) + 16.00 g mol-1 = 30.02 g mol-1

2.Divide the molar mass of the molecular formula by the molar mass of the empirical formula.

C3H6O3