Combustion analysis is a laboratory method used to determine the empirical formula of an organic compound that contains carbon, hydrogen, and sometimes oxygen. A known mass of the unknown sample is burned completely in excess oxygen, converting its carbon into CO2 and its hydrogen into H2O. By measuring the masses of CO2 and H2O produced, chemists can work backward to find how much carbon and hydrogen were in the original sample.
This matters because empirical formulas reveal the simplest whole-number ratio of atoms in a compound.
Key Facts
- Complete combustion converts carbon to CO2 and hydrogen to H2O.
- mol C = mol CO2 because each CO2 molecule contains 1 carbon atom.
- mol H = 2 × mol H2O because each H2O molecule contains 2 hydrogen atoms.
- mass C = mol C × 12.01 g/mol and mass H = mol H × 1.008 g/mol.
- If the compound contains oxygen, mass O = sample mass - mass C - mass H.
- Empirical formula steps: grams to moles, divide by the smallest mole value, multiply to get whole numbers.
Vocabulary
- Combustion analysis
- A method for determining the elemental composition of a compound by burning it and measuring the CO2 and H2O produced.
- Empirical formula
- The simplest whole-number ratio of atoms of each element in a compound.
- Complete combustion
- A reaction in which a substance burns in enough oxygen to convert carbon fully to CO2 and hydrogen fully to H2O.
- Mole ratio
- A ratio comparing the amounts of substances in moles, often taken from a chemical formula or balanced equation.
- Mass percent
- The percentage by mass of a particular element in a compound.
Common Mistakes to Avoid
- Using the mass of CO2 as the mass of carbon is wrong because CO2 also contains oxygen. Convert CO2 to moles, then use the 1 to 1 mole ratio to find moles of carbon.
- Using the mass of H2O as the mass of hydrogen is wrong because water also contains oxygen. Convert H2O to moles, then multiply by 2 to get moles of hydrogen atoms.
- Assuming oxygen comes directly from the measured gases is wrong when the compound itself may contain oxygen. Find oxygen by subtracting the calculated masses of carbon and hydrogen from the original sample mass.
- Rounding mole ratios too early can give an incorrect empirical formula. Keep several significant figures until the final whole-number ratio step.
Practice Questions
- 1 A 0.500 g sample of a compound containing C, H, and O is burned completely to produce 0.733 g CO2 and 0.300 g H2O. Find the empirical formula.
- 2 A hydrocarbon sample with mass 1.20 g produces 3.76 g CO2 and 1.54 g H2O during complete combustion. Determine its empirical formula.
- 3 In a combustion analysis experiment, why is it important that the sample burns completely in excess oxygen, and how would incomplete combustion affect the calculated empirical formula?