1. Combustion Analysis
The composition of a substance is often determined by
using a specified reaction to break down the substance
into known measurable compounds.
Organic compounds are often analyzed by combustion
1. Define combustion
The reaction of a substance with molecular oxygen
2. Identify the products in combustion of a simple
organic compound
carbon dioxide and water

2. For example, assume an unknown hydrocarbon is
analyzed by combustion analysis.
1. Write the equation for the reaction.
CxHy + O2  x CO2 + y H2O
Note that all of the carbons are converted from
the hydrocarbon to carbon dioxide.
Measurement of the carbon dioxide provides a
quantitative value for the amount of carbon in
the substance
Note that all of the hydrogens are converted from
the hydrocarbon to water. Measurement of the
water provides a quantitative value for the
amount of hydrogen in the substance.

3. Organic compounds are often analyzed by combustion
The substance is heated to a high temperature with O2
to insure complete combustion. The resulting CO2 and
H2O are absorbed using appropriate chemicals, and
the amounts of CO2 and H2O are determined by
measuring the increase in mass of the absorbents

4. Assume a sample of an unknown hydrocarbon
is completely burned in the presence of oxygen, and the
resulting carbon dioxide and water are collected and
measured. Results: grams CO2 and 1.65 g H2O.
Determine the empirical formula.
Carbon
3.45 g CO2 1 mol CO2 1 mol C = mol C
44 g mol CO2
Hydrogen
1.65 g H2O 1 mol H2O 2 mol H = mol H
18 g mol H2O

5. Carbon = .0784 mol Hydrogen = 0.183 mol
Carbon is the lowest value = mol
Carbon = Hydrogen = mol = 2.33 mol
Carbon: Hydrogen ratio is 1: 2.33
Hydrogen = 3 (2.33) = 7
Carbon = 3 (1) = 3
Empirical Formula = C3H7

6. Complete combustion of a 0. 523 g of unknown hydrocarbon produces 1
Complete combustion of a g of unknown hydrocarbon produces 1.61 g of carbon dioxide and g of water. In a separate experiment, it is determined that the molar mass of the compound is g. What is the molecular formula?
Carbon
1.61 g CO2 1 mol CO2 1 mol C = mol C
44 g mol CO2
Hydrogen
.743 g H2O 1 mol H2O 2 mol H = mol H
18 g mol H2O

7. Complete combustion of a 0. 523 g of unknown hydrocarbon produces 1
Complete combustion of a g of unknown hydrocarbon produces 1.61 g of carbon dioxide and g of water. In a separate experiment, it is determined that the molar mass of the compound is g. What is the molecular formula?
mol C and mol H =
0.0366
Carbon:Hydrogen ratio is 1: 2.26
Hydrogen (4)  9
Carbon (4) = 4
Empirical Formula is C4 H9

8. Complete combustion of a 0. 523 g of unknown hydrocarbon produces 1
Complete combustion of a g of unknown hydrocarbon produces 1.61 g of carbon dioxide and g of water. In a separate experiment, it is determined that the molar mass of the compound is g. What is the molecular formula?
Empirical Formula is C4 H9 = g/mol
Molecular Formula
114 g/mol = 2
57 g/mol
Molecular Formula = 2 ( C4 H9 ) = C8H18