Determine the % C and % H in an unknown compound.
Determine the empirical formula of the unknown compound.
Determine the molecular formula of the unknown compound.
Carbon-Hydrogen (CH) analysis is performed by burning the unknown sample. As a result of the complete combustion of the compound, all of the carbon in the compound is converted to carbon dioxide gas and all of the hydrogen in the compound is converted to water vapor.
CxHyA + z O2 (g) → x CO2 (g) + y/2 H2O (g) + A
The combustion is conducted in the presence of excess oxygen ( z >> x + y/2 ). The symbol A represents other elements in the system, which burn to produce either a solid residue or an unreactive gas (unreactive for the purposes of this analysis).
The sample is typically wrapped in a tin capsule and inserted into a furnace held at 1200oC. A steady stream of an unreactive gas (e.g., helium) is passed through the furnace and a burst of pure oxygen gas is added as the sample capsule is inserted into the oven. As the sample burns, the temperature rises to approximately 1700oC. The furnace contains a catalyst (Cr2O3) that facilitates the complete combustion of the sample. Copper turnings are also present to ensure that any nitrogen in the sample is reduced to N2, which is unreactive.
As the gas leaves the furnace, the carbon dioxide and water vapor produced by the combustion reaction are carried out of the furnace. The gas stream first passes through a desiccant, typically Mg(ClO4)2, which removes all moisture from the gas stream. Next the gas stream passes through a tube containing NaOH, which removes all carbon dioxide from the gas stream (forming NaHCO3).
By weighing the tubes containing Mg(ClO4)2 and NaOH before and after the combustion reaction, it is possible to determine the amounts of water and carbon dioxide produced by the combustion reaction and thus the amounts of hydrogen and carbon in the unknown compound.