When a nitrogen-containing organic compound is heated with copper oxide in an atmosphere of carbon dioxide, the main product obtained is free nitrogen gas (N2). This reaction is known as the "Dumas method" or "organic nitrogen determination."

Explanation:
1. Dumas Method:
The Dumas method is a technique used to determine the nitrogen content in organic compounds. It involves the combustion of the organic compound in the presence of copper oxide (CuO) and a stream of carbon dioxide (CO2) gas. The reaction proceeds as follows:

Organic compound + CuO + CO2 -> N2 + H2O + Cu

2. Copper Oxide (CuO):
Copper oxide is a catalyst in this reaction. It helps in the decomposition of the organic compound and aids in the conversion of nitrogen-containing compounds to free nitrogen gas.

3. Carbon Dioxide (CO2):
The presence of carbon dioxide in the reaction atmosphere provides an inert environment and prevents the oxidation of nitrogen gas to nitrogen oxides. It also helps in the efficient combustion of the organic compound.

4. Nitrogen (N2):
The primary product obtained from this reaction is free nitrogen gas. This is because the nitrogen present in the organic compound is converted to nitrogen gas, which is then released into the atmosphere. This reaction is based on the principle that nitrogen does not form stable compounds with copper oxide under these conditions.

5. Other Products:
Apart from nitrogen gas, the reaction also produces water vapor (H2O) and copper (Cu). Water is formed due to the combustion of hydrogen present in the organic compound, while copper is a byproduct of the reaction between copper oxide and carbon.

In conclusion, when a nitrogen-containing organic compound is heated with copper oxide in an atmosphere of carbon dioxide, the main product obtained is free nitrogen gas. This reaction is commonly used in the Dumas method for the determination of nitrogen content in organic compounds.