|MadSci Network: Chemistry|
Typically, for a simple molecule like CO, you can look up the standard state enthalpy of formation in a table. If you have a chemistry textbook, there should be such a table in the back of the book. If not, NIST (National Institute of Standards and Technology) has a website with thermodynamic data (check the gas phase option). According to NIST, the DfH° (CO) = -110.5 kJ/mol.
For simple molecules, the enthalpy of formation values are usually determined experimentally with very precise calorimetry measurements. Or, alternately, they could be calculated with Hess's Law, given (1) a reaction involving CO, (2) the overall change in enthalpy, and (3) the enthalpy of formation for all other molecules in the reaction. But, for a simple molecule like CO, the value above should be more accurate than the value from a direct calculation.
Your MAD Scientist,
References: NIST Chemistry Webbook (NIST Standard Reference Database Number 69). http://webbook.nist.gov/chemistry/
Try the links in the MadSci Library for more information on Chemistry.