Stoichiometry and Thermochemistry of Reacting Systems

CO + a O2 → b CO2, (A)

the terms a and b are called stoichiometric coefficients, which are determined using mass or atom conservation. For instance, for C atoms

b = 1, and, similarly, for O atoms

1 + a × 2 = b × 2, i.e., a = 1/2.