Final answer:
The 2 in 2 CO indicates that there are two molecules of carbon monoxide involved in the chemical reaction.
Step-by-step explanation:
The 2 in 2 CO represents the coefficient in a chemical equation which indicates the number of molecules involved in the reaction. This coefficient means that there are 2 molecules of carbon monoxide (CO) reacting. In the case of collisions and reaction rates, such as when carbon monoxide is reacting with oxygen to form carbon dioxide (2 CO(g) + O₂(g) → 2 CO₂(g)), the coefficient ensures that the law of conservation of mass is observed. The ratio implies that for the reaction to be balanced, two molecules of carbon monoxide will react with one molecule of oxygen to produce two molecules of carbon dioxide.