In the given balanced chemical equation: 2CO + O2 -> 2CO2, option 1 states that for every mole of O2 added to the vessel, only 2 moles of CO can be added. This is incorrect, as there is no such restriction in the equation. CO and O2 can react in any ratio, as long as it follows the balanced equation.

Option 2 states that no matter how much of CO and O2 are added to the vessel, only 1 mole of O2 will be consumed. This is incorrect, as the equation clearly shows that 2 moles of CO react with 1 mole of O2 to produce 2 moles of CO2. Thus, the consumption of O2 is directly proportional to the amount of CO present.

Option 3 correctly states that when CO and O2 react, they do so in a 2:1 mole ratio. This means that for every 2 moles of CO, 1 mole of O2 is required for complete reaction.

Option 4 states that if 2 moles of CO and 1 mole of O2 are placed in a vessel, they will react to give 1 mole of CO2. This is incorrect, as the balanced equation shows