A closed flask contains a 0.25 moles of O2 which exerts a pressure of 0.50 atm. If 0.75 moles of CO2 is added?

It remains at 0.50 atm. One of Dalton's Laws, its specific name escapes me. It says each gas exerts its own pressure independent of the other gases in the container, so the O2 will remain at 0.50 regardless of the other gases.