At STP, temperature is 273.15 K and the pressure is at 1 atm. The volume occupied by the system given above can be calculated by using the ideals gas equation assuming that O2 is an ideal gas.
PV = nRT V = nRT / P V = 5.00 mol (0.08206 L atm / mol K ) (273.15) / 1 atm V = 112 L
