The pressure gauge on a tank registers the gauge pressure, which is the difference between the interior and exterior pressure. When the tank is full of oxygen (O2), it contains 15.0 kg of the gas at a gauge pressure of 41.0 atm. Determine the mass of oxygen that has been withdrawn from the tank when the pressure reading is 28.0 atm. Assume the temperature of the tank remains constant. I though you could use the Ideal gas law, converting 15kg into moles, but would the volume be considered constant here? Because if not, then I do not see how you could apply it to PV= nRT.