An oxygen cylinder of volume 30 litres has an initial gauge pressure of 15 atm. And a temperature of `27^(@)C`. After some oxygen is withdrawn from the cylinder, the gauge pressure drops to 11 atm. And its temperature drops to `17^(@)C`. Estimate the mass of oxygen taken out of the cylinder. `(R = 8.1 J "mole"^(-1) K^(-1)`, molecular mass of `O_(2) = 32 u)`.