From ideal gas expressions; n = PV/RT = (4.24*1000*450)/(8.314*303) = 757.4 moles
Now, 75% of 757.4 moles will evaporate leaving 20%. Then, 25% of 757.5 moles... 25% of 757.4 moles = 25/100*757.4 = 189.35 moles Mass of 189.35 moles = 189.35 moles*18 g/mol = 3408.3 g ≈ 3.4 kg