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Estimate the mass of liquid air evaporated in an hour from a poorly pumped Dewar vessel, if the air pressure (at a temperature of $293$ K) remaining between the vessel walls is $0.133$ Pa. The surface of the vessel is $600$ $cm^2$ , the specific heat of vaporization of liquid air is $0.2$ MJ / kg, its temperature is $81$ K. The gap between the vessel walls is small compared to the free path of the molecules.