A rigid tank of 80(ft)3 capacity contains 4,180l (lbm) of saturated liquid water at 430(oF). This amount of liquid almost completely fills the tank, the small remaining volume being occupied by saturated-vapor steam. Because a bit more vapor space to the tank is wanted, a valve at the top of the tank is opened, and saturated-vapor steam is vented lo the atmosphere until the temperature in the tank falls to 420(oF). Assuming no heat transfer to the contents of the tank, determine the mass of steam vented?