A refrigerated food warehouse is insulated with a layer of urea-formaldehyde foam 18 in, thick. There is some residual formaldehyde in the foam and it has a concentration of 0.03 lb formaldehyde/ft3 of foam when the material is manufactured. Formaldehyde can diffuse into the air where workers will be exposed to the vapor. The outside of the foam is covered with an impermeable layer to prevent moisture migration, so the formaldehyde can only diffuse in one direction, which is into the warehouse. There is a system available that will remove 0.4 lb/hr of formaldehyde from the air during recirculation through the refrigeration system. It will return the air to the warehouse with essentially no formaldehyde, and you can assume the concentration of formaldehyde to be zero in the warehouse as long as the removal system is not overloaded. The warehouse has 35,000 ft2 of insulated surface and the effective diffusivity of formaldehyde through the foam is 1.5e-8 ft2/sec. (a) At the time when the removal system just begin to keep the air free of formaldehyde, determine the mass flux of formaldehyde at the surface of the foam next to air. (b) During the period when the semi-infinite model is valid, determine the concentration of formaldehyde at the inner surface (next to the impermeable layer) of the foam. (c) During the period when the semi-infinite model is valid, determine the mass flux of formaldehyde at the inner surface of the foam.