A thick wood has an initial water content of 100 mol/m3. The top surface of the slab is exposed to a wet air stream such that the water concentration at the surface is kept at 2 kmol/m3. Assuming the slab is semi-infinite, the diffusion coefficient of water vapor in wood is 1.2x10-5 m2/s, the slab dimensions (LxWxt) are 200 x 50 x 20 cm, estimate:

a. The water concentration 5 cm below the surface at 1, 5, and 10 min.

b. The time required for the water concentration 10 cm below the surface to reach half the surface concentration.