Q. Molecular diffusivity (DAB) of water vapour in air is 2.6×10-5 m2 s-1 over an effective distance of 3 mm. Density and coefficient of viscosity of air are 1.2 kg m-3 and 2×10-5 kg m-1 s-1, respectively. Sherwood Number (NSh) for water vapour in air is .