Consider the steady, uni-directional diffusion of a binary mixture of A and B across a vertical slab of dimensions 0.2 m × 0.1 m × 0.02 m as shown in the figure. The total molar concentration of A and B is constant at 100 mol m⁻³. The mole fraction of A on the left and right faces of the slab are maintained at 0.8 and 0.2, respectively. If the binary diffusion coefficient D_AB = 1 × 10⁻⁵ m² s⁻¹, the molar flow rate of A in mol s⁻¹, along the horizontal x direction is