An old-fashioned glass apothecary jar contains a patent medicine. The neck is closed with a rubber stopper that is 20 mm tall, with a diameter of 10 mm at the bottom end, widening to 20 mm at the top end. The molar concentration of medicine vapor in the stopper is 2 x 10^-3k mol/m³ at the bottom surface and is negligible at the top surface. If the mass diffusivity of medicine vapor in rubber is 0.2 x 10^-9 m²/s, find the rate (k mol/s) at which vapor exits through the stopper.