An old-fashioned glass apothecary jar contains a patent medicine. The neck is closed with a rubber stopper that is \(20 \mathrm{~mm}\) tall, with a diameter of \(15 \mathrm{~mm}\) at the bottom end, widening to \(20 \mathrm{~mm}\) at the top end. The molar concentration of medicine vapor in the stopper is \(2 \times 10^{-3} \mathrm{kmol} / \mathrm{m}^{3}\) at the bottom surface and is negligible at the top surface. If the mass diffusivity of medicine vapor in rubber is \(0.15 \times 10^{-9} \mathrm{~m}^{2} / \mathrm{s}\), find the rate \((\mathrm{kmol} / \mathrm{s})\) at which vapor exits through the stopper.