Consider an irreversible first-order heterogeneous reaction inside a porous spherical pellet that is coated on the inside with platinum. Let the reactant concentration halfway between the external surface and the center of the pellet (r = R/2) be equal to one-tenth of the concentration of the pellet's external surface. At the external surface, the concentration is 0.001 mol/L, the effective diffusion coefficient is 0.1 cm²/s, and the diameter is 0.002 cm. Assume that the pellet is isothermal. What is the reactant concentration at the center of the pellet?