A tube 2 m long and 10-4 m2 in cross section contains CO, at atmospheric pressure and at a temperature of 300 K. The carbon atoms in one-half of the C02 molecules are the radioactive isotope C”. At time t =0, all of the molecules at the extreme left end of the tube contain radioactive carbon, and the number of such molecules per unit volun1e decreases uniforn1ly to zero at the other end of I he tube. (a) What is I he initial concentration gradient of radioactive molecules? (b) Initially, how many radioactive molecules per second cross a cross section at the midpoint of the tube from left 10 right? (c) How n1any cross fron1 right to left? (d) What is the initial net rate of diffusion of radioactive molecules across the cross section, in n1olecules per second and micrograms per second?