19. At a particular temperature, N2O5 decomposes according to a first-order rate law with a half-life of 3.0 s. If the initial concentration of N2O5 is 1.0 x molecules/cm³, what will be the concentration (in molecules/cm³) after 12.0 s? (A)6.25 (B)25.0 (C)12.5 (D)50.0 (E)3.13