A freshly prepared sample of a certain radioactive isotope has an activity of 15.0 mCi. After 12.00 h, its activity is 2.00 mCi.

(a) Find the decay constant in second. (b) Calculate the atoms of the isotope were contained in the freshly prepared sample. (c) Calculate the sample's activity 20.0 h after it is prepared, in mCi.

Constants: Speed of light c = 3.0 x 108 m s-1 Avogadro's number NA = 6.02 x 1023 mol-1 Planck constant h = 6.63 x 10-34 J s Boltzmann constant k = 1.38 x 10-23 J K-1 Permittivity of free space to = 8.85 x 10-12 F m-l Permeability of free space /do = 4x x 107 H ml Basic charge e = 1.6 x 10-19 C Electron rest-mass me = 9.1 x 10-31 kg Proton rest-mass mp = 1.6725 x 102 kg = 1.0072766 u Neutron rest-mass mn = 1.6748 x 1027kg = 1.0086654 u Bohr's radius a = 5.3 x 10-1 m 1 eV =1.6 x 10-19 J lu = 931 MeV c-2 1 barn = 10-28 m2 1 fm = 10-15 m 1 Ci =3.7 x 1010 s-1