The Rutherford-Soddy law of radioactive decay is :
#N(t)=N_0 e^(-\lambdat); \qquad \lambda = \ln(2)/t_{1/2}#.
Given : #t_{1/2} = 33 sec; => \lambda = 2.1004\times10^{-2} sec^{-1}#
#t=12.5 days=1.08\times10^6 sec; #
#(N(t))/N_o = e^(-\lambdat) = 0.3679\times10^{-5}#
#N_o-# Number of As-81 nuclei at the beginning (#t=0#),
#N(t) -# Number of As-81 nuclei after time #t#,
#M_o-# Mass of As-81 at the beginning (#t=0#)
#M(t) -# Mass of As-81 nuclei after time #t#,
#(M(t))/M_o = (N(t))/N_o = 0.3679\times10^{-5};#
#M(t) = 0.3679\times10^{-5}\timesM_o=0.3679# milli-grams
