The half life of a radioactive isotope is three hours. If the initial mass of the isotope were 256 g, the mass of it remaining undecayed after 18 hours would be
t1/2 = 3 hours
n = T/t1/2 = 18/3 = 6
N = No(1/2)n = 256(1/2)6 = 4 gm
The correct option is D.