Chemical Kinetics

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

  1. 16.0 g
  2. 12.0 g
  3. 8.0 g
  4. 4.0 g

Answer

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.