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

- 16.0 g
- 12.0 g
- 8.0 g
- 4.0 g

**Answer**

t_{1/2} = 3 hours

n = T/t_{1/2} = 18/3 = 6

N = N_{o}(1/2)^{n} = 256(1/2)^{6} = 4 gm

**The correct option is D.**