(a)[2]
Define the term radioactive half-life.
(b(i))[3]
Calculate the number of iodine-131 atoms present in $1.0\,\text{kg}$ of this water.
(b(ii))[2]
Since $1.0\,\text{mol}$ of water has a mass of $18\,\text{g}$, calculate $$\frac{\text{number of molecules of water in }1.0\,\text{kg of water}}{\text{number of atoms of iodine-131 in }1.0\,\text{kg of contaminated water}}.$$
(c)[3]
The permitted activity limit for iodine-131 in water has been fixed at $170\,\text{Bq kg}^{-1}$. Calculate the time, in days, for the activity of the contaminated water to fall to this acceptable level.