A collection of gas particles in a container held at a certain temperature (T) will act such that they will collide with each other as well as with the walls of the container. The translational motion of the gas particles manifests as their kinetic energy (KE) which in turn is related to the gas temperature by the following relation:
-----(1)
where k = Boltzmann constant
Thus, based on equation(1) the average KE is directly proportional to the temperature.
So in one hour half the amount remains (that's what half life means). In two hours 1/4 (or half of half) would remain and in three hours 1/8 would remain. So the answer is 1/8