Pressure inside a box with only a single molecule Suppose that we have a cube with dimensions $25 \times 25 \times 25$ centimeters containing a single hydrogen molecule. How can we calculate the pressure within the cube?
 A: Pressure would not be defined in this case since it is a statistic measure of the force a big number of molecules apply on surfaces when they randomly collide with it. The property of pressure depends indeed on the assumption of a big number of particles involved such that all the macroscopic values converge on the average/expected value and the velocities of singles molecules becomes irrelevant.
So in this case what we call pressure would be the force the hydrogen apply on the cube faces when it collides with them, this force would be discontinuous and would not have any pressure-like symmetry, well the answer I would say is indeed that pressure is not defined in this case since it is only a macroscopic statistical phenomenon.
A: 
I have drawn the velocity vs time diagram for a single molecule bouncing back and forth in a 1D box. Assume the particle keeps moving with some constant speed $v$ between bounce. Since the wall imparts some force on the molecule to change its direction, that is the only place where the velocity changes. In the above image, $T$ is the time between consecutive bounces and $t_c$ is the finite time required to change the velocity from $v$ to $-v$.
Thus the 'average' force from the wall on the molecule would be 
$$F=\dfrac{2mv}{t_c}.$$
Thus to answer your question, we cannot calculate force if you do not know $t_c$; and this is something that you haven't provided in your question. Note that technically there is no link between $T$ and $t_c$. This is because $t_c$ depends on the walls of the container and the molecule in question, while $T$ depends on the length of the container and the speed of the particle. These two quantities are independent. 
Thus your question is not well-posed ergo expecting an answer is wrong.
