It can not be answered without experimental data.
The mole number of helium is 8/4=2 mol. Under same condition, the volume of gas is related to the mole number of the gas. So the mole number of chlorine is also 2 mol which is 142 g.
The molecules are continually colliding with each other and with the walls of the container. When a molecule collides with the wall, they exert small force on the wall The pressure exerted by the gas is due to the sum of all these collision forces. The more particles that hit the walls, the higher the pressure.
