This question already has an answer here:
Consider a cube shaped container with one mole of gas inside.
Acceleration due to gravity would give the difference in force between the top of the container and the bottom to be mg, with net force mg on the bottom, where m is the mass of the molecule.
I think I understand this idea, but what I am confused about is that at any one time, only a subset of the molecules collide with the bottom of the container at any instance in time; therefore, only a subset of all the molecules are contributing to the net difference in force - some, for example are colliding with the sides, some occupy the space within the container. So is the mass of the gas, determined by weighing, less than the actual mass of the gas?
What if I changed the shape of the container, so that the surface area in contact with the balance is greater or smaller? The pressure per unit area is the same for all sides, but the overall force on that surface is different.
I hope that makes sense, I would appreciate any help with this.