Question

why can it be assumed that the total pressure inside the gas collection tube equal to the atmospheric pressure outside of the tube

Answer 1

the layer of atmosphere we live in called troposphere and it is the most densest in terms of gases. As you move up the atmosphere the volume of gases decreases. The way gases behave in the troposphere is very odd, some of the particles of gases vibrate, some rotate and some go in straight line/direction. Gases in the troposphere are the only type of matter to have the most kinetic energy because these particles of gases have so much space to move around that they are literally everywhere, even in places that are quite difficult to get to, for example when you pour water in a empty glass, you automatically assume that the glass is empty but at a molecular level there are tiny particles of different types of gases all moving and bumping around. When you pour water in a glass, what happens is that water pushes all the gases out of the glass and into the atmosphere but not all of the gases gets thrown out, some gases in the form of tiny bubbles gets trapped in the water, they can't really move because the surface tension of the water is so great that it doesn't allow the gas bubbles to escape, it can't really react with water either, so how do these gas bubbles escape, we need to realize that the atmospheric pressure inside the glass is very low compare to outside pressure, so when you give the container a quick shake, the bubbles gain just enough kinetic energy to move to the top and eventually escape from the bottle. Coming back to your point why do we assume that the pressure inside a tube is equal to the outside tube. If the tube is an open top, it's obvious to say that the gases take up the space of an empty container and it fills it with atmospheric gases which includes Nitrogen, Oxygen, Argon and more. So next time when you see an empty container, realize that it contains all the gases there are in the troposphere but on a much smaller scale. The reason it is considered the same is because as i've already given you an example of water in a glass.....meaning if you have an open container, the atmospheric difference of gases will be the same. It means that the pressure that inside the container is the same as outside of the container. In a different situation if you have an empty sealed barrel, even in that kindda situation the pressure inside the barrel will be the same as the outside pressure. When you start to takeout the reaming of the gases from an "empty barrel" you are basically creating a vacuum in the barrel.....meaning if you keep on taking air out of the container and go beyond the critical point of a barrel to stand against the pressure or vacuum what will happen is that the pressure inside the container is gets very low and the out pressure is very high in this process the outside pressure of the gases pushes the walls of the barrel inwards and now because there is no pressure inside the container, it collapse.

Answer 2

sorry if i said too much, it's a review for me too. :)