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Home Concept Builders Gases and Gas Laws Pressure Concepts Help Help 5

Pressure Concepts - Questions 5 Help

A sample of gas has a pressure due to the collisions of moving gas particles with the container walls. This pressure can be understood at the particle level. Each collision of a gas particle with the container wall results in a force on the wall. Because there are so many particles colliding so frequently with the container wall, there is enough force that accumulates to result in a noticeable pressure. Any variable that effects the ratio of the cumulative amount of force per unit of wall area will effect this pressure.

There are two very similar versions of questions in this Question Group. One of the versions is shown below.

Version 1
Container A and Container B are the same size; their volume and surface area is the same. The speed with which the particles move (on average) is the same for both containers. The number of gas particles in each container is the same. But the type of gas in the two containers is different. The gas particles in Container A are less massive particles than those in Container B. Which container will have the greatest gas pressure?
Gases push! They push outward upon the walls of the container that contains them. And this pushing is the result of collisions. Each collision of a gas particle on a container wall contributes to an overall amount of force on the wall. By definition, the pressure is the total force divided by the area of the container walls. So to maximize pressure, this ratio of force per area would have to be maximized. The larger the ratio ... the larger the pressure.

So exactly how can this ratio of force per area be maximized. Of the two components of the pressure ratio, the area is easiest to explain. Making the area as small as possible makes the pressure as large as possible.

The force component of this ratio is more difficult to explain. But to keep it simple, the pressure is maximized if the overall and total forcefulness of the collisions of gas particles with container walls is increased. The forcefulness of the collisions can be increased by collisions that ...
 
  • occur at greater speeds.
  • involve more massive particles (assuming the same or greater speed).
  • involve more particles doing the colliding rather than less particles.
  • occur more frequently (either because there are more particles or the particle speed is faster and they can traverse the distance from wall to wall in less time).

In this question, all variables (particle speed, container wall areas, and number of particles) are the same for the two containers with the exception of the particle mass. The pressure will be greater for more massive particles (assuming the same particle speed). A collision of a more massive particle with a container wall will be a more forceful collision.



In case you were wondering (... or objecting), the condition described for the two containers in this question is possible. The conditions is that one container has more massive particles than the other container but those particles move at the same speed. This is possible provided that the two containers are at different temperatures. For two different gas particles at the same temperature, the more massive particles would have the slower speed. But that's not the case in this question, so we can presume that the temperature of Container A and Container B are different.
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