Why does mass not affect gas pressure in ideal gas law?

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Discussion Overview

The discussion revolves around the relationship between mass and gas pressure in the context of the ideal gas law. Participants explore how different gases, regardless of their molar mass, produce similar effects on pressure when other variables are controlled, such as temperature and volume.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant notes that in a virtual lab, both light and heavy gases produced the same effect on a piston, prompting a question about why mass does not seem to affect gas pressure.
  • Another participant explains that the parameters of gas laws include temperature, pressure, volume, and molarity, asserting that a mole is a mole regardless of the gas type.
  • It is suggested that while molar mass might intuitively seem relevant, a heavier molecule with a given kinetic energy (KE) moves more slowly than a lighter molecule with the same KE.
  • A later reply emphasizes that mass does not affect pressure because gas measurements account for mass through other variables, particularly temperature, which reflects the average kinetic energy of gas molecules.

Areas of Agreement / Disagreement

Participants express varying degrees of understanding regarding the role of mass in gas pressure, with some confusion noted about the implications of kinetic energy and molecular weight. No consensus is reached on the underlying reasons for the observed phenomena.

Contextual Notes

Participants reference the ideal gas law (PV=nRT) and discuss the relationship between temperature and kinetic energy without resolving the complexities of how these variables interact with mass.

ariana0923
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So, I was playing around with this virtual lab online. I could add light gas or heavy gas to try to make the piston move. I got the same results using both gases..in other words, it took the same amount of light gas to move the piston as it did using heavy gas. Why is that? One would think it would make a difference..
 
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Nope. The parameters for the gas laws are temperature, pressure, volume and molarity. A mole is a mole, regardless of the gas; likewise volume. Temperature is a measure of the average kinetic energy of the gas particles--at first glance, it might seem that molar mass would make a difference here, but a heavy molecule with a given KE will move more slowly than a light molecule with the same KE. If you can find a simulation which tracks average particle velocity, you';ll see what I mean.
 
negitron said:
it might seem that molar mass would make a difference here, but a heavy molecule with a given KE will move more slowly than a light molecule with the same KE. .


...that confused me because I thought the mass DIDN'T make a difference?
 
Mass doesnt, because you are measuring the gas in terms of other variables that effectively take into account the mass.

From ideal gas: PV=nRTAs negitron was saying the Temperature is a way of describing the kinetic energy of the molecules. A for a heavy molecule to have the same KE as a light molecule it must be traveling slower.

As pressure depends on the Temp (average energy) of the gas, we don't care about how much it weighs, only the energy it has.
 

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