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

[ariana0923]

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..

 

[negitron]

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.

 

[ariana0923]

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?

 

[xxChrisxx]

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.