How are gases all so similar unlike any other state of matter?

For ex:

1) 1 mole of any gas will occupy 22.4 litres at atmospheric pressure. (Cannot be said of all solids/ liquids)

2) partial pressure of any gas is directly proportional only to mole fraction. ( even weight of atoms is irrelevant.)

I guessed it would be due to negligible intermolecular attractions but I'm not too clear. Could someone explain.

  • 2
    $\begingroup$ Those would only be true if you assume that they are ideal gases, which by definition have no intermolecular forces. For real gases the molar volumes etc. would be a little different. These results can be derived from kinetic theory of gases or statistical mechanics - if you go through the working, you will find that the molar mass of the gas does not come into play. $\endgroup$ – orthocresol Nov 25 '15 at 10:30
  • $\begingroup$ @orthocresol how would you explain pressure being independent of weight of gas? Even in ideal condition gases have different weights, right? $\endgroup$ – Mahathi Vempati Nov 25 '15 at 10:34
  • $\begingroup$ Well, it's not easy to explain qualitatively. If you really wanted to know, then you should look for a derivation of the ideal gas law. A handwavy argument would go, if the molar mass increases, the force exerted by the gas molecules upon collision with the walls would be larger; but the gas molecules are also on average moving more slowly since they are heavier, which reduces the force. These factors balance exactly as long as the temperature (which is a measure of average kinetic energy of a molecule) is kept the same. $\endgroup$ – orthocresol Nov 25 '15 at 10:43
  • $\begingroup$ Related: Gas Pressure and Molecular Mass $\endgroup$ – user7951 Nov 25 '15 at 14:22

The way to get a handle on why gases are all similar is to think about the relationships between how the particles of the gas behave and the macro properties temperature and pressure.

The temperature is a measure of the average kinetic energy of the particles. The pressure is a consequence of the particles of the gas bouncing off the walls of the vessel the gas is contained in or the surfaces in contact with the gas. In a gas made of heavy particles the average speed of the particles will be much lower than for a gas made of light particles. When you do the calculations about the effect at a surface (where the pressure is a result of the particles bouncing elastically off the surface) the effects of the mass cancel out because the heavier particles travel more slowly at the same temperature. Overall this means that, for an ideal gas, the mass of the particles doesn't matter for the pressure and all gases behave the same regardless of the particles they are made from.


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