Please help - getting conflicting stories on V ideal V real P real P ideal anyone!


I am here to ?

Ok guys not sure if this is the place but hopefully it can help others...

regarding gases
the EK books (newest ones) say that V real is greater than V ideal (volume) and p real is less than p ideal (pressure)

The tpr books say the same thing except they claim that BOTH v real and p real are less than their ideal counterparts

How is the volume of a gas judged? the volume of the gas molecules or the space that they take up in a container...

and then after researching this subject I found something regarding that the answer depends....positive deviation due to molecular volume and negative deviation due to IM forces

can someone please clarify this with some explanation....its really bugging the heck out of me


10+ Year Member
Jun 17, 2006
I am not certain about pressure, but volume works this way: The ideal gas law assumes that gas particles take up no volume at all. Therefore the V in the ideal gas constant is the total volume that the gas is enclosed in. The Van der Waals equation makes up for this by subtracting from the total volume the volume that the particles take up. As a result, the real volume is less than the ideal volume.

Pressure also deviates from the ideal law because the ideal law assumes that there are no interactions between particles. In reality, there are collisions between gas particles and charge forces that take effect. I'm not sure whether it decreases or increases, though. Judging from Van der Waals equation, where a variable sum is added to the pressure, I'd say that real pressure is greater than the ideal pressure. However, I'm not sure if the constant is necessarily positive; I'm just using some critical thinking to reason it out here.


10+ Year Member
May 31, 2006
Yeah, i used to get confused on this concept all the time, but here goes:

Pressure ideal > Pressure real due to the fact that gas molecules have intermolecular attractive forces, which causes a decrease in tendency to collide with the container wall

Volume ideal < Volume real due to the fact that the individual gas particles do occupy some volume (which is generally seen when amount of gas increases or the volume of container decreases).

Hope that helps some! :)


nom nom nom
10+ Year Member
Apr 2, 2006
the island of Doctor Moreau
Resident [Any Field]
there are three scenarios:
Attractive forces dominant, Repulsive forces dominant, and no forces (ideal)

If the molecules are of the first type, then the pressure of the system (compared to an ideal gas) will be less... its hard to say that both the volume and pressure will be less, since theyre inversely proportional, but if you hold one constant the other has to be less.

Molecules of the second type have yet another thing to push off of (remember thats what creates the pressure of a balloon - molecules constanty ramming into the walls) and the pressure (or volume) will be increased compared to the ideal world

Theres also one more thing to consider: at extremely high pressures/low volumes, you have to consider the size of the individual gas molecules themselves...this alone will cause a deviation from the ideal PV=nRT...a positive deviation...