Questions on pressure and non-ideal gases

by Bobcent
Tags: gases, nonideal, pressure
Bobcent is offline
May8-13, 07:12 AM
P: 31

I have two questions.

First, how do I calculate the pressure of a non-ideal gas if I know volume, temperature and the number of moles? The gas in question is water vapor. I suspect a constant for the specific gas will be involved. Which constant is this?

And secondly, how do I calculate how much energy is stored in a pressurized gas if I know the mass (or the number of moles) and the pressure of the gas?

Thanks in advance!
Phys.Org News Partner Physics news on
The hemihelix: Scientists discover a new shape using rubber bands (w/ video)
Mapping the road to quantum gravity
Chameleon crystals could enable active camouflage (w/ video)
Nugatory is offline
May8-13, 07:53 AM
Sci Advisor
P: 2,972
Googling for "non-ideal gas presure" brings up a bunch of links. Maybe start with ?
Bobcent is offline
May8-13, 11:36 AM
P: 31
Quote Quote by Nugatory View Post
Googling for "non-ideal gas presure" brings up a bunch of links. Maybe start with ?
Thanks for your post, good explanation! The problem is that I don't seem to be getting the right answer.

Water vapor has a density of 0.6 kg/m3 at 100 degree celsius (373 degree kelvin) and standard atmospheric pressure.

So if I put in all the values for water vapor except pressure, I should get 1 atm as the answer, but instead I get approximately 0.032 atm.

These are the values I used:

P = Unknown
V = 1 Liter
a = 5.464 According to the website you linked
n = 0.6/1000/18 = 3.33*10^-5 moles
b = 0.0305 According to the website you linked
R = 0.08260 According to the website you linked
T = 373 Degree kelvin

Done the calculation several times, don't understand whats wrong...

Thankful for help!

nasu is offline
May8-13, 01:03 PM
P: 1,909

Questions on pressure and non-ideal gases

It's difficult to say what is wrong with your calculation if you don't show it.

But the corrections to the ideal gas equation for the given parameters are really small.
The ideal gas gives approximately 1 atm and so should the van der Waals equation.
There is an error in the equation on this site, I believe. It should not be P squared, it does not make sense (dimensionally).
colliflour is offline
May9-13, 01:34 AM
P: 20
Using an equation of state for a substance with hydrogen bonding, especially water, is never going to give you a good answer. Regardless, I looked up the specific volume of saturated water vapor at 1atm and 373.15K using the NIST steam tables, which was .0332 mol/L and used the ideal gas law to find pressure: .082057*373.15*.0332=P=1.0166 atm. You could use the virial equation or compressibility factor, but why bother. Water has tabulated values, make use of em!
Bobcent is offline
May9-13, 11:59 AM
P: 31
Thanks for the help everyone, got it right now! :)

Register to reply

Related Discussions
Two questions on ideal gases and heat Introductory Physics Homework 4
Quick questions about ideal gases Introductory Physics Homework 1
Voltage Vs Pressure: Ideal Gases- not so ideal? General Physics 0
Ideal Gases/Pressure Introductory Physics Homework 8
and for my next trick...more ideal gases questions. the joy. Introductory Physics Homework 2