Van der Waals Equation Calculator
Real gas pressure, and how far it strays from ideal.
Work out Van der Waals Equation. Real gas pressure, and how far it strays from ideal. Written for the problem set you are stuck on.
CO₂ 3.592, NH₃ 4.170, N₂ 1.390, O₂ 1.360, H₂ 0.2453, He 0.03412.
CO₂ 0.04267, NH₃ 0.03707, N₂ 0.03913, H₂ 0.02651, He 0.02370.
Real gas pressure
34.642 atm
-22.72% against the ideal gas law
The two corrections pull opposite ways, and which one wins is a property of the gas as much as the conditions. Here the attraction term is winning, so the real pressure is below ideal — molecules pulling on each other arrive at the wall with less momentum. That is typical of gases with strong intermolecular forces such as CO₂ and ammonia. Both corrections vanish at low pressure and high temperature, which is exactly when the ideal gas law is safe to use.
How the Van der Waals Equation Calculator works
Enter moles, volume, temperature and the two van der Waals constants to get the real gas pressure alongside the ideal figure. The page shows both correction terms separately, because which one dominates is what decides whether a gas reads above or below ideal.
Also known as: real gas calculator · non ideal gas calculator · van der waals equation of state · deviation from ideal gas calculator
Frequently asked questions
What is the van der Waals equation?
(P + an²/V²)(V − nb) = nRT. The a term corrects for attraction between molecules and the b term for the volume they occupy themselves.
Why do some gases read above ideal and others below?
Because the two corrections oppose each other. Attraction lowers pressure below ideal, excluded volume raises it above. CO₂ and ammonia have strong attractions and read low; hydrogen and helium barely attract and read high.
When can I just use PV = nRT?
At low pressure and high temperature, meaning well above the boiling point and below roughly 10 atm. Under those conditions both corrections are small and the ideal gas law is accurate to well under a percent.
What do a and b actually mean?
a is a measure of intermolecular attraction, in L²·atm/mol², and it tracks polarity and molecular size. b is the excluded volume per mole, in L/mol, roughly four times the actual molecular volume.
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