Rule of thumb · Materials and ElementsNº 97 / 238

Common gases well above their critical temperature are ideal to about 1 percent below ten atmospheres

When is the ideal gas law good enough?

Nitrogen and oxygen at 20 °C are within about 1 percent of ideal below 10 atm (Z = 0.993 for nitrogen at 10 atm), but carbon dioxide, nearer its critical temperature, is 4.5 percent low there (Z = 0.955).

Why it works

The ideal law ignores the size of the molecules, which pushes the pressure up, and the pull between them, which pulls it down. At modest pressure and room temperature the pull wins a little for most gases and Z is just below 1; at high pressure the size wins and Z rises above 1 (helium, which has almost no pull, is 1.19 at 200 atm).

When it fails

These are van der Waals figures, good to a few percent away from the critical point and wrong near it; for design use the measured data or a proper equation of state (the NIST tables). The law also says nothing about a gas that liquefies: carbon dioxide at 20 °C turns to liquid at about 57 bar.

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

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