Chemistry / Chem 1343 · Procedure · 60–90 seconds
Calculating Pressure with the van der Waals Equation
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Calculating a real gas's pressure with the van der Waals equation means substituting moles, volume, temperature, and that gas's own size and attraction constants into the corrected equation, then comparing the result against the ideal gas law's prediction for the same sample.
A 4.25-L flask holds 3.46 mol CO2 at 229 °C (502 K). The ideal gas law predicts about 33.7 atm. Substituting CO2's van der Waals constants (a = 3.59 L²·atm/mol², b = 0.0427 L/mol) into the corrected equation gives about 32.4 atm instead, a modest downward correction from intermolecular attraction.
Expecting a large gap between the two calculated pressures at every condition ignores that the van der Waals correction is small whenever pressure is not very high and temperature is not very low.