Van der Waals Real-Gas Pressure & Z Factor

Real-gas pressure from the van der Waals equation of state (P + a n^2/V^2)(V - n b) = nRT, plus the compressibility factor Z = PV/(nRT). Pick a gas (helium, hydrogen, nitrogen, oxygen, air, carbon dioxide, methane, ammonia, water vapor) and enter moles, volume, and temperature; it returns the real pressure, the ideal-gas pressure for comparison, Z, and the percent deviation. The a (attraction) and b (excluded-volume) constants are the CRC Handbook values. Z drifts furthest from 1 at high pressure and low temperature -- exactly where PV = nRT misreads a compressed-gas cylinder or a sealed vessel. Below 1 attraction dominates (near condensation); above 1 the molecules' own volume dominates. A first-principles chemistry aid; near the critical point a fuller EOS governs.

(P + a n^2/V^2)(V - n b) = nRT, solved for pressure: P = nRT/(V - n b) - a n^2/V^2; R = 0.0820573 L*atm/(mol*K), T in kelvin; a in L^2*atm/mol^2, b in L/mol per gas; compressibility Z = PV/(nRT); deviation vs ideal = (P_real - P_ideal)/P_ideal.

The van der Waals equation of state (van der Waals, 1873); a and b constants from the CRC Handbook of Chemistry & Physics. First principles.

The van der Waals equation is public first-principles chemistry; the a and b constants are published in the public-domain CRC Handbook; moles, volume, and temperature are the user's own measurements.

Verify protocol against your lab's SOP before pipetting. A miscalculated dilution can ruin a run or a sample.

Field names used by the API: gas, moles, volume_l, temperature_c, pressure_real_atm, z_factor

• van der Waals EOS (P + a n^2/V^2)(V - n b) = nRT; R = 0.0820573 L*atm/(mol*K); T in kelvinvan der Waals, 1873
• a and b constants per-gas attraction (a) and excluded-volume (b) constantsCRC Handbook of Chemistry & Physics (public domain)
• Scope single-phase gas; near the critical point or two-phase region a fuller EOS governsscope of this tile