IB Physics HL B.3.8 Ideal gas approximation limits Question Bank

Practise IB Physics HL B.3.8 by evaluating real-gas limits through pressure, density, temperature, liquefaction and intermolecular forces.

Exam points

Evaluate how pressure, density and temperature determine whether real-gas behaviour approximates the ideal model.
Explain deviations through finite molecular volume, intermolecular forces, inelasticity and liquefaction.

[Maximum number: 2]

The equipment shown in the diagram was used by a student to investigate the variation with volume, of the pressure p of air, at constant temperature. The air was trapped in a tube of constant cross-sectional area above a column of oil.

Figure for Question B.3.8—Ideal gas approximation limits question 1 — IB Physics HL

The pump forces oil to move up the tube decreasing the volume of the trapped air.

The equation in (b) may be used to predict the pressure of the air at extremely large values of $\frac{1}{H}$ . Suggest why this will be an unreliable estimate of the pressure.

very large $\frac{1}{H}$ means very small volumes / very high pressures at very small volumes the ideal gas does not apply

OR at very small volumes some of the assumptions of the kinetic theory of gases do not hold

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B.3.8—Ideal gas approximation limits question 1