Read Why Real Gases Deviate from Ideal Behaviour
Real-gas questions test one idea from two directions — the compressibility factor Z tells you whether attractive or repulsive intermolecular forces dominate at the given condition, and kinetic theory tells you how speed, energy and pressure connect back to temperature and molar mass.
A phase change alters the arrangement and motion of particles without creating a new substance.
Particles change their freedom of movement
Heat → particles move more freely
Heating increases particle motion: solid → liquid → gas. During a phase-change plateau, energy separates particles instead of raising temperature.
Reading the compressibility factor Z
Z < 1 means the gas occupies less volume than ideal — attractive intermolecular forces dominate, pulling molecules closer together. Z > 1 means repulsive forces dominate, typically at high pressure where molecular volume itself becomes significant.
Kinetic energy depends on T alone; speed depends on T and molar mass
Average kinetic energy of a gas depends only on absolute temperature, identical for any ideal gas at the same T regardless of molar mass. Average speed, by contrast, depends on both T and molar mass (lighter molecules move faster at the same T) — don't let a molar-mass difference imply a kinetic-energy difference.
Boiling point tracks intermolecular force strength
Comparing boiling points across substances is really comparing intermolecular force strength (hydrogen bonding > dipole-dipole > dispersion, roughly), not molecular weight alone — a lighter molecule with stronger intermolecular attraction can boil higher than a heavier one without it.
Graham's law of effusion: lighter gases escape faster
At the same temperature and pressure the rate of effusion of a gas is inversely proportional to the square root of its molar mass, rate1 / rate2 = sqrt(M2 / M1).
Subtracting nickel (58.7) leaves 109.3 g/mol for the carbonyl groups, and dividing by 28 per CO gives x of about 3.9, which is the nearest whole-number picture of Ni(CO)4. Always square the rate ratio before multiplying by the molar mass, and remember the heavier gas is the slower one.
OlympIQ exam lens
- ✓Z < 1 → attractive forces dominate; Z > 1 → repulsive forces / molecular volume dominates.
- ✓Average kinetic energy depends only on T, the same for any gas at that T.
- ✓Average speed depends on both T and molar mass — lighter molecules move faster at equal T.
- ✓Boiling point ranks by intermolecular force strength, not simply by molecular weight.
- ✓Graham's law: rate1/rate2 = sqrt(M2/M1); square the rate ratio to get the ratio of molar masses, and the heavier gas effuses more slowly
Don't confuse kinetic energy with speed — two gases at the same temperature always have equal average kinetic energy, even though their average speeds differ by molar mass.
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