What is the value of R in different units?

R = 8.314 J/mol·K = 0.0821 L·atm/mol·K = 1.987 cal/mol·K. Choose the value whose units match the problem.

Why does H₂ deviate from ideality less than CO₂?

H₂ has weaker inter-molecular attractions and a smaller molecular size, so both van der Waals constants a and b are small.

What does a compressibility factor Z < 1 indicate?

Attractive forces dominate; the gas occupies less volume than an ideal gas at the same T, P.

Why can't a gas be liquefied above its critical temperature?

Above T_c, molecular kinetic energy exceeds the inter-molecular attractive potential, so no pressure can force the molecules to condense into a liquid phase.

Is air an ideal gas at room conditions?

Approximately yes — at 1 atm and 298 K its compressibility factor is within 0.1% of unity, so the ideal gas law is accurate for most MDCAT calculations.

A gas at 1 atm and 300 K is heated at constant volume to 600 K. Its pressure becomes:

2 atm. P/T = constant → P₂ = P₁(T₂/T₁) = 1 × 2 = 2 atm.

If the rate of effusion of an unknown gas is half that of H₂, its molar mass is:

8. Graham's law: r₁/r₂ = √(M₂/M₁) → ½ = √(2/M) → M = 8 g/mol.

The v_rms of an ideal gas is doubled when the absolute temperature is:

Quadrupled. v_rms ∝ √T; doubling v_rms requires T to be multiplied by 4.

A real gas behaves most like an ideal gas at:

Low P, high T. Low pressure makes molecular volume negligible; high temperature overcomes attractive forces.

In a mixture of 2 mol N₂ and 3 mol O₂ at total pressure 5 atm, the partial pressure of O₂ is:

3 atm. x(O₂) = 3/5; P(O₂) = 0.6 × 5 = 3 atm.

Gases MDCAT 2026 — Notes, Key Concepts & MCQs (Chemistry)

Q: What does a compressibility factor Z < 1 indicate?

Attractive forces dominate; the gas occupies less volume than an ideal gas at the same T, P.

Q: Why can't a gas be liquefied above its critical temperature?

Above T_c, molecular kinetic energy exceeds the inter-molecular attractive potential, so no pressure can force the molecules to condense into a liquid phase.

Q: Is air an ideal gas at room conditions?

Approximately yes — at 1 atm and 298 K its compressibility factor is within 0.1% of unity, so the ideal gas law is accurate for most MDCAT calculations.

Gases

Gases averages 2 MCQs per MDCAT paper — gas laws, the ideal gas equation, and kinetic theory dominate, with occasional questions on real-gas deviations.

Gases is a Chemistry chapter on the official PMDC MDCAT 2026 syllabus, contributing roughly 2 MCQs to the 45-MCQ Chemistry section. Mastering the core concepts below typically secures the full chapter weightage.

The classical gas laws

Boyle (1662): at fixed n, T, PV = constant — pressure and volume are inversely proportional. Charles (1787): at fixed n, P, V/T = constant. Gay-Lussac: at fixed n, V, P/T = constant. Avogadro: equal volumes at the same T and P contain equal numbers of molecules. Combining all four gives the ideal gas equation PV = nRT, where R = 8.314 J/mol·K = 0.0821 L·atm/mol·K. At STP (273.15 K, 1 atm) one mole occupies 22.4 L, the canonical FSc XI Chapter 3 result.

Ideal gas equation in practice

A 2.0 L vessel at 300 K holding 0.10 mol of N₂ exerts P = nRT/V = 0.10 × 0.0821 × 300 / 2.0 ≈ 1.23 atm. Density ρ = PM/RT links density to molar mass — useful for identifying an unknown gas. Graham's law of effusion: rate ∝ 1/√M; H₂ effuses 4× faster than O₂ because √(32/2) = 4. Dalton's law of partial pressures: P_total = ΣP_i, with P_i = x_iP_total where x_i is the mole fraction.

Kinetic molecular theory

The five postulates from Atkins Chapter 1: gases are particles in random motion; their volumes are negligible compared to the container; collisions are perfectly elastic; there are no inter-molecular forces; average KE depends only on absolute temperature. From these, PV = ⅓Nm⟨v²⟩, leading to the root-mean-square speed v_rms = √(3RT/M). For O₂ at 300 K: v_rms = √(3 × 8.314 × 300 / 0.032) ≈ 484 m/s. Average KE per molecule = (3/2)k_BT, with k_B = 1.381×10⁻²³ J/K.

Real gases and deviations from ideality

Real gases deviate at high pressure (where molecular volume becomes significant) and at low temperature (where attractive forces dominate). Van der Waals corrected the ideal equation: (P + an²/V²)(V − nb) = nRT, where a accounts for inter-molecular attraction and b for finite molecular size. Compressibility factor Z = PV/nRT: Z = 1 for ideal, Z < 1 when attractions dominate (moderate P), Z > 1 when repulsions dominate (very high P). H₂ and He show Z > 1 at most conditions because their attractive forces are very weak.

Liquefaction and critical phenomena

Above the critical temperature T_c, no amount of pressure liquefies a gas — liquid and vapour phases become indistinguishable. CO₂ has T_c = 304 K and P_c = 73 atm; this is why it is sold as a liquid in cylinders at room temperature. The Linde process uses Joule-Thomson cooling (gas expands adiabatically, cools below inversion temperature, eventually liquefies). For H₂ and He the inversion temperature is below room temperature, so they must be pre-cooled before throttling — a detail Atkins highlights as the bridge to thermodynamics.

Key Concepts

Worked MCQs

Frequently Asked Questions

How Gases Is Tested

MDCAT questions on Gases are a mix of recall (definitions, classifications), application (predict outcomes, interpret diagrams), and basic numerical/analytical reasoning. PMDC papers from 2020–2025 emphasized the concepts above; older UHS papers (2008–2019) tested them too, with slight variations in question framing.