Ideal Gas Law MCQ – Objective Questions with Answers

Ideal Gas Law MCQ – Objective Questions with Answers

Mechanical Engineering › Thermodynamics | Free practice MCQs with detailed explanations

Last Updated: June 2026

📌 About this MCQ Set

The ideal gas law, pV = mRT (or pV = nRuT), relates the pressure, volume and temperature of an ideal gas. It combines Boyle’s, Charles’s and Gay-Lussac’s laws.

These MCQs cover the equation of state, gas constants, and the assumptions behind ideal gas behaviour.

8 questions • every answer comes with a worked explanation. Click Show Answer to check yourself.

📖 New to this topic? Read the full concept guide: Ideal Gas Law →

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Ideal Gas Law MCQs

Q1. The ideal gas equation of state is:

  1. pV = mRT
  2. pV = mR/T
  3. p/V = mRT
  4. pVT = mR
Show Answer

Answer: A. pV = mRT

pV = mRT, where R is the specific gas constant and T is absolute temperature.

Q2. Boyle’s law states that at constant temperature, pressure is ____ to volume.

  1. Directly proportional
  2. Inversely proportional
  3. Equal
  4. Unrelated
Show Answer

Answer: B. Inversely proportional

At constant temperature p ∝ 1/V (Boyle’s law).

Q3. Charles’s law states that at constant pressure, volume is ____ to absolute temperature.

  1. Inversely proportional
  2. Directly proportional
  3. Equal
  4. Unrelated
Show Answer

Answer: B. Directly proportional

At constant pressure V ∝ T (Charles’s law).

Q4. The universal gas constant Ru is approximately:

  1. 8.314 J/mol·K
  2. 287 J/kg·K
  3. 1.4 J/mol·K
  4. 100 J/mol·K
Show Answer

Answer: A. 8.314 J/mol·K

Ru = 8.314 J/mol·K; the specific gas constant R = Ru/M.

Q5. The specific gas constant for air is approximately:

  1. 8.314 J/kg·K
  2. 287 J/kg·K
  3. 461 J/kg·K
  4. 2077 J/kg·K
Show Answer

Answer: B. 287 J/kg·K

For air R ≈ 287 J/kg·K (Ru divided by the molar mass of air ≈ 28.97 g/mol).

Q6. An ideal gas assumes that intermolecular forces are:

  1. Strong
  2. Negligible
  3. Attractive only
  4. Repulsive only
Show Answer

Answer: B. Negligible

Ideal gas molecules are assumed to have negligible intermolecular forces and negligible molecular volume.

Q7. Real gases behave most like ideal gases at:

  1. Low temperature, high pressure
  2. High temperature, low pressure
  3. High pressure only
  4. Low temperature only
Show Answer

Answer: B. High temperature, low pressure

At high temperature and low pressure, molecules are far apart and the ideal gas assumptions hold well.

Q8. For an ideal gas, the relation between specific heats is:

  1. Cp − Cv = R
  2. Cp + Cv = R
  3. Cp/Cv = R
  4. Cp × Cv = R
Show Answer

Answer: A. Cp − Cv = R

Mayer’s relation: Cp − Cv = R for an ideal gas.

Frequently Asked Questions

What is the ideal gas law?

It is the equation of state pV = mRT relating pressure, volume and absolute temperature of an ideal gas.

When do real gases behave ideally?

At high temperatures and low pressures, where molecules are far apart and intermolecular forces are negligible.

What is the difference between the universal and specific gas constants?

The universal gas constant Ru (8.314 J/mol·K) is the same for all gases; the specific gas constant R = Ru/M depends on the gas’s molar mass.

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