Ideal Gas

• Assumptions of Ideal Gas

  1. Gases are made of molecules which are in a constant random motion in straight lines
  2. The molecules behave as rigid spheres
  3. Pressure is due to collisions between molecules & the walls of the container
  4. All collisions are perfectly elastic (no loss in kinetic energy during the collision)
  5. Temperature of gas is proportional to average kinetic energy of molecules

  Two most important assumptions:

  1. No (or negligible) intermolecular forces between gas molecules
  2. Volume occupied by molecules is negligible relative to volume of the container
• Limitations of Ideal Gas Law

  At very low temperature and pressures, real gases do not obey the kinetic theory as under these conditions:

  • Molecules are close to each other
  • Instantaneous dipole-dipole or permanent dipole-dipole forces between molecules
  • These attractive forces pull the molecules away from the container wall
  • Volume of molecules is not negligible
• Ideal gas equation


  pV = nRT

  p= pressure in Pa

  V = volume in m³

  n = moles

  R = Gas constant 8.314 Jmol⁻¹K⁻¹

  T = temperature in Kelvins