Gas Laws: MCQ 4

Kinetic Theory of Gases

• The kinetic theory is based on 5 assumptions:
  • Gases are made up of particles with negligible volumes compared to distances between them
  • No attractive or repulsive forces between gases
  • Particles are in constant rapid random motion, colliding with each other and the container walls
  • Average kinetic energy of particles is proportional to the Kelvin temperature
  • All collisions are perfectly elastic

Real Gases and Ideal Gases

• Real gases (e.g., O₂, N₂, Cl₂, CO₂) do not fully obey the kinetic theory assumptions
• Ideal gases obey all the assumptions of the kinetic theory under all conditions of temperature and pressure
• Real gases deviate most from the kinetic theory at low temperatures and high pressures

Behaviour of Real Gases

• At low temperatures and high pressures, particles become closer together, and their volumes are no longer negligible
• Attractive forces between particles cause deviations from ideal gas behaviour
• Real gases behave most like ideal gases at high temperatures and low pressures

Equation of State for an Ideal Gas

• PV = nRT, derived from a combination of Boyle's, Charles', and Avogadro's Laws
• n = number of moles
• R = universal gas constant (8.31 K.K⁻¹.mol⁻¹)
• Units: Volume (m³), Pressure (Pa), Temperature (K)

Calculations

• Example 1: Calculating the volume occupied by 10g of oxygen gas at 25°C and 200kPa
• Example 2: Calculating the Mr of a volatile liquid from the volume of vapour measured at 100°C and 100kPa

Experiment to Find the Mr of a Volatile Liquid

• A volatile liquid is one that vaporises easily (low boiling point)
• The most accurate method is using a mass spectrometer, but a simpler method can be used in the school lab
• The experiment involves measuring the mass of the condensed liquid, volume of the flask, atmospheric pressure, and temperature of the vapour
• The measurements are then used to calculate the number of moles and Mr of the volatile liquid