Chem 20: Unit B

• Ideal Gas Law


  • STP: Standard Temperature and Pressure - P = 100.00 kPa, V = 22.4 L, T = 273.15 K
  • SATP: Standard Ambient Temperature and Pressure - P = 100.00 kPa, V = 298.15 K
• More molecules mean more collisions, fewer molecules mean fewer collisions
• Units – Pressure

  • Pressure is caused by collisions of molecules with walls or objects
  • Atmospheric pressure is the pressure exerted by the air
  • Pressure and number of molecules are directly related
  • Doubling number of molecules doubles pressure
  • Removing molecules reduces pressure
  • Barometer measures pressure in mm Hg
• STP: Standard Temperature and Pressure - P = 100.00 kPa, V = 22.4 L, T = 273.15 K
• Volume is the size of the container occupied by a gas
• Units – Temperature

  • Temperature is a measure of kinetic energy
  • Increased temperature means increased kinetic energy
  • Measured in Kelvin
  • Conversion formula: x K = x °C + 273.15
• Kinetic Molecular Theory helps describe how gases behave
• Conversion: x K = x °C + 273.15
• The first device for measuring atmospheric pressure was the barometer
• SATP: Standard Ambient Temperature and Pressure - P = 100.00 kPa, V = 298.15 K
• Kinetic Molecular Theory

  • Gases exert pressure due to collisions with objects in their path
  • Gases consist of tiny particles
  • Volume of individual particles can be assumed negligible
  • Particles are in constant random motion, colliding with the walls of the container
  • Particles are assumed not to attract or repel each other
  • Average kinetic energy of gas particles is directly proportional to temperature
  • Gases expand to fill any container
  • Gases are fluids
  • Gases have very low densities
  • Gases can be compressed
  • Gases undergo diffusion
  • Gases are affected by changes in pressure, temperature, and volume
• Pressure represents force exerted over an area
• Units – Volume


  • Volume is the amount of space occupied by a gas
  • Units used are liters and milliliters
  • Conversion: 1000 mL = 1 L, 10^-3 L = 1 mL
• 1000 mL = 1 L, 10^-3 L = 1 mL
• Temperature is measured in Kelvin
• Particles in a real gas

  • Have volume
  • Have inelastic collisions
  • Are in constant, random, nonlinear motion
  • Do attract or repel each other
  • Do condense into liquids at low enough temperatures due to forces of attraction
• Temperature of an ideal gas for which 0.270 mol occupies 15.0 L at 2.54 atm
• PV = nRT
• Pressure of an ideal gas if 1.04 mol occupies 21.8 L at 25.0 oC
• Particles in an ideal gas

  • Have no volume
  • Have elastic collisions
  • Are in constant, random, linear motion
  • Don’t attract or repel each other
  • Have an average Ek directly related to temperature (K)
  • Behave perfectly under all conditions of temperature, pressure, and volume
  • Small gas molecules behave more ideally than large gas molecules
  • Do not condense into liquids
• Number of moles of 1.50 L of an ideal gas at 37.0 oC and 725.0 Torr
• Mathematically: The value of the constant will change depending on the temperature (T) and the quantity of gas (n)
• STP – Standard Temperature and Pressure
• 1000 mL = 1 L
• SATP – Standard Ambient Temperature and Pressure
• Volume occupied by 6.72 x 10-3 mol of an ideal gas at 265.0 oC and pressure of 23.0 torr
• Real gas law deviations

  • Likely to behave nearly ideally when: Gases at high temperature and low pressure, Adequate energy to overcome IMF attractions and low frequency of collisions result in lower energy lost, Small non-polar gas molecules, Lower IMFs (only small LD)
  • Likely not to behave ideally when: Gases at low temperature and high pressure, Inadequate energy to overcome IMF attractions and high frequency of collisions result in greater energy lost, Large polar gas molecules, Greater IMFs (increasingly large LD + DD and HB)
• Units used for volume


  • liters (L)
  • milliliters (mL)
• Boyle’s Law: Pressure is inversely proportional to volume when temperature and number of moles are held constant
• Charles’ Law: The big picture – as temperature increases, volume increases
• Boyle’s Law

  The value of the constant will change depending on the temperature (T) and the quantity (n) of the gas sample
• Combined Gas Law: The combined gas law expresses the relationship between pressure, volume, and temperature of a fixed amount of gas – moles are held constant
• Boyle’s Law: The big picture – as volume decreases, pressure increases
• 1 cm³ = 1mL
• Charles’ Law: Volume is directly proportional to temperature when the quantity (n) and pressure (P) of a gas are held constant
• Law of Combining Volumes

  For every 2 mol of NH3 there is 1 mol of N2 and 3 mol of H2. The change in moles during reactions can be used to determine how volumes respond
• Law of Combining Volumes: One mole of ethane gas, C2H6(g), undergoes a combustion reaction
• Law of Combining Volumes

  • Hydrogen gas + oxygen gas → water vapour
  • 2 H2(g) + O2(g) → 2 H2O(g)
  • 100 mL + 50 mL → 100 mL 50 mL + 25 mL → 50 mL 2 mL + 1 mL → 2 mL
• Combined Gas Law


  Volume for a gas at different pressures and temperatures can be calculated using the formula P1V1/T1 = P2V2/T2
• Law of Combining Volumes

  Determine the molar volume ratio between compounds to solve for the unknown volume