Gases & the kinetic molecular theory




Gases & the kinetic molecular theory


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Gases & the kinetic molecular theory




Gases & the Kinetic Molecular Theory


Properties of Gases

  • Matter exists in three states: solids, liquids, and gases.
  • A vapor is a gas that has formed either by the evaporation of a liquid or the sublimation of a solid.
  • The density of gases is significantly less than liquids and solids – this indicates that the gas molecules are far apart relative to their size and their interactions are weak
  • Gases are easily compressed and fill their container
  • The interactions of gases would be minimal b/c of the molecules are so far apart if were not for their rapid motion.


  • Properties of Gases:
    • Gases can be compressed into smaller volumes; their densities can be increased by increasing pressure
    • Gases exert pressure on their surroundings; therefore, pressure must be exerted to be confined
    • Gases expand w/out limits; so gases can uniformly and completely occupy the volume of any container
    • Gases diffuse into one another; meaning that gases placed in the same container mix completely and cannot separate on standing
    • The amounts and properties of gases are describe in terms of temperature, pressure, volume occupied and number of molecules



  • Pressure = force/area
  • What is a barometer?
  • What is a manometer?
  • Know the units of pressure: torr, mm Hg, and atm
  • Be able to convert b/t all 3 units of pressure: 1 atm = 760 mm Hg at 0oC = 760 torr
  • The SI unit of pressure is the pascal (Pa)
  • 1 atm = 101.325 kPa


Boyle’s Law

  • Know Boyle’s Law: expresses the relationship between volume and pressure

P1V1 = P2V2  (constant n, T)  Remember:  n = moles



Charles’s Law

  • Know Charles’s Law: expresses the relationship between volume and temperature (in Kelvin)


(constant n, P)





  • Standard Temperature and Pressure (STP) = 0oC (273 K) and 1 atm


Combined Gas Law

  • Know the Combined Gas Law


      (at constant n)




Avogadro’s Law

  • Know Avogadro’s Law


    (at constant T,P)



  • Avogadro’s Law led to the Standard Molar Volume: 1 mole of a gas = 22.4 L at STP



Ideal Gas Law

  • Know the Ideal Gas Law


    • PV = nRT


    • R is the Universal Gas Constant
    • R = 0.0821 L atm/ mol K
    • R = 8.314J/mol K
    • You decide which R to use based on the units


Additional Information

  • Be able to do solve for molecular weights of gases using the Ideal Gas Law
  • By solving for n you can get moles; then using the provided mass solve for the molecular weight


Dalton’s Law

  • Know Dalton’s Law of Partial Pressure – the total pressure exerted by a mixture of gases is the sum of the partial pressures of the each gas.

Ptotal = P1 + P2 + P3 + …


  • For gases collected over water you must take into consideration that the gas is saturated with water vapor. (Patm is the atmospheric pressure)

Patm = Pgas + Pwater

  • So if you are trying to solve for the pressure of the gas:

Pgas = Patm - Pwater


Gas Stoiciometry

  • Be able to complete stoichiometric calculations for reactions with gases.
    • Remember that for gases 22.4 L = 1 mol at STP
  • If the conditions are not at STP you must use the Ideal Gas Law to solve for moles and then do your stoichiometry




  • Know the Kinetic Molecular Theory
    • #1.  Gas is composed of particles- usually molecules or atoms
    • #2.  Particles in a gas move rapidly in constant random motion
    • #3.  Collisions are perfectly elastic, no energy is lost or gained
    • #4.  Between collisions, the molecules exert no attractive or repulsive forces on each other



  • Know what diffusion is.
  • Know what effusion is.
  • How does molecular size affect effusion/diffusion
  • From a group of substances be able to determine which would have greatest rate of effusion/diffusion
  • Know Graham’s Law


Real Gases

  • Know how real gases deviate from ideal gases
  • Non-ideal gas behavior is greatest when the gas is at high pressure and/or low temp. ( near liquefaction)




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Gases & the kinetic molecular theory