Chapter 5: The Gaseous State

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Chapter 5 The Gaseous State
Chemistry 1061 Principles of Chemistry I Andy
Aspaas, Instructor
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Pressure

• Pressure force per unit area
• Force exerted on a surface area by molecules in
  motion
• Units
• 1 atmosphere 14.7 psi
• 1 atmosphere 760 mm Hg
• 1 atmosphere 101,325 Pascals
• 1 Pascal 1 kg/ms2

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Boyles law

• Relates volume and pressure of a gas
• PV constant
• PiViPfVf

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Charless Law

• Relates volume and temperature of a gas
• V/T constant
• Vi / Ti Vf / Tf

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Combined gas law

• Combines Charless and Boyles laws to give one
  equation
• (PiVi)/Ti (PfVf)/Tf

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Avogadros Law

• Relates volume with amount of gas
• Equal volumes of any two gases at the same
  temperature and pressure contain the same number
  of molecules
• Volume of 1 mole of gas Vm, molar gas volume
• 22.4 L/mol at STP (standard temp pressure, 0 C
  and 1 atm)
• Nearly the same for any ideal gas!

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Ideal gases

• An ideal gas follows the empirical gas laws
  exactly
• A theoretical gas whose molecules have no volume
  of their own, and whose molecules do not interact
  with each other
• R molar gas constant, the constant of the
  combined gas equation when 1 mol of gas is used
• Vm R x (T / P)
• PV nRT
• R 0.0821 (L atm)/(K mol)

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Using the ideal gas law

• If moles gas can be calculated
• Moles ? mass ? density (if MW is known)
• Moles ? MW (if mass is given)
• Etc.
• Stoiciometry in equations can give number of
  moles
• Moles ? Liters by ideal gas law

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Law of partial pressures

• The sum of partial pressures (PA) of all of the
  different gases in a mixture is equal to the
  total pressure of the mixture
• Related to mole fraction of a component of a
  mixture
• Mole fraction fraction of moles of a certain
  substance in a mixture of several substances
• Mole fraction (nA/n) (PA/P)
• Vapor pressure of water, when collecting gases
  over water

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Molecular speed and effusion

• Average molecular speed, (u) is dependent on the
  temperature and Molar mass of the gas
• u (3RT) / (Mm)1/2
• Effusion process in which a gas flows through a
  small hole in a container
• Rate of effusion is proportional to 1/ (Mm)1/2

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Real gases

• Ideal gas equation does not hold up, especially
  at high pressures
• Real volume is larger than the ideal gas law
  predicts, since molecules themselves take up
  space
• Actual pressure is smaller than ideal gas law
  predicts, since intermolecular attractions weaken
  collisions against the walls of the vessel

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van der Waals equation

• V becomes V nb
• P becomes P n2a / V2
• van der Waals equation
• (P n2a / V2)(V nb) nRT
• a and b values for many gases are known in the
  literature