Introduction to the Gas Laws

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Introduction to the Gas Laws

• CPS Chemistry

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What is a Gas?

• A state of matter where there is indefinite
  volume (will fill its container) and indefinite
  density (can be compressed)
• Gasses are fluids, because like liquids they have
  the ability to flow
• Gasses are made up of randomly moving molecules,
  but are mostly empty space

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Temperature

• Temperature is the measure of how hot or cold an
  object is relative to something else
• Temperature in a gas relates to the kinetic
  energy of the molecules, the hotter the
  temperature, the faster they move
• Temperature in gas law problems is measured in C
  or K
• Standard temperature is 0C or 273K

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Pressure

• Pressure is force/area, N/m2 or a Pascal
• Pressure is directly proportional to applied
  force
• Pressure is indirectly proportional to area
• Pressure in gasses is the result of the number of
  collisions that the molecules make with the
  container, so the greater the temperature, the
  greater the pressure

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Pressure cont.

• Units of pressure are
• torr
• mm of Hg (millimeters of mercury)
• atm (atmospheres)
• Standard Pressure is
• 760 torr
• 760 mm of Hg
• 1 atmosphere

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Volume

• Volume is the amount of space that gasses take
  up, gasses will fully fill any container that
  they are in
• Two units of volume are
• Cubic centimeters cc or cm3
• Millileters mL
• 1 cc 1 cm3 1 mL

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Boyles Law

• Pressure and volume are inversely proportional,
  if you increase one, you decrease the other

citation
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The equation

• P1V1 P2V2
• This works when the temperature is kept
  constant.
• Before you start working with ANY gas law
  problem, make sure that you have all the same
  units!

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Example 1

• You have 2.0L of hydrogen gas at 3.0 atmospheres,
  you increase the pressure to 12.0 atm, what is
  your new volume?

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Example 2

• You have 30cm3 of chlorine gas at 1000 torr you
  change the volume to 90cm3 what is your new
  pressure? (solve for torr, mm of Hg and atm)

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Charles Law

• When pressure is kept constant the relationship
  between Temperature and volume is

citation
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The Formula

• The formula is

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Example 1

• If pressure is held constant, and a 2.0L of Neon
  gas is heated from 0 to 100, what is the new
  volume?

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Example 2

• If pressure is held constant, and a 850cc of
  Argon gas is cooled from 50C to 10C, what is
  the new volume?

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Combined Gas Law

• When you have all three variables involved
  (Pressure, Temperature, and Volume) you use the
  combined gas law

P1V1
P2V2

T1
T2
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Example 1

• A 50 mL sample of fluorine gas at 15C that rises
  in temperature to 80C, if the original pressure
  is 2 atm and increases to 8 atm, what is the new
  volume?

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Example 2

• A 500 cc sample of boron gas at 85C that cools
  to 0C, if the original pressure is 768 torr and
  increases to 850 torr what is the new volume?

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Partial Pressures

• The total pressure in a combination of gasses is
  the sum of all the pressures of each individual
  gas.
• PtotP1P2P3
• all gas pressures must be in the same unit

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Example 1

• The total pressure for air in a scuba tank is
  1200 torr, if the oxygen has a pressure of 500
  torr, the Nitrogen is 350 torr, what is the
  pressure of the argon gas component?

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Ideal Gas Law

• All the prior gas laws involve a combination of
  pressure, temperature, or volume. The ideal gas
  law also takes into account the number of
  molecules of gas involved.
• PVnRT

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Ideal gas law cont.

• P pressure atmospheres
• V volume Liters
• T temperature Kelvin
• N number of moles mol
• R gas constant .08021

L atm mol K
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Example 1

• A rigid cylinder with a volume of 10.0 liters is
  filled with neon gas at a pressure of 250. atm at
  30C, how many moles are in the gas?

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Example 2

• A 12.0 liter flask at 45C has 5.00 moles of
  helium gas, what is the pressure?