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Chapter 5
Gases
Gases: General Properties
• Gas molecules: far apart and free to move
• Specify state of a gas by four properties
–
–
–
–
Pressure
(atmospheres;
Volume
(liters)
Temperature
(K)
Number of moles
• STP = standard T and P
1 atm = 760 mm Hg)
(273 K, 1 atm)
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Gas Laws
• What happens to volume of gas when its
temperature decreases?
Gas Laws
• What happens to volume of gas when pressure on
the gas is decreased?
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The Gas Laws
• Describe this behavior mathematically
– Volume decreases when temperature decreases
• Charles’s Law:
– Volume increases when pressure decreases
• Boyle’s Law:
P1V1 = P2V2
– Combined gas law:
Gas Laws
• Example; A gas occupies 1.00 L at 1.00 atm. It expands
to 5.00 L at constant T. What is new pressure?
• Example: A gas occupies 1.00 L at 25oC. What is new
volume if temperature is raised to 48oC at constant P?
• Example: If a gas at 1800 mm Hg and 25oC occupies 5.00
L, what volume will it have at STP?
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Ideal Gas Equation
• Generalization of the gas laws
• PV = nRT
• Example: What is the pressure of 8.32 moles of a gas
occupying 2.5 L at 100oC?
Ideal Gas Equation
• PV = nRT
• Example: What is the molar mass of a gas if 2.10 g
occupies 1.287 L at STP?
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Ideal Gas Equation
• A gas has the empirical formula CH3
• At 25oC and 750 mm Hg, 1.00 g occupies 825 mL
• What is molecular formula of the gas?
Gas Stoichiometry
• 2 N2 (g) + 3 H2 (g) --> 2 NH3 (g)
• React 5.00 g hydrogen gas with excess nitrogen.
• What volume of ammonia is produced at 200oC and 10.0 atm
pressure?
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Gas Stoichiometry
• Air bags in automobiles contain sodium azide, NaN3,
that decomposes according to this equation.
–
2 NaN3 (s) ----> 2 Na (s) + 3 N2 (g)
• Question: If we want the gas to fill a 25.0 L air bag to
a pressure of 1000 mm Hg at 25oC, how many grams
of NaN3 are required?
Partial Pressures
• Dalton’s Law: Total pressure of mixture of
gases = sum of partial pressures of its
components
• P = PA + PB
• Example: A 1.00 L cylinder contains 0.500 mole of He
and 0.700 mole of Ar at 298 K. What is the pressure
inside of the cylinder?
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Kinetic Molecular Theory of Gases
• A molecular model of ideal gases
– Gas molecules are very small and far apart
– Gas molecules move randomly
– There are no forces between molecules
• Collisions are elastic
• Molecule not affected by a collision (other than speed and
direction)
– Average kinetic energy of a molecule is proportional to
temperature
Average Molecular Speed
• From data below, what can we say about how
average molecular speed is affected by
– Temperature?
– Molecular mass?
Constant temperature
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Average Molecular Speed
• On the average
– High temperatures mean higher speeds
– Heavier molecules are slower than lighter molecules
GC
• Remember gas-liquid chromatography
• Retention time: time a compound remains in column
• Retention time depends on attraction of column for compound
– Weak attraction: short retention time
– Strong attraction: long retention time
• How does column temperature affect the retention time?
• How does molar mass of compound affect its retention time?
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Real Gases
• Ideal gas model makes two assumption
– No intermolecular forces
– Molecules have zero volume (small and far apart)
• Only approximate
• Question: Would real gases be more ideal at low or
high temperatures? Why?
Real Gases
• Ideal gas model makes two assumption
– No intermolecular forces
– Molecules have zero volume (small and far apart)
• Only approximate
• Question: Would real gases be more ideal at low or
high pressure? Why?
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