Gases

1
Gases

• Chapters 10 11

2
Kinetic-Molecular Theory

• Used to describe the properties of solids,
  liquids, gases
• Based on the idea that particles are always in
  motion
• Ideal gas an imaginary gas that perfectly fits
  all the assumptions of the kinetic-molecular
  theory

3
The Kinetic-Molecular theory assumes the
following about gases

• Gas particles do not attract or repel each other.
• Gas particles are much smaller than the distances
  between them.
• Gas particles are in constant, random motion.
• No kinetic energy is lost when gas particles
  collide with each other.
• All gases have the same average kinetic energy at
  a given temperature.

4
Gas Laws

• Most gases obey 3 simple laws.
• When a gas obeys these laws, it is an ideal gas.
• These laws include
• Boyles Law
• Charless Law
• Gay-Lussacs Law

5
(1) Boyles Law

• If temperature is constant, then volume is
  inversely proportional to pressure
• P1V1 P2V2

6
ProblemA sample of oxygen gas has a volume of
150mL when its pressure is 0.947 atm. What will
the volume of the gas be at a pressure of 0.987
atm if the temperature remains constant?
7
(2) Charles Law

• If pressure is constant, then volume is
  proportional to temperature
• V1 V2
• T1 T2

8
Temperature must be expressed in Kelvin
units.Tkelvin 273 Tcelsius
9
ProblemA sample of neon gas occupies a volume
of 752mL at 25C. What volume will the gas occupy
at 50C if the pressure remains constant?
10
(3) Gay-Lussacs Law

• If volume is constant, then pressure is
  proportional to temperature
• P1 P2
• T1 T2

11
ProblemThe gas in an aerosol can is at a
pressure of 3.00 atm at 25C. Directions on the
can warn the user not to keep the can in a place
where the temperature exceeds 52C. What would
the gas pressure in the can be at 52C?
12
Combined Gas Law

• If all 3 variables change none are held
  constant, the combined gas law is used to state
  the relationship among pressure, volume,
  temperature.
• P1V1 P2V2
• T1 T2

13
ProblemA helium-filled balloon has a volume of
50L at 25C and 1.08 atm. What volume will it
have at 0.855 atm and 10C?
14
Avogadros Principle

• States that equal volume of gases at the same
  temperature pressure contain the same number of
  particles

15
Molar Volume

• Avogadros principle allows us to find the molar
  volume of molecules of gas
• Molar volume the volume that 1 mole occupies at
  0C (273K) 1 atm
• This specific temperature pressure is called
  standard temperature pressure (STP).

16
Contd

• 1 mole of any gas at STP will occupy a volume of
  22.4L.
• This conversion factor is very useful when you
  want to find the number of moles, the mass, or
  the number of particles in a gas sample

17
Problem

• How many moles are there in 3.72L of a gas?

18
Ideal Gas Law

• Describes the physical behavior of an ideal gas
  in terms of pressure, volume, temperature,
  number of moles of gas present
• PV nRT
• P pressure V volume n number of
  moles
• R ideal gas constant
• T temperature in Kelvin units

19
Ideal Gas Constant

• The value of R (the ideal gas constant) depends
  on the units of pressure
• If pressure is in atm, R 0.0821
• If pressure is in kPa, R 8.314
• If pressure is in mmHg or torr, R 62.4

20
The ideal gas law can also be used to find the
molar mass density of a gas.d P x MM R x
Tddensity PpressureMM molar mass
Rideal gas constant Ttemperature
21
ProblemWhat is the pressure in atmospheres
exerted by a 0.500 mol sample of nitrogen gas in
a 10.0L container at 298K?
22
Daltons Law of Partial Pressure

• States that the total pressure of a mixture of
  gases is equal to the sum of the pressures of all
  gases in the mixture
• Ptotal P1 P2 P3 Pn

23
ProblemWhat is the pressure of sample
containing oxygen, hydrogen, nitrogen at 0.125
atm, 0.087 atm, 0.442 atm respectively?