Properties of Gases

1
Properties of Gases

• expand to completely fill their container
• take the shape of their container
• low density
• much less than solid or liquid state
• compressible
• mixtures of gases are always homogeneous
• fluid

2
Kinetic Molecular Theory
3
Measuring Air Pressure

• use a barometer
• column of mercury supported by air pressure
• force of the air on the surface of the mercury
  balanced by the pull of gravity on the column of
  mercury

4
Common Units of Pressure
5
ExampleA high-performance road bicycle is
inflated to a total pressure of 125 psi. What is
the pressure in millimeters of mercury?

• Information
• Given 125 psi
• Find ? mmHg
• CF 14.7 psi 760 mmHg
• SM psi ? mmHg

• Apply the Solution Map

6.46259 x 103 mmHg

• Sig. Figs. Round

6.46 x 103 mmHg
the 760 is an exact number and does not effect
the significant figures
6
Boyles Law

• pressure of a gas is inversely proportional to
  its volume
• constant T and amount of gas
• graph P vs V is curve
• as P increases, V decreases by the same factor P
  x V constant
• P1 x V1 P2 x V2

7
Boyles Law
8
Boyles Experiment, P x V
9
A cylinder equipped with a moveable piston has an
applied pressure of 4.0 atm and a volume of 6.0
L. What is the volume if the applied pressure
is decreased to 1.0 atm?

• Information
• Given P1 4.0 atm V1 6.0 L
• P2 1.0 atm
• Find V2 ? L
• Eqn P1 V1 P2 V2
• SM P1, V1, P2 ? V2

10
Standard Conditions, STP

• Common reference points for comparing
• standard pressure 1.00 atm
• standard temperature 0C or 273 K
• Molar Volume at STP is 22.4L/mol

11
Charles Law

• volume is directly proportional to temperature
• constant P and amount of gas
• graph of V vs T is straight line
• as T increases, V also increases
• Kelvin T Celsius T 273
• V constant x T
• if T measured in Kelvin

12
Volume and Temperature
13
Charles Law
William Thomson, the Lord of Kelvin, extrapolated
the line graphs of Volume vs. Temp. to determine
the theoretical temp. a gas would have a volume
of 0.
14
A gas has a volume of 2.80 L at an unknown
temperature. When the sample is at 0C, its
volume decreases to 2.57 L. What was the initial
temperature in kelvin and in Celsius?

• Information
• Given V1 2.80 L
• V2 2.57 L t2 0C
• Find temp1 in K and C
• Eqn
• SM V1, V2 T2 ? T1

15
Gay-Lussacs Law

• Pressure of a gas is directly related to its
  Kelvin temperature while the volume and number of
  moles of gas remain constant.

16
The Combined Gas Law
17
Avogadros Law

• volume directly proportional to the number of gas
  molecules
• V constant x n
• constant P and T
• more gas molecules larger volume
• count number of gas molecules by moles
• equal volumes of gases contain equal numbers of
  molecules

18
Avogadros Law
19
Daltons Law of Partial Pressures

• each gas in the mixture exerts a pressure
  independent of the other gases in the mixture
• the pressure of an component gas in a mixture is
  called a partial pressure
• the sum of the partial pressures of all the gases
  in a mixture equals the total pressure
• Ptotal Pgas A Pgas B Pgas C ...

20
Daltons Law of Partial Pressures

• A mixture of He, Ne Ar has a total pressure of
  558 mm Hg. If the partial pressure of He is 341
  mm Hg, and of Ne is 112 mm Hg, what is the
  partial pressure of Ar ?
• Ptot PHe PNe PAr
• PAr Ptot - PHe - PNe 558-341-112 105
• PAr 105 mm Hg

21
Finding Partial Pressure

• to find the partial pressure of a gas, multiply
  the total pressure of the mixture by the
  fractional composition of the gas
• for example, in a gas mixture that is 80.0 He
  and 20.0 Ne that has a total pressure of 1.0
  atm, the partial pressure of He would be
• PHe (0.800)(1.0 atm) 0.80 atm