1. Gases

2. Properties of Gases Pressure Barometric Pressure
Gases uniformly fill any container
Gases are easily compressed
Mix completely with any other gas
Exert pressure on their surroundings
Pressure = force/area
Barometric Pressure
mm Hg (torr)
Newtons/meter2 = pascal (Pa)
Atmospheres (atm)
Standard Pressure
760 torr
101,325 Pa
1.0 atm
760 mm Hg

3. Gas Stoichiometry Standard temperature and pressure (STP) Molar volume
0˚C, 273 K
1 atm, 760 torr
Molar volume
One mole of an ideal gas occupies L of volume (at STP)
True for ANY gas
Useful things to remember
Density = mass/volume
n = grams of substance/molar mass

4. Dalton’s law of partial pressures
The total pressure of a gaseous mixture is the sum of the partial pressure of all the gases.
PT = P1 + P2 + P
Air is a mixture of gases - each adds it own pressure to the total.
Pair = PN2 + PO2 + PAr + PCO2 + PH2O

5. Partial Pressure Example
Mixtures of helium and oxygen are used in scuba diving tanks to help prevent “the bends.”
For a particular dive, 46 liters of O2 and 12 liters of He were pumped into a 5 liter tank. Both gases were added at 1.0 atm pressure at 25oC.
Determine the partial pressure for both gases in the scuba tank at 25oC.

6. Partial pressure example
First calculate the number of moles of each gas using PV = nRT.
Rearrange to solve for n
n = P V R T
nO2 = = 1.9 mol
nHe = = 0.49 mol
(1.0 atm) (46 L)
( L atm K-1 mol-1)(298.15K)
(1.0 atm) (12 L)
( L atm K-1 mol-1)(298.15K)

7. Partial pressure example
Calculate the partial pressures of each.
PO2 = = 9.3 atm
PHe = = 2.4 atm
Total pressure in the tank is 11.7 atm.
(1.9 mol) ( K) ( L atm K-1 mol-1)
(5.0 L)
(0.49 mol) ( K) ( L atm K-1 mol-1)
(5.0 L)

8. Mole Fraction XA = nA = PA nT PT Directly related to pressures!
If a gas is responsible for 25% of pressure, it is 25% of moles

9. Butane gas, P1, and
water vapor, P2, are
in the tube.
Experiment
Air Pressure, PT
PT =P1+P2
Butane lighter

10. Butane gas is collected by water displacement at 25°C
Butane gas is collected by water displacement at 25°C. If the atmospheric pressure is 755mm Hg, what is the pressure of the butane gas? (Given vapor pressure of water is 24 mm Hg)

11. Patmosphere = Pbutane+ PH2O
755 mm Hg = Pbutane + 24 mm Hg
Pbutane = 731 mm Hg

12. Example of Dalton’s Law
O2 gas is collected by water displacement at 15ºC. If the atmospheric pressure is 644 mm Hg, what is the pressure of the O2 gas, given P of water is 13 mm Hg?
(631mm )

13. Avogadro’s law If you have more moles of a gas, it takes up more
space at the same temperature and pressure.

14. Dalton & Avogadro’s Laws
2NH4OH (s) N2 (g) + O2 (g) + 4H2O (g)
When 10.0 g NH4OH decomposes, the total pressure of the mixture of gaseous products is 800. mm Hg. What is the partial pressure of each gas?
Ptotal = PN2 + PO2 + PH2O
ntotal = nN2 + nO2 + nH2O
PN nN2
Ptotal ntotal . =

15. Dalton & Avogadro’s Laws
2NH4OH (s) N2 (g) + O2 (g) + 4H2O (g)

16. Dalton & Avogadro’s Laws

17. Kinetic-molecular theory
This theory explains the behavior of gases.
Gases consist of very small particles (molecules) which are separated by large distances.
Gas molecules move at very high speeds - hydrogen molecules travel at almost 4000 mph at 25oC.
Pressure is the result of molecules hitting the container. At 25 oC and 1 atm, a molecule hits another molecule and average of 1010 times/sec.

18. Kinetic-molecular theory
No attractive forces exist between ideal gas molecules or the container they are in.
Energy of motion is called kinetic energy.
Average kinetic energy = mv 2
Because gas molecules hit each other frequently, their speed and direction are constantly changing.
The distribution of gas molecule speeds can be calculated for various temperatures. 1 2

19. Kinetic-molecular theory
Fraction having each speed
Molecular speed (m/s)
O2 at 25oC
O2 at 700oC
H2 at 25oC
Average speed

20. Diffusion versus Effusion
The random and spontaneous mixing of molecules.
Effusion
The escape of molecules through small holes in a barrier.

21. Graham’s law Rate A MM B Rate B MM A =
Relates the rates of effusion of two gases to their molar masses.
This law notes that larger molecules move more slowly.
Rate A MM B
Rate B MM A =

22. Graham’s law
Helium is placed in scuba tanks so it effuses into your blood instead of N2. How much faster is He than nitrogen?
Rate He
Rate N =
= 2.6 X faster

23. Graham’s law
What is the molar mass of an unknown gas that effuses times faster than pentane, C5H12, through a porous barrier?
Rate unknown gas = =
Rate C5H x
x = 16 g / mole

24. Graham’s law
CH4 and H2 effuse through a porous barrier. How much faster will hydrogen effuse than methane?
Rate H
Rate CH =
= 2.8 times faster

25. Ideal Gas Law Limitations
Works well at low pressures and high temperatures
Most gases do NOT behave ideally above 1 atm pressure
Does not work well near the condensation conditions of a gas
Assumes:
Molecules don’t attract or repel each other
Volume of molecules is negligible (volume is a vacuum)
Both untrue

26. Real Gases
As pressure approaches zero, all gases approach ideal behavior.
At high pressure, gases deviate significantly from ideal behavior.
Why?
Attractive forces actually do exist between molecules.
Molecules are not points -- they have volume.