1. Gases
Edward Wen, PhD

2. 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 when pressure is changed.
mixtures of gases are always homogeneous (common air)
fluid

3. Properties of Gas: Indefinite Shape & Volume
Gas molecules:
have enough kinetic energy and little attractions
 keep moving around and spreading out
 fill the container of whatever shape

4. Comparison: Gas, Liquid, Solid

5. Kinetic Molecular Theory

6. Pressure: Gases Pushing
What are Gas molecules doing?
constantly in motion
as they move and strike a surface, they push on that surface
push = force
Pressure of gas: total amount of force exerted by gas molecules hitting the entire surface at any one instant
pressure = force per unit area

7. Measuring Air Pressure
use a barometer: column of mercury supported by air pressure
Force of the air on the surface of the mercury  Gravity on the column of mercury
gravity

8. Common Units of Pressure
Average Air Pressure at Sea Level
pascal (Pa)
101,325
kilopascal (kPa)
atmosphere (atm)
1 (exactly)
millimeters of mercury (mmHg)
760 (exactly)
torr (torr)
pounds per square inch (psi, lbs./in2)
14.7

9. Practice: Convert Pressure between units
735.0 mmHg = ? atm
35. psi = ? torr
Ans: atm
Ans: 1.8 × 103 torr

10. The Effect of Gas Pressure
whenever there is a Pressure difference, a gas will flow from area of High pressure  area of Low pressure
the bigger the difference in pressure, the stronger the flow of the gas
if there is something in the gas’ path, the gas will try to push it along as the gas flows

11. Gas Pressure in Soda Straws
Straw at idle:
Pressure of the
air inside the straw = Pressure
of the air outside
the straw  liquid levels is
the same on both
sides
Suction of the straw:
Pressure of the
air inside the straw is
< Pressure
of the air outside
the straw 
liquid is pushed
up the straw by
the outside air

12. Atmospheric Pressure & Altitude
Altitude↑ Atmospheric pressure↓
At the surface, P = 14.7 psi,
At 10,000 ft altitude, P = 10.0 psi
Rapid changes in atmospheric pressure may cause your ears to “pop”  an imbalance in pressure on either side of your ear drum (driving or flying)
Demo: Can you make a piece of paper uphold a bottle of water?

13. Boyle’s Law For the gas contained at constant temperature:
Pressure of a gas is inversely proportional to its volume:
P /V
Or P x V = constant
P1 x V1 = _______

14. When you double the pressure on a gas,
the volume reduces to one half, (as long as the
temperature and amount of gas do not change)

15. Example: 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
Answer: 24 L

16. We’re losing altitude.
Quick Professor, give your
lecture on Charles’ Law!

17. Charles’ Law For the gas contained and at constant Pressure:
Volume is directly proportional to temperature V T
constant P and amount of gas
graph of V vs T is straight line
as T increases, V also increases
Kelvin K = °C + 273
V = constant x T
if T measured in Kelvin

18. Charles’ Law in Action https://www.youtube.com/watch?v=al5f9q845q0
Egg sucked into bottle:
The density of common air depends on the temperature. Higher T, lower Density
Why the air vents for the air conditioning system are located at the ceiling?

19. Example: A gas has a volume of 2. 80 L at an unknown temperature
Example: A gas has a volume of 2.80 L at an unknown temperature. When the sample is at 0°C, 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 = 0°C
Find: temp1 in K and °C
T1 = 297 K = 24 °C

21. Gay-Lussac’s Law For the gas contained at constant Volume:
Pressure is directly proportional to temperature P T
constant V and amount of gas
graph of P vs T is straight line
as T increases, P also increases
Kelvin K = °C + 273
P = constant x T
if T measured in Kelvin

22. Gay-Lussac’s Law in Action
The pressure of gas in a sealed container depends on the temperature. Higher T, higher Pressure
Keep the propane container in a cool place, avoid from direct sunlight.
If the storage of propane container is on fire, the pressure of propane gas will increase!!! So what to do?
RUN! Call 911, Tell the FD about it!

24. Avogadro’s 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
the gas doesn’t matter

25. Avogadro’s Law

26. Combined Gas Law Boyle’s Law : Pressure and Volume
at constant temperature
Charles’ Law : Volume and absolute Temperature
at constant pressure
 Volume of a sample of gas when both the Pressure and Temperature change

27. Example: A sample of gas has a volume of 158 mL at a pressure of 755 mmHg and a temperature of 34°C. The gas is compressed to a volume of 108 mL and heated to 85°C, what is the final pressure in mmHg?
Information
Given: V1 = 158 mL, P1 = 755 mmHg,
t1 = 34°C
V2 = 108 mL, t2 = 85°C
Find: P2, mmHg
P2 = 1.29  103 mmHg

28. Ideal Gas Law Combined Gas Law + Avgadro’s Law  Ideal Gas Law
R is called the Gas Constant
the value of R depends on the units of P and V
R = atm/K · mol
convert P to atm and V to L
Application of Ideal Gas law: when T, P, V of a gas all changes

29. Information
Given: V = 3.2 L, P = 24.2 psi,
t = 25°C
Find: n, mol
Example: Calculate the number of moles of gas in a basketball inflated to a total pressure of 24.2 psi with a volume of 3.2 L at 25°C

30. Air: Mixtures of Gases Air is a mixture (N2 , O2)
Each gas in the mixture behaves independently of the other gases
though all gases in the mixture have the same volume and temperature
all gases completely occupy the container, so all gases in the mixture have the volume of the container
Gas
% in Air, by volume
nitrogen, N2 78
argon, Ar
0.9
oxygen, O2 21
carbon dioxide, CO2
0.03

31. Collecting gas over water
Zn metal reacts
with HCl(aq) to
produce H2(g).
The gas flows
through the tube
and bubbles into
the jar, where it
displaces the
water in the jar.
Pgas = PH2O + PH2
Because water
evaporates, some
water vapor gets
mixed in with
the H2.

32. Standard Conditions (STP)
Common reference points for comparing
Standard Temperature & Pressure
Standard Pressure = 1.00 atm
Standard Temperature = 0°C = 273 K

33. Molar Volume of a Gas at STP
Definition: The volume of 1 (exact) mole gas at STP
Use the Ideal Gas Law: PV = nRT
1 mole of any gas at STP will occupy 22.4 L
==> Molar volume
can be used as a conversion factor
as long as you work at STP
1 mol  22.4 L

34. Molar Volume So much empty space between molecules in the gas state,
 the volume of the
gas is not effected
by the size of the
molecules, (under
ideal conditions).

35. Density of Gas at STP
Since every exactly one mole of any gas has a volume of 22.4 L, whereas the mass of such gas would be as the molar mass in grams
Density of Gas = Molar mass / Molar Volume
Example: Find the Density of Oxygen gas at STP.
1.43 g/L

36. Density of Common Gases
At STP, the density of common gases (in g/L) as: H He CH N
Air O CO Cl
Which one, hydrogen gas or helium gas, is better in blimps in providing lift?
Why carbon dioxide is used in putting out fire? What if its density is less than the air?

37. Real Gases Ideal gas laws assume
No Attractions between gas molecules
No Volume: gas molecules do not take up space
based on the Kinetic-Molecular Theory
Real gases: often do not behave like Ideal gases at High pressure (“Squeezed”) or Low temperature (“Frozen”)

38. Ideal vs. Real

39. The tire on a bicycle stored in a cool garage at 18C had a pressure of 30. psi. What is the pressure inside the tire after riding the bike at 35C? Assume the volume of the tire remains constant.
Information
Given: T1 = 18°C, P1 = 30. psi
T2 = 35°C
Find: P2 in psi.
T1 = 291 K, T2 = 308 K, P2 = 32 psi