1. Chapter 9: Gases: Their Properties and Behavior
4/22/2019
Chapter 9
Gases
Copyright © 2008 Pearson Prentice Hall, Inc.

2. Gases and Gas Pressure Gas mixtures are homogeneous and compressible
Chemistry: McMurry and Fay, 6th Edition
Chapter 9: Gases: Their Properties and Behavior
4/22/2019 3:30:46 PM
Gases and Gas Pressure
Gas mixtures are homogeneous and compressible
Gas molecules do not interact with each other
Copyright © 2011 Pearson Prentice Hall, Inc.

3. Atmospheric Pressure Atmospheric pressure
is the pressure exerted by a column of air from the top of the atmosphere to the surface of the Earth.

4. Pressure Imbalance in Ear
If there is a difference
in pressure across
the eardrum membrane,
the membrane will be
pushed out – what we
commonly call a
“popped eardrum.”
picture from Tro Intro Chem 2nd ed.

5. Gases and Gas Pressure A barometer
Chemistry: McMurry and Fay, 6th Edition
Chapter 9: Gases: Their Properties and Behavior
4/22/2019 3:30:46 PM
Gases and Gas Pressure
A barometer
measures the pressure exerted by the gases in the atmosphere.
indicates atmospheric pressure as the height in mm of the mercury column.
Copyright © 2011 Pearson Prentice Hall, Inc.

6. Gas Pressure Gas pressure is a force acting on a specific area.
Pressure (P) = force
area
has units of atm, mmHg, torr, lb/in.2, and kilopascals(kPa).
1 atm = mm Hg (exact)
1 atm = 760 torr
1 atm = lb/in.2
1 atm = Pa
1 atm = kPa

7. Gases and Gas Pressure Monometer Pgas > Patm Pgas < Patm
Chemistry: McMurry and Fay, 6th Edition
Chapter 9: Gases: Their Properties and Behavior
4/22/2019 3:30:46 PM
Gases and Gas Pressure
Monometer
Pgas < Patm
Pgas = Patm - PHg
Pgas > Patm
Pgas = Patm + PHg
Copyright © 2011 Pearson Prentice Hall, Inc.

8. Chapter 9: Gases: Their Properties and Behavior
4/22/2019
The Gas Laws
Ideal Gas: A gas whose behavior follows the gas laws exactly.
The physical properties of a gas can be defined by four variables:
P pressure
T temperature (calculation must be in Kelvin)
V volume
n number of moles
Copyright © 2008 Pearson Prentice Hall, Inc.

9. The Gas Laws 1 V a (constant n and T) P
Chemistry: McMurry and Fay, 6th Edition
Chapter 9: Gases: Their Properties and Behavior
4/22/2019 3:30:46 PM
The Gas Laws
Boyle’s Law a V P 1
(constant n
and T)
PinitialVinitial = PfinalVfinal
One goes up, the other one goes down.
Copyright © 2011 Pearson Prentice Hall, Inc.

10. The Gas Laws Boyle’s Law 1 V a (constant n and T) P
Chemistry: McMurry and Fay, 6th Edition
Chapter 9: Gases: Their Properties and Behavior
4/22/2019 3:30:46 PM
The Gas Laws
Boyle’s Law a V P 1
(constant n and T)
One goes up, the other one goes down.
Copyright © 2011 Pearson Prentice Hall, Inc.

11. PV Constant in Boyle’s Law
In Boyle’s law, the product P x V is constant as long
as T and n do not change.
P1V1 = atm x L = atm L
P2V2 = atm x L = atm L
P3V3 = atm x L = atm L
Boyle’s law can be stated as
P1V1 = P2V2 (T, n constant)

12. Boyle’s Law and Diving
since water is denser than air, for each 10 m you dive below the surface, the pressure on your lungs increases 1 atm
at 20 m the total pressure is 3 atm
if your tank contained air at 1 atm pressure you would not be able to inhale it into your lungs

13. Boyles’ Law and Breathing
During an inhalation,
the lungs expand.
the pressure in the lungs decreases.
air flows towards the lower pressure in the lungs.

14. Boyles’ Law and Breathing
During an exhalation,
lung volume decreases.
pressure within the lungs increases.
air flows from the higher pressure in the lungs to the outside.

15. Chapter 9: Gases: Their Properties and Behavior
4/22/2019
The Gas Laws
V a T
Constant n and P
In Charles’s Law,
the Kelvin temperature of a gas is directly related to the volume.
P and n are constant.
when the temperature of a gas increases, its volume increases.
One goes up, the other one goes up.
Copyright © 2008 Pearson Prentice Hall, Inc.

16. The Gas Laws = Tfinal Vfinal Tinitial Vinitial Charles’s Law
Chemistry: McMurry and Fay, 6th Edition
Chapter 9: Gases: Their Properties and Behavior
4/22/2019 3:30:46 PM
The Gas Laws
Charles’s Law =
Tfinal
Vfinal
Tinitial
Vinitial
V a T
(constant n and P)
Copyright © 2011 Pearson Prentice Hall, Inc.

17. Chapter 9: Gases: Their Properties and Behavior
4/22/2019
The Gas Laws
Avogadro’s Law
V a n
(constant T and P)
= k n V =
nfinal
Vfinal
ninitial
Vinitial
One goes up, the other one goes up.
Copyright © 2008 Pearson Prentice Hall, Inc.

18. Chapter 9: Gases: Their Properties and Behavior
The Ideal Gas Law
4/22/2019
Summary
Boyle’s Law:
PinitialVinitial = PfinalVfinal =
Tfinal
Vfinal
Tinitial
Vinitial
Charles’ Law:
ninitial
Vinitial
nfinal
Vfinal
Avogadro’s Law: =
It is not necessary to know the names of the gas laws in order to solve problems. If one remembers the relationships between pressure, volume, temperature, and moles, then one can determine how to work the problem.
Copyright © 2008 Pearson Prentice Hall, Inc.

19. Examples
If a sample of helium gas has a volume of 120 mL and a pressure of 850 mmHg, what is the new volume if the pressure is changed to 425 mmHg?
A sample of oxygen gas has a volume of 420 mL at a temperature of 18 °C. At what temperature (in °C) will the volume of the oxygen be 640 mL (P and n constant)?
If 0.75 mole of helium gas occupies a volume of 1.5 L, what volume will 1.2 moles of helium occupy at the same temperature and pressure?

20. Combine Gas Law
is an expression obtained by mathematically combining Boyle’s and Charles’ law
P1V1 = P2V2 @ constant n
T T2
can predict P, V or T when condition is changed

21. Examples
A gas has a volume of 675 mL at 35 °C and atm pressure. What is the volume (mL) of the gas at -95 °C and a pressure of 802 mmHg? (n constant)

22. Chapter 9: Gases: Their Properties and Behavior
4/22/2019
The Ideal Gas Law
Ideal Gas Law:
PV = nRT
R is the gas constant and is the same for all gases.
R =
K mol
L atm
T = 0 °C ( K)
Standard Temperature and Pressure (STP) for Gases
Standard pressure is actually 1 bar.
Gas law problems must use Kelvin!!!!!!
P = 1 atm
Copyright © 2008 Pearson Prentice Hall, Inc.

23. Examples
What volume is occupied by 25.7 g of carbon dioxide gas at 25.0oC and 371 torr?
Determine the molar mass of a gas with a density of g/L at 80.0oC

24. The Ideal Gas Law What is the volume of 1 mol of gas at STP? (1 atm)
Chemistry: McMurry and Fay, 6th Edition
Chapter 9: Gases: Their Properties and Behavior
The Ideal Gas Law
4/22/2019 3:30:46 PM
What is the volume of 1 mol of gas at STP?
(1 atm)
(1 mol)
K mol
L atm
( K) P
nRT
V = =
= L
The closer the molar volume of a gas is to L at STP, the more ideal the behavior.
Copyright © 2011 Pearson Prentice Hall, Inc.

25. Molar Volume
At standard temperature and pressure (STP), 1 mole of a gas occupies a volume of 22.4 L, which is called its molar volume.

26. Example A. What is the volume at STP of 4.00 g of CH4?
1) L 2) L 3) 44.8 L
B. How many g of He are present in 8.00 L of gas at STP?
1) g 2) g 3) 1.43 g