1. Chapter 5 Gases

2. Air Pressure & Shallow Wells
Gases
•Are mostly empty space
• Occupy containers uniformly and completely
• The densities of gases are much smaller than those of liquids and solid and highly variable depending on temperature and pressure
Because there is a lot of unoccupied space in the structure of a gas, gases do not have a lot of mass in a given volume, the result is they have low density
Expand infinitely
• Diffuse and mix rapidly
Lower density
Higher density

3. Gases Pushing gas molecules are constantly in motion
as they move and strike a surface, they push on that surface
push = force
if we could measure the total amount of force exerted by gas molecules hitting the entire surface at any one instant, we would know the pressure the gas is exerting
pressure = force per unit area
Unit area
Force
Pressure:

4. Atmospheric Pressure Effects
differences in air pressure result in weather and wind patterns
the higher up in the atmosphere you climb, the lower the atmospheric pressure is around you
at the surface the atmospheric pressure is 14.7 psi, but at 10,000 ft it is only 10.0 psi
rapid changes in atmospheric pressure may cause your ears to “pop” due to an imbalance in pressure on either side of your ear drum

5. The Pressure of a Gas
result of the constant movement of the gas molecules and their collisions with the surfaces around them
the pressure of a gas depends on several factors
number of gas particles in a given volume
volume of the container
average speed of the gas particles

6. Measuring Air Pressure
Chemistry: McMurry and Fay, 6th Edition
Chapter 9: Gases: Their Properties and Behavior
Measuring Air Pressure
11/23/2018 4:18:17 PM
Units
Pa (SI unit)
torr
mm Hg
atm
bar
Conversions
1 atm = 760 mm Hg
(exact)
1 torr = 1 mm Hg
(exact)
1 bar = 1 x 105 Pa
(exact)
1 atm = Pa
Barometer
1 atm = 14.7 psi
Copyright © 2011 Pearson Prentice Hall, Inc.

7. Gases and Gas Pressure Chemistry: McMurry and Fay, 6th Edition
Chapter 9: Gases: Their Properties and Behavior
Gases and Gas Pressure
11/23/2018 4:18:17 PM
Copyright © 2011 Pearson Prentice Hall, Inc.

8. Boyle’s law 1 V a (constant n and T) P
Chemistry: McMurry and Fay, 6th Edition
Chapter 9: Gases: Their Properties and Behavior
Boyle’s law
11/23/2018 4:18:17 PM
pressure of a gas is inversely proportional to its volume
constant T and amount of gas
as P increases, V decreases by the same factor a V P 1
(constant n
and T)
One goes up, the other one goes down.
Two sets of conditions
P1 x V1 = P2 x V2
Copyright © 2011 Pearson Prentice Hall, Inc.

9. 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.

10. Charles’s Law 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.
For two conditions, Charles’s law is written
V1 = V (P and n constant)
T T2
Charles’s Law can be used to approximate absolute zero. At a temperature of absolute
zero (0K), theoretically an ideal gas has no volume.

11. Chapter 9: Gases: Their Properties and Behavior
Avogadro’s Law
11/23/2018
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.

12. Examples
A cylinder with a movable piston has a volume of 7.25 L at 4.52 atm. What is the volume at 1.21 atm?
A gas has a volume of 2.57 L at 0.00°C. What was the temperature at L?
A mol sample of He has a volume of 4.65 L. How many moles must be added to give 6.48 L?

13. Chapter 9: Gases: Their Properties and Behavior
11/23/2018
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 (atm)
T temperature (calculation must be in Kelvin)
V volume (L)
n number of moles
The Ideal Gas Law, PV = nRT,
- models the behavior of ideal gases. Other gas laws can be derived from the Ideal Gas Law for either one set of conditions or for two sets of conditions (initial and final conditions).
To derive gas laws for two sets of conditions, solve the Ideal Gas Law for R PV
---- = R nT
R =
K mol
L atm
Copyright © 2008 Pearson Prentice Hall, Inc.

14. Examples
A mol sample of argon gas has a volume of 9.00L at a pressure of 875 mmHg. What is the temperature (in oC) of the gas?
What volume is occupied by 25.7 g of carbon dioxide gas at 25.0oC and 371 torr?

15. Chapter 9: Gases: Their Properties and Behavior
11/23/2018
The Ideal Gas Law
since the volume of a gas varies with pressure and temperature, chemists have agreed on a set of conditions to report our measurements so that comparison is easy – we call these standard conditions
T = 0 °C ( K)
Standard Temperature and Pressure (STP) for Gases
P = 1 atm
Standard pressure is actually 1 bar.
Gas law problems must use Kelvin!!!!!!
(1 atm)
(1 mol)
K mol
L atm
( K) P
nRT =
= L
Copyright © 2008 Pearson Prentice Hall, Inc.

16. Molar Volume
solving the ideal gas equation for the volume of 1 mol of gas at STP gives 22.4 L
6.022 x 1023 molecules of gas
we call the volume of 1 mole of gas at STP the molar volume
it is important to recognize that one mole of different gases have different masses, even though they have the same volume

17. Examples What is the volume occupied by 2.75 moles of N2 gas at STP?
Assuming ideal behavior, which of the following gas samples will have the greatest volume at STP?
1 g H2 b. 1 g O2
c g Ar

18. Gas Density and Molar Mass
The density of a gas is proportional to its molar mass. As the molar mass of a gas increases, so does the density of the gas. Matter often separates according to its density, with less dense matter floating on matter of higher density

19. Examples
Calculate the density of gaseous hydrogen at a pressure of 1.32 atm and a temperature of -45.0oC.
A sample of gas has a mass of 0.827g. Its volume is 0.270L at a temperature of 88.0oC and a pressure of 975 mmHg. Find its molar mass

20. Partial Pressure
when gases are mixed together, their molecules behave independent of each other
the pressure of a single gas in a mixture of gases is called its partial pressure
we can calculate the partial pressure of a gas if
the sum of the partial pressures of all the gases in the mixture equals the total pressure
Dalton’s Law of Partial Pressures
PT = P1 + P2 + P

21. Mole Fraction
the fraction of the total pressure that a single gas contributes is equal to the fraction of the total number of moles that a single gas contributes
the ratio of the moles of a single component to the total number of moles in the mixture is called the mole fraction, c
the partial pressure of a gas is equal to the mole fraction of that gas times the total pressure

22. Example
Find the partial pressure of neon in a mixture with total pressure 3.9 atm, volume 8.7 L, temperature 598 K, and 0.17 moles Xe.

23. Collecting Gases
gases are often collected by having them displace water from a container
the problem is that since water evaporates, there is also water vapor in the collected gas
the partial pressure of the water vapor, called the vapor pressure, depends only on the temperature

24. Vapor Pressure of Water

25. Examples
1.02 L of O2 collected over water at 293 K with a total pressure of mmHg. Find mass O2.
0.12 moles of H2 is collected over water in a 10.0 L container at 323 K. Find the total pressure.