1. Gases
Chapter 14

2. Review from Chapter 13 Pressure
Describing gases: To describe a gas fully, you need to state the 4 measurable quantities:
1. Volume Temperature
2. # of molecules 4. Pressure
Definition: The force per unit of area on a surface
Equation:

3. Measuring Pressure
A barometer is a device used to measure atmospheric pressure
Introduced by Torricelli with experiments involving mercury (Hg)
He determined the air (atmosphere) could support a column of Hg 760 mm high
The height of the Hg depends on the air pressure
What would happen to the height of the column in the mountains?
What would happen to the height of the column 100ft under water?

4. Units of Pressure Pressure can be measured in many units
Most common: mm of Hg
1 mm Hg = 1 torr (in honor of Torricelli)
Atmospheric Pressure at sea level and 0oC is 760 mm Hg
Other units of pressure include: Atmospheres (atm) and Pascal (Pa)

5. CONVERSIONS 760 mm Hg= = 760 torr = 1 atm = 29.92 in Hg = 14.7 psi
= Pa
= kPa

6. Standard Temperature and Pressure (STP)
To compare volume of gases, it is necessary to know the pressure at which the volume is measured
For purpose of comparison, scientists have agreed on standard conditions
STP= 1 atm pressure and 0oC

7. Review Calculation
The atmospheric pressure in Denver is atm. Express this in mmHg and kPa.

8. Boyle’s Law
Robert Boyle studied the relationship between pressure and volume
Boyle’s Law: States that the volume of a fixed mass of gas varies inversely with the pressure at constant temperature
Can be written as: P1V1= P2V2

10. Practice Problem P1V1= P2V2
A sample of oxygen gas has a volume of 150. mL when its pressure is atm. What will the volume of the gas be at a pressure of atm if the temperature remains constant?
P1=
V1=
P2=
V2=

11. Charles’s Law
Jacques Charles studied the relationship between volume and temperature
Charle’s Law: States that the volume of a fixed mass of gas at constant pressure varies directly with the Kelvin temperature.
Can be written as:

12. Kelvin Temperature? The Kelvin scale is K= 273 + oC
-273 oC is the lowest possible temperature to achieve (absolute zero)
Absolute zero is given the value of zero on the Kelvin scale

14. Practice Problem
A sample of neon gas occupies a volume of 752 mL at 25 oC. What will the volume of the gas occupy at 50oC if the pressure remains constant?
V1=
T1=
V2=
T2=

15. Gay-Lussac’s Law
Gay-Lussac determined the relationship between temperature and pressure
Gay-Lussac’s Law: The pressure of a fixed mass of gas at constant volume varies directly with the Kelvin temperature
Can be written as:
Pressure (atm)

16. Practice Problem
The gas in an aerosol can is at a pressure of 3.00 atm at 25oC. Directions on the can warn the user not to keep in a place where the temperature exceeds 52oC. What would the pressure in the can be at the 52oC?
P1=
T1=
P2=
T2=

17. The Combined Gas Law
A gas sample often undergoes changes in temperature, pressure and volume.
Combining all three (Boyle’s, Charles’, and Gay-Lussac’s) will give us a valid equation
Can be written as:

18. Practice Problem
A helium filled balloon has a volume of 50.0 L at 25oC and 1.08 atm. What volume will it have at atm and 10.oC?
P1=
T1=
V1 =
P2=
T2=
V2 =

19. Avogadro’s Principal
Equal volumes of gases at the same temperature and pressure contain equal numbers of particles
From Chapter 11: 1 mole= 6.02 x1023 particles
Molar volume for a gas is the volume that one mole occupies at 0.00oC and 1.00 atm pressure. (STP conditions)
Avogadro showed experimentally that one mole of any gas will occupy a volume of 22.4L at STP.
Conversion Factor:

20. Practice Problem
What volume will g of krypton gas occupy at STP?

21. The Ideal Gas Law PV=nRT
Describes the physical behavior of an ideal gas in terms of pressure, volume, temperature, and the number of moles of gas present
PV=nRT
R represents an experimentally determined constant that is referred to as the ideal gas constant (depends on the units used for pressure)

22. Numerical Values of the Gas Constant, R
Units of R
Numerical Value of R P V T n
0.0821
atm L K
mol
8.314
kPa
62.4
mm Hg

23. Real vs. Ideal Gas
An ideal gas is one whose particles take up no space and have no intermolecular attractive forces
In the real world, no gas is truly ideal.
Real gases deviate most from ideal gas behavior at extremely high pressures and low temperatures

24. Practice Problem
What is the pressure in atm exerted by a mol samples of nitrogen gas in a 10.0L container at 298K?

25. Applying the Ideal Gas Law
Rearranging the PV=nRT equation allows you to also calculate the molar mass and density of a gas sample if the mass of the sample is known
Recall from Chapter 12-
n (moles) = m (mass)/ M (molar mass)
D= Density (mass/volume)

26. Practice Problem
Calculate the grams of N2 gas present in a L sample kept at 1.00 atm pressure and a temperature of 22.0oC.

27. Gas Stoichiometry Gas volume A moles A moles B mass B
Why are we using stoichiometry?
Suppose we need to determine the volume of something other than our known (the given), we can apply stoichiometry to achieve the desired products/reactants
“Plan of Attack”
Start with a BALANCED equation.
Use stoichiometry first to get into the desired substance
Use the Ideal Gas Law (IGL) to convert into volume of that substance
Gas volume A moles A moles B mass B
Mass A moles A moles B gas volume B

28. Practice Problem
What volume of chlorine gas at 38oC and 1.63 atm is needed to react completely with 10.4 g of sodium to form NaCl? (Cl2 + 2Na  2NaCl)