1. Unit 8: Gases
Chapter 12

2. Key Concepts Properties of Gases, Kinetic-Molecular Theory
Barometer, Manometer
Units of Pressure (mmHg, atm)
Units of Temperature (K)
Standard Temperature Pressure (STP)
Boyle’s Law, Charles’ Law, Gay-Lussac’s Law
Combined Gas Law
Ideal Gas Law
Dalton’s Law, Graham’s Law

3. Properties of Gases Kinetic-Molecular Theory:
Gas particles are far apart
Fluidity and compressibility- gas particles can easily move around one another or move closer together because they are farther apart than liquid or solid particles
Gas particles are in constant motion
They collide with each other and the walls of their container, and those collisions cause pressure.

4. Properties of Gases
More space to move around Less space to move around
Less collisions = less pressure More collisions = more pressure

5. Measuring Pressure Barometer: measures atmospheric pressure
Atmospheric pressure pushes down on the mercury.
The mercury is pushed up the glass tube.
Atmospheric pressure pushes down on the mercury, the mercury is pushed up the glass tube.
The more atmospheric pressure, the higher the mercury moves up the glass tube.
760 mmHg
mmHg is a measurement of atmospheric pressure
760 mmHg is the atmospheric pressure at sea level

6. Measuring Pressure
Manometer: measures the difference between atmospheric pressure and the gas pressure in a container h
Patm
Pgas < Patm
Pgas = Patm - h h
Patm
Pgas > Patm
Pgas = Patm + h
Atmospheric pressure pushes
down on the mercury.
Gas pressure pushes down
on the mercury.
The height is the difference between
the atmosphere and gas pressures
Pressure of the atmosphere is pushing down on the mercury on the right side, and the pressure of the gas is pushing down on the left side.
The height is the difference in pressure between the atmosphere and the gas.

7. Measuring Pressure Units of Pressure torr pounds per square inch (psi)
pascals (Pa)
millimeters mercury (mmHg)
atmospheres (atm)
Standard pressure at sea level is 760 mmHg or 1 atm

8. Measuring Temperature
Units of Temperature
fahrenheit (°F)
celsius (°C)
kelvin (K)
Standard temperature is 0°C or 273K

9. STP STP: standard temperature & pressure
Standard Pressure: 1atm = 760mmHg
Standard Temperature: 0°C = 273K
____ °C = K
We use Kelvins instead of Celsius for temperature because Kelvins start at absolute zero (lowest temp possible), so there is never a negative measurement of heat.

10. PhET Simulation Follow along with the PhET gas properties simulation
Fill out the worksheet

11. Gas Lab
Trash Bag
Egg in Flask
Crushing Cans
Soap in Microwave

12. Gas Laws Boyle’s Law Gay-Lussac’s Law Charles’ Law Combined Gas Law
Ideal Gas Law
Dalton’s Law
Graham’s Law

13. Boyle’s Law P1V1 = P2V2 Boyle’s Law: pressure and volume
Inversely proportional relationship (opposite): if volume decreases, pressure increases
P1V1 = P2V2

14. Boyle’s Law
Ex. A balloon has a volume of 1.2L and a pressure of 760mmHg. If the pressure increases to 800mmHg, what is the new volume?
P1V1 = P2V2
V2 = (P1V1)/P2 = (760mmHg x 1.2L) / (800mmHg) = 1.14L

15. Gay-Lussac’s Law P1 = P2 T1 T2
Gay-Lussac’s Law: pressure and temperature
Directly proportional relationship (same): if temperature increases, pressure increases
P1 = P2
T T2

16. Gay-Lussac’s Law
Ex. A container has gas with a pressure of 760mmHg at a temperature of 20°C, and the temperature increases to 40°C. What is the new pressure?
Hint: use kelvin!
P1/T1 = P2/T2
P2 = T2 x (P1/T1) = (40°C+273) x (760mmHg/(20°C+273)) = 812mmHg

17. Charles’ Law V1 = V2 T1 T2 Charles’ Law: temperature and volume
Directly proportional relationship (same): if temperature increases, volume increases
V1 = V2
T T2

18. Charles’ Law
Ex. A 5L balloon at 10°C is heated to a temperature of 70°C. What is the new volume?
Hint: use kelvin!
V1/T1 = V2/T2
V2 = T2 x V1/T1 = (70°C+273) x (5L/(10°C+273)) = 6.06L

19. Combine the gas laws… P1V1 = P2V2 V1 = V2 P1 = P2 T1 T2 T1 T2 Boyle’s
Put the three equations together to form the combined gas law
Gay-Lussac’s
Charles’

20. Boyle’s Law, Gay-Lussac’s Law, & Charles’ Law
Combined Gas Law
Combined Gas Law: pressure, volume, and temperature
Boyle’s Law, Gay-Lussac’s Law, & Charles’ Law
P1V1 = P2V2
T T2

21. Combined Gas Law
Ex. A 10L balloon at sea level with a pressure of 1atm and a temperature of 30°C is taken to the top of Mauna Kea, where the pressure is 0.6atm and the temperature is 15°C. What is the new volume of the balloon?
(P1V2)/T1 = (P2V2)/T2
V2 = (P1V1T2)/(T1P2) = (1atm x 10L x (15°C+273)) / ((30°C+273) x 0.6atm) = 15.8L

22. Ideal Gas Law PV = nRT Ideal Gas Law: equation for an “ideal gas”
Pressure (P), volume (V), temperature (T), moles (n), and the gas constant (R)
PV = nRT
Ideal Gas: follows all of the gas laws perfectly
Use the gas constant (R) that matches the unit of pressure (mmHg or atm)
*NOTE: the gas constant (R)’s unit of volume is L, so you must convert ml to L
R = 62.4 LmmHg/molK or
R = Latm/molK
Use the gas constant (R) that matches
the unit of pressure (mmHg or atm)

23. Ideal Gas Law
Ex. What is the volume of 2mol of gas at 0.8atm and 20°C?
PV = nRT
V = nRT/P = [(2mol)(0.0821Latm/molK)(293K)] / (0.8atm) = 60.1L
R = 62.4 LmmHg/molK or
R = Latm/molK
Use the gas constant (R) that matches
the unit of pressure (mmHg or atm)

24. Ideal Gas Law
Ex. A sample of gas occupies 5L at a pressure of 740mmHg and a temperature of 25°C. How many moles are in the sample of gas?
PV = nRT
n = PV/RT = [(740mmHg)(5L)] / [(62.4LmmHg/molK)(298K)] = 0.2 mol
R = 62.4 LmmHg/molK or
R = Latm/molK
Use the gas constant (R) that matches
the unit of pressure (mmHg or atm)

25. Dalton’s Law Ptotal = P1 + P2 + P3 + …
Dalton’s Law: pressure of a mixture of gases
Pressure depends on the number of gas particles
Add all the pressures together to get the total
Ptotal = P1 + P2 + P3 + …

26. Dalton’s Law
Ex. A sample of gas contains oxygen, nitrogen, and helium gas. The oxygen exerts a pressure of 300mmHg, the nitrogen exerts a pressure of 450mmHg, and the helium exerts a pressure of 100mmHg. What is the total pressure?
Ptotal = P1 + P2 + P3
Ptotal = 300mmHg + 450mmHg + 100mmHg = 850mmHg

27. Graham’s Law Graham’s Law: speed of gases at the same temperature…
Heavy molecules move slower
Light molecules move faster
*Compare atomic masses from the periodic table

28. Graham’s Law
Ex. A sample of helium and a sample of argon have a temperature of 25°C, which gas is moving faster?
He= 4 g/mol, Ar=40 g/mol. Helium is lighter so it moves faster.
If the gases are at different temperatures, the hotter one moves faster and the colder one moves slower.