1. Gas Pressure, and Gas Laws
Chapter Gases (p )
Properties of Gases,
Gas Pressure, and Gas Laws

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.
Mixtures of gases are always homogeneous. 2

3. Air Pressure
Pressure: a measure of the force applied by gas molecules on objects
This can be felt by the walls of a container
or on objects in air

4. Pressure What causes gas pressure in a closed container?
Pressure is the result of a force distributed over an area.
Collisions between particles of a gas and the walls of the container cause the pressure in a closed container of gas.

5. Helium Filled Balloon
The helium atoms in a balloon are constantly moving.
When many particles collide with the walls of a container at the same time, they produce a measurable pressure.
The more frequent the collisions, the greater the pressure is.
The speed of the particles and their mass also affect the pressure.

6. 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.

7. Factors That Affect Gas Pressure
Factors that affect the pressure of an enclosed gas are its temperature, its volume, and the number of its particles.

8. Temperature P= 60 kPa P= 72 kPa
Raising the temperature of a gas will increase its pressure if the volume of the gas and the number of particles are constant. P=
60 kPa P=
72 kPa

9. Effect of Temperature on Pressure in a Fixed Volume
As the temperature rises, the average kinetic energy of the particles in the air increases.
With increased kinetic energy, the particles move faster and collide more often with the inner walls of the container.
Faster-moving particles hit the walls with greater force.
More collisions and increased force cause the pressure of the gas in the container to rise.

10. Pressure and Volume Boyle’s Law
Reducing the volume of a gas increases its pressure if the temperature of the gas and the number of particles are constant. 10

11. Boyle’s Law Boyle’s law states that
the pressure of a gas is inversely related to its volume when T and the amount of molecules are constant.
if volume decreases, the pressure increases. 11

12. In the next lab, we will test…
When you double the pressure on a gas,
the volume is cut in half (as long as the
temperature and amount of gas do not change).
Tro's Introductory Chemistry, Chapter 11 12

13. Boyle’s Law
Robert Boyle described the relationship between the pressure and volume of a gas. The graph shows an inverse relationship between the volume of a gas and the pressure of the gas.

14. 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, number of molecules (n) constant) 14

15. Solving for a Gas Law Factor
The equation for Boyle’s law can be rearranged to
solve for any factor.
P1V1 = P2V Boyle’s law
To solve for V2 , divide both sides by P2.
P1V1 = P2V2
P P2
V1 x P1 = V2 P2 15

16. Learning Check 1) pressure decreases 2) pressure increases
For a cylinder containing helium gas, indicate if cylinder A or cylinder B represents the new volume for the following changes (n and T are constant).
1) pressure decreases
2) pressure increases
Copyright © by Pearson Education, Inc. 16

17. Solution 1) pressure decreases B 2) pressure increases A
For a cylinder containing helium gas, indicate if cylinder A or cylinder B represents the new volume for the following changes (n and T are constant).
1) pressure decreases B
2) pressure increases A
Copyright © by Pearson Education, Inc. 17

18. Learning Check
A sample of helium gas in a balloon has a volume of 6.4 L at a pressure of 0.70 atm. At 1.40 atm (T is constant), is the new volume represented by A, B, or C?
Copyright © by Pearson Education, Inc. 18

19. Solution
A sample of helium gas in a balloon has a volume of 6.4 L at a pressure of 0.70 atm. At a higher pressure (T constant), the new volume is represented by the smaller balloon A. 19

20. Calculations with Boyle’s Law20

21. Calculation with Boyle’s Law
Freon-12, CCl2F2, is used in refrigeration systems. What is the new volume (L) of a 8.0 L sample of Freon gas initially at 550 mmHg after its pressure is changed to 2200 mmHg at constant T and n?
1. Set up a data table:
Conditions 1 Conditions 2
P1 = mmHg P = mmHg
V1 = L V2 = ? 21

22. Calculation with Boyle’s Law (continued)
2. When pressure increases, volume decreases.
Solve Boyle’s law for V2:
P1V1 = P2V2
V = V1 x P1 P2
V = L x 550 mmHg = L
2200 mmHg
pressure ratio
decreases volume 22

23. Example 1 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?
1) 60 mL ) 120 mL 3) 240 mL 23

24. Example 1 Answer P1 = 850 mmHg P2 = 425 mmHg V1 = 120 mL V2 = ??
Choice 3) 240 mL
P1 = 850 mmHg P2 = 425 mmHg
V1 = 120 mL V2 = ??
V2 = V1 x P1 = mL x mmHg = 240 mL
P mmHg Pressure ratio
increases volume 24

25. Example 2
A Cylinder with a Movable Piston Has a Volume of 6.0 L at 4.0 atm. What Is the Volume at 1.0 atm? 25

26. Example 2: A Cylinder with a Movable Piston Has a Volume of 6.0 L at 4.0 atm. What Is the Volume at 1.0 atm?
P1 = 4.0 atm P2 = 1.0 atm
V1 = 6.0 L V2 = ??
V2 = V1 x P1 = L x atm = 24 L
P atm Pressure ratio
increases volume
Check: Since P and V are inversely proportional, when the pressure decreases ~4x, the volume should increase ~4x, and it does. 26

27. Example 3
If the sample of helium gas has a volume of 6.4 L at a pressure of 0.70 atm, what is the new volume when the pressure is increased to 1.40 atm (T constant)?
A) 3.2 L B) 6.4 L C) 12.8 L 27

28. Example 3: Solution Choice A) 3.2 L V2 = V1 x P1 P2
V2 = 6.4 L x atm = L
1.40 atm
Volume decreases when there is an increase in the pressure (temperature is constant.)
P1 = 0.7 atm P2 = atm
V1 = 6.4 L V2 = ?? 28

29. Example 4 1) 200. mmHg 2) 400. mmHg 3) 1200 mmHg
A sample of oxygen gas has a volume of 12.0 L at 600. mmHg. What is the new pressure when the volume changes to 36.0 L? (T and n constant).
1) mmHg
2) mmHg
3) 1200 mmHg
Copyright © by Pearson Education, Inc. 29

30. Example 4: Solution Choice 1) 200. mmHg Data Table
Conditions 1 Conditions 2
P1 = mmHg P2 = ???
V1 = L V2 = L
P2 = P1 x V1 V2
600. mmHg x L = mmHg L 30

31. Temperature and Volume
Charles’s Law 31

32. Tro's Introductory Chemistry, Chapter 11
Standard Conditions
When doing gas problems, always have temperatures in kelvin.
K = °C + 273
Common reference points for comparing.
Standard pressure = 1.00 atm.
Standard temperature = 0 °C= 273 K.
STP.
Tro's Introductory Chemistry, Chapter 11 32

33. Charles’s Law In Charles’s Law,
the Kelvin temperature of a gas is directly related to the volume.
P and amount of molecules (n) are constant.
when the temperature of a gas increases, its volume increases.
Copyright © by Pearson Education, Inc. 33

34. Volume and Temperature
As a gas is heated, it expands.
This causes the density of the
gas to decrease.
Because the hot air in the
balloon is less dense than the
surrounding air, it rises. 34

35. Charles’s Law: V and T For two conditions, Charles’s law is written
V1 = V (P and n constant)
T T2
Rearranging Charles’s law to solve for V2:
T2 x V1 = V2 x T1
T T1
V2 = V1 x T2 T1 35

36. Learning Check
Solve Charles’s law expression for T2.
V1 = V2
T T2 36

37. Solution V1 = V2 T1 T2 Cross-multiply to give: V1T2 = V2T1
Isolate T2 by dividing through by V1:
V V1
T2 = T1 x V2 V1 37

38. E1: Using Charles’s Law
A balloon has a volume of 785 mL at 21 °C. If the
temperature drops to 0 °C, what is the new volume of
the balloon (P constant)?
1. Set up data table:
Conditions 1 Conditions 2
V1 = 785 mL V2 = ?
T1 = 21 °C = 294 K T2 = 0 °C = 273 K
Be sure to use the Kelvin (K) temperature in
gas calculations. 38

39. E1: Solution 2. Solve Charles’s law for V2: V1 = V2 T1 T2 V2 = V1 x T2
V2 = 785 mL x K = 729 mL
294 K 39

40. E2
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)?
1) °C
2) 170 °C
3) °C 40

41. E2: Solution 1. Set up data table: Conditions 1 Conditions 2
V1 = 420 mL V2 = 640 mL
T1 = 18 °C +273= 291 K T2 = ? °C
T2 = T1 x V2 V1
T2 = K x mL = K
420 mL
= 443 K – = °C 41

42. Learning Check Use the gas laws to complete each sentence with
1) increases or 2) decreases.
A. Pressure _______ when V decreases.
B. When T decreases, V _______.
C. Pressure _______ when V changes from 12 L to 24 L.
D. Volume _______when T changes from 15 °C to 45 °C. 42

43. Solution Use the gas laws to complete each sentence with
1) increases or 2) decreases.
A. Pressure 1) increases when V decreases.
B. When T decreases, V 2) decreases.
C. Pressure 2) decreases when V changes from 12 L to 24 L.
D. Volume 1) increases when T changes from 15 °C to 45 °C. 43