1. Gases doing all of these things!
BEHAVIOR OF GASES
Gases have weight
Gases take up space
Gases exert pressure
Gases fill their containers
Gases doing all of these things!

2. Kinetic Theory of Gases The basic assumptions of the kinetic molecular theory are:
Gases are mostly empty space
The molecules in a gas are separate, very small and very far apart

3. Kinetic Theory of Gases The basic assumptions of the kinetic molecular theory are:
Gas molecules are in constant, chaotic motion
Collisions between gas molecules are elastic (there is no energy gain or loss)

4. Kinetic Theory of Gases The basic assumptions of the kinetic molecular theory are:
The average kinetic energy of gas molecules is directly proportional to the absolute temperature
Gas pressure is caused by collisions of molecules with the walls of the container

5. Measurements of Gases
To describe a gas, its volume, amount, temperature, and pressure are measured.
Volume: measured in L, mL, cm3 (1 mL = 1 cm3)
Amount: measured in moles (mol), grams (g)
Temperature: measured in KELVIN (K)
K = ºC + 273
Pressure: measured in mm Hg, torr, atm, etc.
P = F / A (force per unit area)

6. P = F /A Moderate Force (about 100 lbs) Small Area (0.0625 in2)
Enormous Pressure (1600 psi)

7. Large Surface Area (lots of nails)
Bed of Nails
Moderate Force
Small Pressure
P = F / A
Large Surface Area (lots of nails)

8. Units of Pressure Units of Pressure: 1 atm = 760 mm Hg
1 atm = 760 torr
1 atm = x 105 Pa
1 atm = kPa
1 atm = bar

9. Boyle’s Law
For a given number of molecules of gas at a constant temperature, the volume of the gas varies inversely with the pressure.
As P, V (when T and n are constant) and vice versa…. INVERSE RELATIONSHIP
V  1/P
P1V1 = P2V2

10. P1V1 = P2V2 (1.2 atm)(12 L) = (3.6 atm)V2 V2 = 4.0 L
Example: A sample of gas occupies 12 L under a pressure of 1.2 atm. What would its volume be if the pressure were increased to 3.6 atm? (assume temp is constant)
P1V1 = P2V2
(1.2 atm)(12 L) = (3.6 atm)V2
V2 = 4.0 L

11. Charles’ Law
Jacques Charles ( )
The volume of a given number of molecules
is directly proportional to the Kelvin temperature.
As T, V  (when P and n are constant) and vice versa…. DIRECT RELATIONSHIP
V  T

12. V1 / T1= V2 / T2 (117 mL) / (373 K) = (234 mL) / T2 T2 = 746 K
Example: A sample of nitrogen gas occupies 117 mL at 100.°C. At what temperature would it occupy 234 mL if the pressure does not change? (express answer in K and °C)
V1 / T1= V2 / T2
(117 mL) / (373 K) = (234 mL) / T2
T2 = 746 K
T2 = 473 ºC

13. Combined gas law
Combining Boyle’s law (pressure-volume) with Charles’ Law (volume-temp):
This is for one gas undergoing changing conditions of temp, pressure, and volume.

14. (985 torr)(105 L)(273K) = (760torr)(V2)(300K)
Example 1: A sample of neon gas occupies 105 L at 27°C under a pressure of 985 torr. What volume would it occupy at standard conditions?
P1 = 985 torr
V1 = 105 L
T1 = 27 °C = 300. K
P2 = 1 atm = 760 torr
V2 = ?
T2 = 0 °C = 273 K
P1V1T2 = P2V2T1
(985 torr)(105 L)(273K) = (760torr)(V2)(300K)
V2= 124 L

15. (80.0kPa)(10.0L)(T2) = (107kPa)(20.0L)(513K)
Example 2: A sample of gas occupies 10.0 L at 240°C under a pressure of 80.0 kPa. At what temperature would the gas occupy 20.0 L if we increased the pressure to 107 kPa?
P1 = 80.0 kPa
V1 = 10.0 L
T1 = 240 °C = 513 K
P2 = 107 kPa
V2 = 20.0 L
T2 = ?
P1V1T2 = P2V2T1
(80.0kPa)(10.0L)(T2) = (107kPa)(20.0L)(513K)
T2= 1372K≈ 1370K

16. Example 3: A sample of oxygen gas occupies 23. 2 L at 22. 2 °C and 1
Example 3: A sample of oxygen gas occupies 23.2 L at 22.2 °C and 1.3 atm. At what pressure (in mm Hg) would the gas occupy 11.6 L if the temperature were lowered to 12.5 °C?
P1 = 1.3 atm = 988 mmHg
V1 = 23.2 L
T1 = 22.2 °C = K
P2 = ?
V2 = 11.6 L
T2 = 12.5 °C = K
P1V1T2 = P2V2T1
(988mm Hg)(23.2L)(285.5K) = (P2)(11.6L)(295.2K)
P2= 1938 mm Hg ≈ 1900 mmHg

17. Gases: Standard Molar Volume & The Ideal Gas Law
Avogadro’s Law: at the same temperature and pressure, equal volumes of all gases contain the same # of molecules (& moles).
Standard molar volume = 22.4
This is true of “ideal” gases at reasonable temperatures and pressures ,the behavior of many “real” gases is nearly ideal.

18. Example: 1. 00 mole of a gas occupies 36. 5 L, and its density is 1
Example: 1.00 mole of a gas occupies 36.5 L, and its density is 1.36 g/L at a given temperature & pressure.
a) What is its molecular weight (molar mass)?

19. b) What is the density of the gas under standard conditions?
Example: 1.00 mole of a gas occupies 36.5 L, and its density is 1.36 g/L at a given temperature & pressure.
b) What is the density of the gas under standard conditions?

20. The units of R depend on the units used for P, V & T
The IDEAL GAS LAW
Shows the relationship among the pressure, volume, temp. and # moles in a sample of gas.
P = pressure (atm)
V = volume (L)
n = # moles
T = temp (K)
R = universal gas constant =
PV=nRT
The units of R depend on the units used for P, V & T

21. PV = nRT V = 23.6 L P = (1820 torr)(1 atm/760 torr) = 2.39 atm V = ?
Example 1: What volume would 50.0 g of ethane, C2H6, occupy at 140 ºC under a pressure of 1820 torr?
P = (1820 torr)(1 atm/760 torr) = 2.39 atm
V = ?
n = (50.0 g)(1 mol / 30.0 g) = 1.67 mol
T = 140 °C = 413 K
PV = nRT
(2.39 atm)(V) = (1.67 mol)( L·atm/mol·K)(413 K)
V = 23.6 L

22. Example 2: Calculate (a) the # moles in, and (b) the mass of an 8
Example 2: Calculate (a) the # moles in, and (b) the mass of an 8.96 L sample of methane, CH4, measured at standard conditions.
P = 1.00 atm
V = 8.96 L
n = ?
T = 273 K
(a)
PV = nRT
(1 atm)(8.96 L) = (n)( L·atm/mol·K)(273 K)
n = mol

23. Example 2: Calculate (a) the # moles in, and (b) the mass of an 8
Example 2: Calculate (a) the # moles in, and (b) the mass of an 8.96 L sample of methane, CH4, measured at standard conditions.
(a)
Or the easier way…

24. (b) Convert moles to grams…
Example 2: Calculate (a) the # moles in, and (b) the mass of an 8.96 L sample of methane, CH4, measured at standard conditions.
(b)
Convert moles to grams…

25. Example 3: Calculate the pressure exerted by 50
Example 3: Calculate the pressure exerted by 50.0 g ethane, C2H6, in a 25.0 L container at 25 ºC?
P = ?
V = 25.0 L
n = (50.0 g)(1 mol / 30.0 g) = 1.67 mol
T = 25 °C = 298 K
PV = nRT
(P)(25.0 L) = (1.67 mol)( L·atm/mol·K)(298 K)
P = 1.63 atm

26. Determining Molecular Weights & Molecular Formulas of Gases:
If the mass of a volume of gas is known, we can use this info. to determine the molecular formula for a compound.

27. Example 1: A g sample of pure gaseous compound occupies 112 mL at 100. ºC and 750. torr. What is the molecular weight of the compound?
First find # moles
Then use mass to determine g/mol…
PV = nRT
(0.99 atm)(0.112 L) = (n)( L·atm/mol·K)(373 K)
n = mol

28. Example 2: A compound that contains only C and H is 80. 0% C and 20
Example 2: A compound that contains only C and H is 80.0% C and 20.0% H by mass. At STP, 546 mL of the gas has a mass of g. What is the molecular (true) formula for the compound?
First find the empirical formula…
Empirical formula = CH3

29. Example 2: A compound that contains only C and H is 80. 0% C and 20
Example 2: A compound that contains only C and H is 80.0% C and 20.0% H by mass. At STP, 546 mL of the gas has a mass of g. What is the molecular (true) formula for the compound?
Next, determine the MW of the sample…
0.024 mol

30. CH3 x 2 = C2H6 Empirical formula = CH3  15.0 g/mol
Example 2: A compound that contains only C and H is 80.0% C and 20.0% H by mass. At STP, 546 mL of the gas has a mass of g. What is the molecular (true) formula for the compound?
Finally, determine the molecular formula…
Empirical formula = CH3  15.0 g/mol
True MW = 30.0 g/mol
CH3 x 2 = C2H6

31. Partial Pressures and Mole Fractions
In a mixture of gases each gas exerts the pressure it would exert if it occupied the volume alone.
The total pressure exerted by a mixture of gases is the sum of the partial pressures of the individual gases:
Ptotal = P1 + P2 + P3 + …

32. Notice the two gases are measured at the same temp. and vol.
Example: If mL of hydrogen gas, measured at 25C and 3.00 atm, and mL of oxygen, measured at 25C and 2.00 atm, what sould be the pressure of the mixture of gases?
Notice the two gases are measured at the same temp. and vol.
Ptotal = P1 + P2 + P3 + …
PT = 3.00 atm atm
PT = 5.00 atm

33. Vapor Pressure of a Liquid
The pressure exerted by its gaseous molecules in equilibrium with the liquid; increases with temperature

34. Vapor Pressure of a Liquid
Patm = Pgas + PH2O or
Pgas = Patm - PH2O

35. Vapor Pressure of a Liquid
Temp. (C)
v.p. of water
(mm Hg) 18
15.48 21
18.65 19
16.48 22
19.83 20
17.54 23
21.07
See A-2 for a complete table

36. PH2 = Patm - PH2O PH2 =748 mm Hg – 23.76 mm Hg PH2 = 724.24 mm Hg
Example 1: A sample of hydrogen gas was collected by displacement of water at 25 C. The atmospheric pressure was 748 mm Hg. What pressure would the dry hydrogen exert in the same conditions?
PH2 = Patm - PH2O
PH2 =748 mm Hg – mm Hg
PH2 = mm Hg
PH2  724 mm Hg

37. Example 2: A sample of oxygen gas was collected by displacement of water. The oxygen occupied 742 mL at 27 C. The barometric pressure was 753 mm Hg. What volume would the dry oxygen occupy at STP?
PO2 = Patm - PH2O
PO2 =753 mm Hg – mm Hg
PO2 = 726 mm Hg
P1V1T2 = P2V2T1
(726 mm Hg)(0.742 L)(273K) = (760 mm Hg)(V2)(300K)
V2 = L

38. PH2 = Patm - PH2O PH2 =758 mm Hg – 23.76 mm Hg PH2 = 734 mm Hg
Example 3: A student prepares a sample of hydrogen gas by electrolyzing water at 25 C. She collects 152 mL of H2 at a total pressure of 758 mm Hg. Calculate: (a) the partial pressure of hydrogen, and (b) the number of moles of hydrogen collected.
PH2 = Patm - PH2O
PH2 =758 mm Hg – mm Hg
PH2 = 734 mm Hg

39. Example 3: A student prepares a sample of hydrogen gas by electrolyzing water at 25 C. She collects 152 mL of H2 at a total pressure of 758 mm Hg. Calculate: (a) the partial pressure of hydrogen, and (b) the number of moles of hydrogen collected.
PV = nRT
(0.966 atm)(0.152 L) = (n)( L·atm/mol·K)(298 K)
n = mol H2

40. Graham’s Law of Diffusion & Effusion
Where,
Rate = rate of diffusion or effusion
MM=molar mass

41. Stoichiometry of Gaseous Reactions
A balanced equation can be used to relate moles or grams of substances taking part in a reaction. (AND VOLUME!)

42. Example: Hydrogen peroxide is the active ingredient in commercial preparations for bleaching hair. What mass of hydrogen peroxide must be used to produce 1.00 L of oxygen gas at 25 C and 1.00 atm?
2H2O2  O2 + 2H2O
PV = nRT
(1.00 atm)(1.00 L) = (n)( L·atm/mol·K)(298 K)
n = mol O2

43. Real Gases
&
Ideal Gases

44. The “Ideal” in the ideal gas law refers to the gas in question
The “Ideal” in the ideal gas law refers to the gas in question. The assumption is that all gasses behave in ideal ways. In real life, such is not the case.

45. For the most precise work, we need to correct for this deviation from ideal behavior.

46. Deviation from Ideal Behavior
The attractive forces among molecules operate at relatively short distances. For a gas at atmospheric pressure, the molecules are far apart and these attractive forces are negligible.
At high pressures, the density of the gas increases, and the molecules are much closer to one another.
Then intermolecular forces can become significant enough to affect the motion of the molecules, and the gas will no longer behave ideally.

47. Another way to observe the nonideality of gases is to lower the temperature. Cooling a gas decreases the molecules’ average kinetic energy, which in a sense deprives molecules of the drive they need to break away from their mutual attractive influences.
Hot is more ideal!

48. The nonideality of gases requires some modifications to the ideal gas law. Such modifications were first made by me. J.D. van der Walls in 1873.
Dutch Physicist

49. Net Force
The speed of a molecule that is moving toward the container wall is reduced by the attractive forces exerted by its neighbors. Consequently, the impact this molecule will make on the wall is not as great as it would be if no intermolecular forces were present.

50. The measured gas pressure is always lower than the pressure the gas would have if it behaved ideally.

51. Pressure Correction
The interaction between molecules that gives rise to non-ideal behavior depends on how frequently any two molecules approach each other closely.
The number of such “encounters” increases as the square of the number of molecules per unit volume, (n/V)2, because the presence of each of the two molecules in a particular region is proportional to n/V.

52. Pressure Correction
The quantity Pideal is the pressure we would measure if there were no intermolecular attraction. The quantity a then is just a proportionally constant in the correction term for pressure.

53. Volume Correction
Each real molecule occupies a small intrinsic volume, so the effective volume of a gas becomes
(V-nb) where…
n is the number of moles
b is a constant (the volume of a single molecule)
The term nb represents the volume occupied by n moles of of the gas.

54. Van der Waals Equation
Having taken into account the corrections for pressure and volume, we can write the ideal gas law as …..
This equation is known as van der Waals equation. The constants a and b are selected for each gas to give the best possible agreement between the equation and actual observed behavior.

55. Get Real!