Gases: Their Properties and Behavior

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Chapter 9

• Gases Their Properties and Behavior

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(No Transcript)
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Gases and Gas Pressure
Gas by Vol Gas by Vol
Nitrogen 78.084 Krypton 0.0001
Oxygen 20.948 Carbon monoxide 0.00001
Argon 0.934 Xenon 0.000008
Carbon dioxide 0.033 Ozone 0.000002
Neon 0.00182 Ammonia 0.000001
Hydrogen 0.0010 Nitrogen dioxide 0.0000001
Helium 0.00052 Sulfur dioxide 0.00000002
Methane 0.0002
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Properties of Gases

• Gases mix completely with one another to form
  homogenous mixtures
• Gases can be compressed (keyboard cleaner)
• Gases exert pressure on what ever is around them
  (balloon, film canister)
• Gases expand into whatever volume is available
  (coke bottle and balloon)
• Gases are described in terms of their temperature
  and pressure, the volume occupied and the amount
  of gas present (gas properties)

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Properties of Gases

• Pressure (P)
• Pressure force / area
• Force mass x acceleration
• Units of Pressure
• Atmosphere (atm)
• Torr
• Pascals (Pa)
• mmHg
• Pressure conversions
• 1 atm 1.01325 x 105 Pa
• 1 atm 760 torr
• 1 atm 760 mmHg

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Problem

• Convert these pressure values.
• 120 mmHg to atm
• 100 kPa to mmHg
• 270 torr to atm

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Properties of Gases

• Volume (V)
• mL
• L
• cm3
• Amount of gas (n) moles
• Temperature (T) - Kelvins

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Kinetic Molecular Theory

• A gas is composed of molecules whose size is much
  smaller than the distance between them
• Gas molecules move randomly at various speeds and
  in every possible direction

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Kinetic Molecular Theory

• Except when gas molecules collide, forces of
  attraction and repulsion between them are
  negligible
• When collisions between molecules occur, they are
  elastic
• The average kinetic energy of gas molecules is
  proportional to the absolute temperature (liquid
  N2 ballons, can, solar bag)
• Ek ½ (mass)(speed)2

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Distribution of molecular speeds at three
temperatures.
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Relationship between molar mass and molecular
speed.
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Problem

• Place these gases in order of increasing average
  molecular speed at 25oC Kr, CH4, N2, and CH2Cl2

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The Gas Laws

• Gas properties
• Gases are described in terms of their temperature
  and pressure, the volume occupied and the amount
  of gas present (gas properties)
• Gas Laws can be derived using
• Kinetic Molecular Theory

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The Gas Laws

• The Pressure-Volume Relationship Boyles Law
• The volume (V) of an ideal gas varies inversely
  with the applied pressure (P) when temperature
  (T) and the amount (n, moles) are constant
• PiVi PfVf

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Problems

• A sample of nitrogen gas at 298 K and 745 torr
  has a volume of 37.42 L. What volume will it
  occupy if the pressure is increased to 894 torr
  at constant temperature?
• A)22.3 L
• B)31.2 L
• C)44.9 L
• D)112 L
• E)380 L

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Problems

• A sample of carbon dioxide gas at 125C and 248
  torr occupies a volume of 275 L. What will the
  gas pressure be if the volume is increased to 321
  L at 125C?
• A)212 torr
• B)289 torr
• C)356 torr
• D)441 torr
• E)359 torr

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The Gas Laws

• The Temperature-Volume Relationship Charless
  Law
• The volume (V) of an ideal gas varies directly
  with absolute temperature (T) when pressure (P)
  and amount (n) are constant.
• Vi / Ti Vf / Tf

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Problems

• A sample container of carbon monoxide occupies a
  volume of 435 mL at a pressure of 785 torr and a
  temperature of 298 K. What would its temperature
  be if the volume were changed to 265 mL at a
  pressure of 785 torr?
• A)182 K
• B)298 K
• C)387 K
• D)489 K
• E)538 K

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Problems

• A 0.850-mole sample of nitrous oxide, a gas used
  as an anesthetic by dentists, has a volume of
  20.46 L at 123C and 1.35 atm. What would be its
  volume at 468C and 1.35 atm?
• A)5.38 L
• B)10.9 L
• C)19.0 L
• D)38.3 L
• E)77.9 L

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The Combined Gas Law

• Combined Gas Law used when a specific amount
  of gas is exposed to two different conditions
• P1V1 / T1 P2V2 / T2

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Problems

• A sample of propane, a component of LP gas, has a
  volume of 35.3 L at 315 K and 922 torr. What is
  its volume at STP?
• A)25.2 L
• B)30.6 L
• C)33.6 L
• D)37.1 L
• E)49.2 L

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Problems

• Calculate the pressure of a helium sample at
  -207.3C and 768 mL if it exerts a pressure of
  175 kPa at 25.0C and 925 mL.
• A)32.1 kPa
• B)46.6 kPa
• C)657 kPa
• D)953 kPa
• E)340 kPa

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Problems

• A carbon dioxide sample weighing 44.0g occupies
  32.68 L at 65C and 645 torr. What is its volume
  at STP?
• A)22.4 L
• B)31.1 L
• C)34.3 L
• D)35.2 L
• E)47.7 L

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The Gas Laws

• The Amount-Volume Relationship Avogadros Law
• The volume (V) of an ideal gas varies directly
  with amount (n) when temperature (T) and pressure
  (P) are constant
• V1 / n1 V2 / n2

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The Ideal Gas Law

• Boyles, Charless and Avogadros Laws can be
  combined to form the Ideal Gas Law
• PV nRT
• R ideal gas constant
• R 0.0821 atm L / mol K
• R 62.36 torr L / mol K
• R 8.314 J / mol K

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Problems

• A sample of nitrogen gas is confined to a 14.0 L
  container at 375 torr and 37.0C. How many moles
  of nitrogen are in the container?
• A)0.271 mol
• B)2.27 mol
• C)3.69 mo1
• D)206 mol
• E)227 mol

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Stoichiometric Relationships with Gases

• Various questions can be asked that relate gas
  laws to stoichiometry.

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Problems

• A 250.0-mL sample of ammonia, NH3(g), exerts a
  pressure of 833 torr at 42.4C. What mass of
  ammonia is in the container?
• A)0.0787 g
• B)0.180 g
• C)8.04 g
• D)17.0 g
• E)59.8 g

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Stoichiometric Relationships with Gases

• The ideal gas law can be used to determine
  density if the molar mass of the gas is known or
  the molar mass if the mass of gas is known
• d m / V PM / RT
• Density increases with molar mass

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Problems

• 9What is the density of carbon dioxide gas at
  -25.2C and 98.0 kPa?
• A)0.232 g/L
• B)0.279 g/L
• C)0.994 g/L
• D)1.74 g/L
• E)2.09 g/L

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Problems

• A flask with a volume of 3.16 L contains 9.33
  grams of an unknown gas at 32.0C and 1.00 atm.
  What is the molar mass of the gas?
• A)7.76 g/mol
• B)66.1 g/mol
• C)74.0 g/mol
• D)81.4 g/mol
• E)144 g/mol

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Problems

• Dr. I. M. A. Brightguy adds 0.1727 g of an
  unknown gas to a 125-mL flask. If Dr. B finds the
  pressure to be 736 torr at 20.0C, is the gas
  likely to be methane, CH4, nitrogen, N2, oxygen,
  O2, neon, Ne, or argon, Ar?
• A)CH4
• B)N2
• C)Ne
• D)Ar
• E)O2

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Partial Pressures and Daltons Law

• Daltons Law of Partial Pressures the total
  pressure exerted by a mixture of gases is the sum
  of the partial pressures of the individual gases
  in the mixture.
• Since all gases in a mixture occupy the same
  volume and are at the same temperature then the
  pressure is directly related to the moles of gas
• ntotal ngas1 ngas2 ..

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Partial Pressures and Daltons Law

• Placing ntotal into the ideal gas law
• PtotalV ntotalRT
• Ptotal ntotalRT / V
• Ptotal Pgas1 Pgas2 ..,

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Partial Pressures and Daltons Law

• Pgas1 / Ptotal ngas1 / ntotal
• ngas1 / ntotal Xgas1
• Xgas1 is called the mole fraction
• All mole fractions 1

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Problems

• What is the pressure in a 7.50-L flask if 0.15
  mol of carbon dioxide is added to 0.33 mol of
  oxygen? The temperature of the mixture is 48.0C.
  
• A)0.252 atm
• B)0.592 atm
• C)1.69 atm
• D)3.96 atm
• E)4.80 atm

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Problems

• If 0.750 L of argon at 1.50 atm and 177C and
  0.235 L of sulfur dioxide at 95.0 kPa and 63.0C
  are added to a 1.00-L flask and the flask's
  temperature is adjusted to 25.0C, what is the
  resulting pressure in the flask?
• A)0.0851 atm
• B)0.244 atm
• C)0.946 atm
• D)1.74 atm
• E)1.86 atm

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Partial Pressures and Daltons Law

• Collecting Gas over water most common way of
  determining the moles of a gas sample as long
  as the gas is not water soluble.

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Problems

• Small quantities of hydrogen can be prepared by
  the addition of hydrochloric acid to zinc. A
  sample of 195 mL of hydrogen was collected over
  water at 25C and 753 torr. What mass of hydrogen
  was collected? (Pwater 24 torr at 25C)
• A)0.00765 g
• B)0.0154 g
• C)0.0159 g
• D)0.0164 g
• E)0.159 g

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Behavior of Ideal Gases

• Ideal Gas Law provides fairly accurate
  predictions for the pressure, volumes and
  temperatures of most gases except
• At extremely high pressures
• Extremely low temperatures

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Optional Homework

• Text 9.26, 9.28, 9.36, 9.44, 9.46, 9.48, 9.50,
  9.52, 9.54, 9.56, 9.62, 9.64, 9.66, 9.74, 9.78,
  9.90, 9.96, 9.104, 9.106
• Chapter 9 Homework on the website

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Required Homework

• Chapter 9 Assignment