Chapter 12 Gases

1
Chapter 12Gases

• The Combined Gas Law
• Volume and Moles
• (Avogadros Law)
• Partial Pressures

2
Combined Gas Law

• Combines Boyles Law,
• Charles Law and Gay Lussacs Law

3
Combined Gas Law Problem

• A sample of helium gas has a volume of 0.180 L,
  a pressure of 0.800 atm and a temperature of
  29C. What is the new temperature(C) of the
  gas at a volume of 90.0 mL and a pressure of 3.20
  atm?

4
Data Table

• Set up Data Table
• P1 0.800 atm P2 3.20 atm
• V1 0.180 L V2 90.0 mL
• T1 302 K T2 ??
• Fill in with increase or decrease
• P change will _____________ T.
• V change will _____________ T.

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Solution

• Enter data for T and P, then T and V
• T2 302 K x atm x mL
  K
• atm mL
• T2 K - 273 C

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Calculation

• Solve for T2
• T2 302 K x 3.20 atm x 90.0 mL 604 K
• 0.800 atm 180.0 mL
• (inc. T) (dec. T)
• T2 604 K - 273 331 C

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Learning Check C1

• A gas has a volume of 675 mL at 35C and 0.850
  atm pressure. What is the temperature in C when
  the gas has a volume of 0.315 L and a pressure of
  802 mm Hg?

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Solution C1

• T1 308 K T2 ?
• V1 675 mL V2 0.315 L 315 mL
• P1 0.850 atm P2 802 mm Hg
• 646 mm Hg
• T2 308 K x 802 mm Hg x 315 mL
• 646 mm Hg
  675 mL
• P inc, T inc V dec,
  T dec
• 178 K - 273 - 95C

9
Volume and Moles

• How does adding more molecules of a gas change
  the volume of the air in a tire?
• If a tire has a leak, how does the loss of air
  (gas) molecules change the volume?

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Learning Check C2

• True (1) or False(2)
• 1.___The P exerted by a gas at constant V is not
  affected by the T of the gas.
• 2.___ At constant P, the V of a gas is directly
  proportional to the absolute T
• 3.___ At constant T, doubling the P will cause
  the V of the gas sample to decrease to one-half
  its original V.

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Solution C2

• True (1) or False(2)
• 1. (2)The P exerted by a gas at constant V is not
  affected by the T of the gas.
• 2. (1) At constant P, the V of a gas is directly
  proportional to the absolute T
• 3. (1) At constant T, doubling the P will cause
  the V of the gas sample to decrease to one-half
  its original V.

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Avogadros Law

• When a gas is at constant T and P, the V is
  directly proportional to the number of moles (n)
  of gas
• V1 V2
• n1 n2
• initial final
  

13
STP

• The volumes of gases can be compared when they
  have the same temperature and pressure (STP).
• Standard temperature 0C or 273 K
• Standard pressure 1 atm (760 mm Hg)

14
Learning Check C3

• A sample of neon gas used in a neon sign has a
  volume of 15 L at STP. What is the volume (L) of
  the neon gas at 2.0 atm and 25C?
• P1 V1 T1 K
• P2 V2 ?? T2 K
• V2 15 L x atm x K
  6.8 L
• atm K

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Solution C3

• P1 1.0 atm V1 15 L T1 273 K
  
• P2 2.0 atm V2 ?? T2 248 K
• V2 15 L x 1.0 atm x 248 K
  6.8 L
• 2.0 atm 273 K

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Molar Volume

• At STP
• 4.0 g He 16.0 g CH4 44.0 g CO2
• 1 mole 1 mole 1mole
• (STP) (STP) (STP)
• V 22.4 L V 22.4 L V
  22.4 L

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Molar Volume Factor

• 1 mole of a gas at STP 22.4 L
• 22.4 L and 1 mole
• 1 mole 22.4 L

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Learning Check C4

• A.What is the volume at STP of 4.00 g of CH4?
• 1) 5.60 L 2) 11.2 L 3) 44.8 L
• B. How many grams of He are present in 8.0 L of
  gas at STP?
• 1) 25.6 g 2) 0.357 g 3) 1.43 g

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Solution C4

• A.What is the volume at STP of 4.00 g of CH4?
• 4.00 g CH4 x 1 mole CH4 x 22.4 L (STP) 5.60 L
• 16.0 g CH4 1 mole
  CH4
• B. How many grams of He are present in 8.0 L of
  gas at STP?
• 8.00 L x 1 mole He x 4.00 g He 1.43
  g He
• 22.4 He 1 mole
  He

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Partial Pressure

• Pressure that each gas in a mixture would exert
  if it were the only gas in the container

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Daltons Law of Partial Pressures

• The total pressure exerted by a gas mixture is
  the sum of the partial pressures of the gases in
  that mixture.
• PT P1 P2 P3 .....

22
Gases in the Air

• The of gases in air Partial pressure (STP)
  
• 78.08 N2 593.4 mm Hg
• 20.95 O2 159.2 mm Hg
• 0.94 Ar 7.1 mm Hg
• 0.03 CO2 0.2 mm Hg
• PAIR PN PO PAr PCO 760 mm Hg
• 2 2
  2

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Learning Check C5

• A.If the atmospheric pressure today is 745 mm Hg,
  what is the partial pressure (mm Hg) of O2 in the
  air?
• 1) 35.6 2) 156 3) 760
• B. At an atmospheric pressure of 714, what is the
  partial pressure (mm Hg) N2 in the air?
• 1) 557 2) 9.14 3) 0.109

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Solution C5

• A.If the atmospheric pressure today is 745 mm Hg,
  what is the partial pressure (mm Hg) of O2 in the
  air?
• 2) 156
• B. At an atmospheric pressure of 714, what is the
  partial pressure (mm Hg) N2 in the air?
• 1) 557

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Partial Pressures

• The total pressure of a gas mixture depends
• on the total number of gas particles, not on
• the types of particles.
• P 1.00 atm P 1.00
  atm

1 mole H2
0.5 mole O2 0.3 mole He 0.2 mole Ar
26
Health Note

• When a scuba diver goes under water, the high
  pressure of the water causes more N2 (g) to
  dissolve in the blood. If the diver rises too
  fast, the dissolved N2 will form bubbles in the
  blood, a dangerous and painful condition called
  "the bends". Helium, which is inert, less dense,
  and does not dissolve in the blood, is mixed with
  O2 in scuba tanks used for deep descents.

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Learning Check C6

• A scuba tank contains O2 with a pressure of
  0.540 atm and He at 855 mm Hg? What is the
  total pressure in mm Hg in the tank?

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Solution C6

• 0.450 atm x 760 mm Hg 342 mm Hg
• 1 atm
• 342 mm Hg 855 mm Hg 1197 mm Hg