Gases

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Chapter 5

• Gases

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Hurricanes, such as this one off the coast of
Florida, are evidence of the powerful forces
present in the earth's atmosphere.
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Important Characteristics of Gases
1) Gases are highly compressible An external
force compresses the gas sample and decreases
its volume, removing the external force
allows the gas volume to increase. 2) Gases
are thermally expandable When a gas sample
is heated, its volume increases, and when it is
cooled its volume decreases. 3) Gases have
low viscosity Gases flow much easier than
liquids or solids. 4) Most Gases have low
densities Gas densities are on the order of
grams per liter whereas liquids and solids
are grams per cubic cm, 1000 times greater. 5)
Gases are infinitely miscible
Gases mix in any proportion such as in air, a
mixture of many gases.
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Substances that are Gases under
Normal Conditions
Substance Formula
MM(g/mol)

• Helium He 4.0
• Neon Ne 20.2
• Argon Ar 39.9
• Hydrogen H2 2.0
• Nitrogen N2 28.0
• Nitrogen Monoxide NO
  30.0
• Oxygen O2 32.0
• Hydrogen Chloride HCL 36.5
• Ozone O3 48.0
• Ammonia NH3
  17.0
• Methane CH4 16.0

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Some Important Industrial Gases
Name - Formula Origin and
use
Methane (CH4) Natural
deposits domestic fuel Ammonia (NH3)
From N2 H2 fertilizers,
explosives Chlorine (Cl2)
Electrolysis of seawater bleaching
and
disinfecting Oxygen (O2)
Liquefied air steelmaking Ethylene (C2H4)
High-temperature decomposition of

natural gas plastics
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Pressure of the Atmosphere

• Called Atmospheric pressure, or the force
  exerted upon us by the atmosphere above us.
• A measure of the weight of the atmosphere
  pressing down upon us.
• Measured using a Barometer! - A device that can
  weigh the atmosphere above us!

Force Area
Pressure
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Figure 5.1 A torricellian barometer.
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Construct a Barometer using Water!

• Density of water 1.00 g/cm3
• Density of Mercury 13.6 g/cm3
• Height of water column Hw
• Hw Height of Hg x Density of Mercury
• 
  
• Hw 760 mm Hg x 13.6/1.00 1.03 x 104 mm
• Hw 10.3 m __________ ft

HeightWater HeightMercury
DensityMercury DensityWater

Density of Water
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Common Units of Pressure
Unit Atmospheric Pressure
Scientific Field Used
Pascal (Pa) 1.01325 x 105 Pa
SI unit physics, kilopascal (kPa)
101.325 kPa
chemistry Atmosphere (atm) 1
atm Chemistry Millimet
ers of mercury 760 mmHg
Chemistry, medicine (mmHg)

biology Torr
760 torr
Chemistry Pounds per square inch 14.7
lb/in2 Engineering (psl or
lb/in2) Bar
1.01325 bar Meteorology, chemistry
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Converting Units of Pressure
Problem A chemist collects a sample of Carbon
dioxide from the decomposition of Limestone
(CaCO3) in a closed end manometer, the height of
the mercury is 341.6 mm Hg. Calculate the CO2
pressure in torr, atmospheres, and
kilopascals. Plan The pressure is in mmHg, so we
use the conversion factors from Table 5.2(p.178)
to find the pressure in the other units. Solution
converting from mmHg to torr
1 torr 1 mm Hg
PCO2 (torr) 341.6 mm Hg x
341.6 torr
converting from torr to atm
1 atm 760 torr
PCO2( atm) 341.6 torr x
0.4495 atm
converting from atm to kPa
101.325 kPa 1 atm
PCO2(kPa) 0.4495 atm x
_____________ kPa
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Figure 5.2 A simple manometer, a device for
measuring the pressure of a gas in a container.
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Figure 5.3 A J-tube similar to the one used by
Boyle.
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Boyles Law P - V relationship

• Pressure is inversely proportional to Volume
• P or V
  or PVk
• Change of Conditions Problems
• if n and T are constant !
• P1V1 k P2V2 k
• k k
• Then
• P1V1 P2V2

k V
k P
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Figure 5.4 Plotting Boyles data from Table
5.1.
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Figure 5.5 Plot of PV versus P for several
gases.
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Applying Boyles Law to Gas Problems
Problem A gas sample at a pressure of 1.23 atm
has a volume of 15.8 cm3, what will be the
volume if the pressure is increased to 3.16
atm? Plan We begin by converting the volume that
is in cm3 to ml and then to liters, then we do
the pressure change to obtain the final
volume! Solution
V1 (cm3)
P1 1.23 atm P2 3.16 atm V1 15.8 cm3
V2 unknown T and n remain constant
1cm3 1 mL
V1 (ml)
1000mL 1L
1 mL 1 cm3
1 L 1000mL
V1 15.8 cm3 x x
0.0158 L
V1 (L)
x P1/P2
P1 P2
1.23 atm 3.16 atm
V2 V1 x 0.0158 L x
________ L
V2 (L)
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Boyles Law - A gas bubble in the ocean!
A bubble of gas is released by the submarine
Alvin at a depth of 6000 ft in the ocean, as
part of a research expedition to study under-
water volcanism. Assume that the ocean is
isothermal (the same temperature throughout) ,a
gas bubble is released that had an initial
volume of 1.00 cm3, what size will it be at the
surface at a pressure of 1.00 atm?(We will
assume that the density of sea water is 1.026
g/cm3, and use the mass of Hg in a barometer for
comparison!)
Initial Conditions
Final Conditions
V 1 1.00 cm3
V 2 ?
P 1 ?
P 2 1.00 atm
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Calculation Continued
0.3048 m 1 ft
100 cm 1 m
1.026 g SH2O 1 cm3
Pressure at depth 6 x 103 ft x
x x
Pressure at depth 187,634.88 g pressure from
SH2O
For a Mercury Barometer 760 mm Hg 1.00 atm,
assume that the cross-section of the barometer
column is 1 cm2.
The mass of Mercury in a barometer is
10 mm 1 cm
Area 1 cm2
1.00 cm3 Hg 13.6 g Hg
1.00 atm 760 mm Hg
Pressure x x
x x

187,635 g
Pressure _____ atm Due to the added atmospheric
pressure ___ atm!
V1 x P1 P2
1.00 cm3 x ____ atm 1.00 atm
V2
____ cm3 liters
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Boyles Law Balloon

• A balloon has a volume of 0.55 L at sea level
• (1.0 atm) and is allowed to rise to an altitude
  of 6.5 km, where the pressure is 0.40 atm. Assume
  that the temperature remains constant (which
  obviously is not true). What is the final volume
  of the balloon?
• P1 1.0 atm P2 0.40 atm
• V1 0.55 L V2 ?
• V2 V1 x P1/P2 (0.55 L) x (1.0 atm / 0.40 atm)
• V2 __________ L

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Figure 5.6 PV plot verses P for 1 mole of
ammonia
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