Properties Of Gases
Chapter 5
Nature Of gases
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11 elements are gases under normal
conditions. The six noble gases are
monoatomic. Other gases are diatomic or
polyatomic molecules. Organic gases include
methane(CH4) ,propane(C3H8) e.t.c.
Inorganic gases include ammonia(NH3), and
sulfur dioxide(SO2).
Pressure
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A sample of gas can be specified by the
following properties:
number of moles of molecules present,
volume of the sample, the temperature and the
pressure. If we change any of these properties
by adding more molecules, compressing the
sample, or changing the temperature then we
change the state of the gas.
Pressure (P) = force/area =F/A
Units Of Pressure
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Force=mass x gravity/A =mg/A
The SI unit of pressure is Pascal (Pa)
1Pa =1kg/m.s2 = 1N/m²
1 bar = 105 Pa = 100kPa
1atm =1.01325 x 105 Pa =101.325kPa
1atm =760 Torr
1atm= 14.7 lb/inch²
The Gas Laws
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Boyles Law: If a gas is compressed at a
constant temperature, the pressure of the gas
increased in a certain way.
Pressure ∞ 1/volume or P = constant/V
This law states that the pressure of a fixed
amount of gas at constant temperature is
inversely proportional to the volume.
Boyle’s Law
Class Practice
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What pressure would a sample of argon exert
when compressed from 500ml to 300ml at
constant temperature, given that the initial
pressure is 750 Torr ?
Charles’s Law
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For a fixed amount of gas under constant pressure,
the volume varies linearly with the temperature.
Volume ∞ absolute temperature
or V=constant x T
Pressure ∞ absolute temperature
or P= constant x T
The volume of a fixed amount of gas at constant
pressure is directly proportional to the absolute
temperature.
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The pressure of a fixed amount of gas at constant
volume is proportional to the absolute temperature.
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Charles's Law - under conditions of constant
pressure and volume, there is a direct relationship
between the volume and absolute temperature for an
ideal gas.V1 / T1 = V2 / T2
Charles's Law may also be called Gay-Lussac's Law.
Charles did the original work, which was verified by
Gay-Lussac.
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Class Practice
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In a study of an internal combustion engine,
nitrogen gas in a 125 ml cylinder at 18°C is
heated to 322°C. What is the final volume of
the gas?
Avogadro’s Principle
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At constant temperature and pressure, the volume of
a gas is proportional to the amount of gas. This can
be done by comparing the molar volumes of gases.
Molar volume= volume occupied/number of moles
V m= V/n
Volume occupied ∞ number of moles
V= constant x n
Avogadro’s principle
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The volume occupied by a sample of gas at
constant pressure and temperature is
proportional to the number of moles of
molecules present.
Class Practice
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A 5.00L tank of carbon dioxide gas contains
0.360 mol of CO2(g). A chemist needs 0.0072
mol CO2 (g) at the same temperature and
pressure as the tank.What volume container
will the chemist need for the sample?
The Ideal Gas Law
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Charles law and Avogadro’s principle concluded that
volume of a gas is directly proportional to
temperature. V∞T and the number of moles V∞ n.
Boyle’s law has told us that volume is inversely
proportional to the pressure V ∞ 1/P
Combining the three equation:
V∞ n x (T/P) or PV ∞ nT or PV= nRT
This is the ideal gas law equation.
R=PV/ nT R=0.082057 L.atm/K.mol
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The ideal gas law, a limiting law contains all
the relations describing the response of ideal
gases to changes in pressure, volume,
temperature, and moles of molecules.
Molar Volume
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The ideal gas law can be used to predict the molar
volume of gas under any conditions of temperature
and pressure.
Vm=V/n=nRT/P/n=RT/P
At standard temperature and pressure (STP) which
means)0°C(273.15K) and 1.00atm the molar volume
is
Vm=(0.82058L.atm/K.mol) x
(273.5K)/1.00atm=22.41L/mol
Class Practice
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Calculate the volume occupied by 1.0 kg of
hydrogen gas at STP?
Ans 1.1x104L
Stoichiometry Of Reacting Gases
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Gay Lussac was the first to summarize the
relation between volumes of reacting gases in
his law of combining volumes.
N2(g)+2O2(g)-2NO2(g)
In which 1 mol nitrogen reacts with 2 mol of
oxygen to form 2 mol of nitrogen dioxide.
Class practice
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Calculate the volume of ethyne (acetylene)
C2H2 produced at 25°C and 1.00atm when 10
g of calcium carbide reacts completely with
water in the reaction
CaC2 (s)+2H2O(l) - Ca(OH)2 (s)+C2H2(g)
Ans 3.8 L
Gas Density
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The densities of gases increase with
increasing pressure and decreasing
temperature and are proportional to the molar
mass.
Determining molar mass from gas
density
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The volatile organic compound geraniol a
component of oil of roses is used in
perfumes.The density of the vapor at 260°C is
0.480gL when the pressure is 103 Torr. What
is the molar mass of geraniol?
Diffusion and Effusion
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The gradual dispersal of one substance
through another substance such as the odor of
perfume,spreading through the air is called as
diffusion.
The escape of one substance through a small
hole into a vacuum, like the escape of air
through a small hole in a spacecraft is called
effusion.
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Diffusion
Effusion
Rate Of Effusion ∞ √(1/molar mass)
 Or rate ∞ √1/M
 Effusion time ∞ √molar mass or
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teffuse∞√M
 Effusion time of A/Effusion time of B
= √(molar mass of A/molar mass of B)
or teffuse(A)/teffuse(B)= √ (MA/MB)
 The time it takes the molecules of a gas to effuse through an
opening or diffuse through another gas is directly
proportional to the square root of it’s molar mass
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If it takes a certain amount of helium atoms
10 s to effuse through a porous barrier, how
long does it take the same amount of methane
molecules CH4 under the same conditions?
Home work
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Page 211
5.28,5.32
Page 213
5.56,5.60