Lecture #13
Measuring the Pressure of a Gas
and
Gas Laws of Boyle, Charles and
Avogadro
Chemistry 142 B
Autumn Quarter, 2004
J. B. Callis, Instructor
The Three States of Matter for the Element Bromine
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. 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.
Substances that are Gases under
Normal Conditions
Substance
Helium
Neon
Argon
Hydrogen
Nitrogen
Nitrogen Monoxide
Oxygen
Hydrogen Chloride
Ozone
Ammonia
Methane
Formula
He
Ne
Ar
H2
N2
NO
O2
HCl
O3
NH3
CH4
MM(g/mol)
4.0
20.2
39.9
2.0
28.0
30.0
32.0
36.5
48.0
17.0
16.0
Pressure of the Atmosphere
• Called “atmospheric pressure,” or the force
exerted upon us by the atmosphere above.
• A measure of the weight of the atmosphere
pressing down upon us.
Force
Pressure 
Area
• Measured using a barometer - A device that
can weigh the atmosphere above us.
The Effect of Atmospheric Pressure on
objects at the earth’s surface
A Mercury Barometer
Construct a Barometer using Water
Density of water =
Density of Mercury =
Height of water column = Hw
Hw =
Hw =
A Closed End Manometer
An Open ended Manometer
A J-tube similar
to the one used
by Boyle
Common Units of Pressure
Unit
Atmospheric Pressure
pascal (Pa);
1.01325 x 105 Pa
kilopascal(kPa)
101.325 kPa
Scientific Field
SI unit; physics,
Chemistry
atmosphere (atm)
Chemistry
1 atm*
millimeters of mercury
( mm Hg )
760 mmHg*
Chemistry, medicine,
biology
torr
760 torr*
Chemistry
pounds per square inch
( psi or lb/in2 )
bar
14.7 lb/in2
Engineering
1.01325 bar
Meteorology, chemistry,
physics
Problem 13-1: 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
The preceding table to find the pressure in the other units.
Solution:
converting from mm Hg to torr:
(a) PCO2 (torr) =
converting from torr to atm:
(b) PCO2( atm) =
converting from atm to kPa:
(c) PCO2(kPa) =
Boyle’s Law : P - V Relationship
Pressure is inversely proportional to volume at fixed
temperature and fixed amount:
P= K
or V = K
or PV=K
V
P
Where:
P is the pressure exerted by the sample on the
surroundings.
V is the volume occupied by the sample.
K is a constant of proportionality.
Problem 13-2: Applying Boyles Law
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
3 = 1 mL
1cm
3
V1 = 15.8 cm
V2 = unknown
T and n remain constant
V (ml)
1
V1 =
1000mL = 1L
V1 (L)
V2 =
x P1/P2
V2 (L)
Deviations From
Boyle’s Law
Extrapolation of PV
Data to Zero Pressure
Yields Ideal Behavior
Charles’s Law : V - T Relationship
Volume is directly proportional to temperature at
fixed pressure and fixed amount:
V = C
V = CT
or
T
Where:
V is the volume occupied by the sample.
T is the temperature in degrees Kelvin.
C is a constant of proportionality.
Charles’s Law
Behavior for
Several Gases
Problem 13-3: Charles Law
A sample of carbon monoxide, a poisonous gas, occupies 3.20
L at 125 oC. Calculate the temperature (oC) at which the
gas will occupy 1.54 L if the pressure remains constant.
V1 =
T1 =
V2 =
T2 =
T2 =
T2 =
oC
=
Problem 13-4: Amonton’s Law
Problem: A copper tank is compressed to a pressure of 4.28 atm at a
temperature of 0.185 oF. What will be the pressure if the temperature is
raised to 95.6 oC?
Plan: The volume of the tank is not changed, and we only have to deal
with the temperature change, and the pressure, so convert to SI units,
and calculate the change in pressure from the Temp.and Pressure change.
Solution:
T1 =
T2 =
P2 =
Avogadro’s Law : V - n Relationship
Volume is directly proportional to moles of gas at
fixed pressure and temperature:
V = B
V = Bn
or
n
Where:
V is the volume occupied by the sample.
n is the number of moles of gas.
B is a constant of proportionality.
Problem 13-5: Avogadro’s Law
Problem: Sulfur hexafluoride is a gas used to trace pollutant plumes in
the atmosphere, if the volume of 2.67 g of SF6 at 1.143 atm and 28.5 oC
is 2.93 m3, what will be the mass of SF6 in a container whose volume is
543.9 m3 at 1.143 atm and 28.5 oC?
Plan: Since the temperature and pressure are the same, it is a V - n
problem, so we can use Avogadro’s Law to calculate the moles of the
gas, then use the molecular mass to calculate the mass of the gas.
Solution: Molar mass SF6 = 146.07 g/mol
Standard Temperature and Pressure (STP)
A set of Standard conditions have been chosen to make it easier to
understand the gas laws, and gas behavior.
Standard Temperature = 00 C = 273.15 K
Standard Pressure = 1 atmosphere = 760 mm Mercury
At these “standard” conditions 1.0 mole of a gas will
occupy a “standard molar volume”.
Standard Molar Volume = 22.414 Liters = 22.4 L
Answers to Problems in Lecture #13
1. 341.6 torr, 0.4495 atm, 45.54 kPa
2. 0.00615 L
3. -81oC
4. 6.18 atm
5. 496 g SF6