Chapter 11
Gases
Conditions that affect gases
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volume
pressure
temperature
# of particles
Pressure and Force
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Pressure
=
Example of pressure:
force
area
Pressure
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Atmospheric pressure- 14.7 psi
78% nitrogen, 21% oxygen and 1% other gas
Pressure

Ways to measure pressure:
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Open end manometer
Closed end manometer
Barometer
Pressure
Pressure
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Barometer-measures atmospheric pressure
Pressure
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Units of Pressure: see pg.364
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Millimeters of mercury
Kilopascals,pascals
atmospheres
torr
760 mmHg = 1 atm= 101.325 kPa = 760 torr
Pressure Conversions

Examples:


Convert the pressure of 775 mm Hg to kPa and to
atm.
Practice:

Convert a pressure of 750. torr to
a. Atm
b. kPa
Standard Conditions
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STP- Standard Temperature and Pressure
Agreed upon standard
0°C and 1 atm
Dalton’s Law of Partial Pressures
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Each individual gas has its own pressure
(partial pressure)
Ptotal = Pa + Pb + Pc + …..
Sample problems for Dalton’s Law

Example: Calculate the partial pressure in
mm Hg exerted by the four main gases in air:
nitrogen, oxygen, argon and carbon dioxide.
Their abundance by volume is 78.08%,
20.95%, .934% and .035 % respectively.
Sample problems for Dalton’s Law

Example : In a sample containing carbon
dioxide, nitrogen and oxygen at exactly 1
atm. The partial pressure of CO2 is .285 torr
and the partial pressure of N2 is 593.525 torr.
What is the partial pressure of O2?
Dalton’s Law of Partial Pressures
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Collecting gas over water:
-useful for gases that do not dissolve or react
with water
- water has a partial pressure, it increases
with increased temp.
-must use a table for the exact value pg. 859
Patm = Pgas + PH20
Dalton’s Law of Partial Pressures
Sample problems for Dalton’s Law

Example: Oxygen gas was collected from
the decomposition of potassium chlorate.
The atmospheric pressure was 731.0 torr and
the temperature of the water was 20.0 C.
What was the partial pressure of the oxygen
gas collected?
Sample problems for Dalton’s Law

Practice: A sample of nitrogen gas is
collected over water at a temperature of
23.0 °C. What is the pressure of the nitrogen
gas if the atmospheric pressure is 785 mm
Hg?
The Simple Gas Laws
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Boyle’s Law
Charles’s Law
Gay-Lussac’s Law
The Combined Gas Law
Boyle’s Law

When temp. and # of particles are constant
the pressure and volume of a gas are
inversely proportional

P1V1= P2V2
Boyle’s Law -practice

Example: A sample of gas has a volume of
150.0 mL when its pressure is .947 atm. What
will the volume of the gas be at a pressure of
0.987 atm if the pressure and amount of gas
remain constant?

Practice: A helium filled balloon contains 125
mL of gas at a pressure of .974 atm What
volume will the gas occupy at standard
pressure?
Charles’s Law

With constant amount of gas and pressure
the volume and temperature of a gas will
have a directly proportional relationship

V1 = V2
T1
T2

NOTE: TEMPERATURE MUST BE IN KELVIN

K= ° C + 273
Charles's Law Practice

Example: A sample of neon gas occupies a
volume of 752 mL at 25 ° C. What volume will
the gas occupy at 50° C if the pressure remains
constant?

Practice: A balloon filled with oxygen gas
occupies a volume of 5.5 L at 25° C. What
volume will the gas occupy at 100. ° C?
Gay-Lussac’s Law

Pressure and temperature of a gas are
directly proportional if the amount and volume
of gas remain constant

P1 = P2
T1
T2
Gay-Lussac’s Law Practice

Example: The gas in an aerosol can is at a
pressure of 3.00 atm at 25°C. What would the
pressure of the gas be if the can was heated to
52° C?

Practice: The temperature within a car tire at
the beginning of a long trip is 25° C. At the end
of the trip the tire has a pressure of 1.80 atm.
What is the final temperature within the tire if its
original pressure was 1.75 atm?
The Combined Gas Law

Expresses all of the simple gas laws, relating
temperature, pressure and volume of a gas
as long as the amount of gas remains
constant.

P1V1 =
T1

P2V2
T2
MEMORIZE this law, cross out the factors
that remain constant.
The Combined Gas Law Practice

Example:A helium filled balloon has a volume of
50.0L at 25° C and 1.08 atm. What will the new
volume be if its pressure is decreased to .855
atm and the temperature decreases to 10. ° C?

Practice: The volume of a gas at 27.0 ° C and
.200 atm is 80.0 mL. What volume will the same
gas sample occupy at standard conditions?
END of MATERIAL FOR TEST 1
Measuring and Comparing the
Volumes of Reacting Gases

Law of Combining Volumes
 at constant temperature and pressure, the volumes of gases
react and can be expressed as whole numbers

Try this: Write and balance and equation for each.
a.
hydrogen gas and oxygen gas produces water vapor
b.
hydrogen gas and chlorine gas produce hydrogen chloride gas
Now go to pg.378 and check the results
Measuring and Comparing the
Volumes of Reacting Gases
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Avogadro’s Law
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Equal volumes of gases at the same temperature
and pressure contain equal # of molecules
standard molar volume of a gas
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one mole of any gas at STP = 22.4L of that gas
Measuring and Comparing the
Volumes of Reacting Gases
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Example: at STP
2H2(g) + O2(g)  2H2O(g)
__ L
__ L
__ L
2 mL
1 mL
2 mL
__ molecules__ molecule__ molecules
__ moles 1 mole
__ moles
44.8 L
22.4 L
__ L
Using Molar Volume of a Gas

You are planning an experiment that requires
.0580 mol of nitrogen monoxide gas. What
volume would you need at STP?
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Practice: A chemical reaction produces .0680
mol of oxygen gas. What volume in liters is
occupied by this gas at STP?
Using Molar Volume of a Gas
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Challenge: Suppose you need 4.22 g of
chlorine gas,Cl2. What volume at STP would
you expect to use?
Stoichiometry of Gases
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- if constant temp and pressure, just use the
molar ratio from the equation!!
- volume and moles are the same ratio as
the equation!!
Stoichiometry of Gases

Example: Propane is C3H8.

Write the combustion reaction for propane.
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a. What will be the volume, in liters of oxygen
required for the complete combustion of .350 L of
propane? Assume STP

b. What will be the volume of carbon dioxide
produced in the same reaction starting with .350 L
of propane? Assume STP
Stoichiometry of Gases

Practice: Xenon gas reacts with fluorine gas
to produce the gas xenon hexafluoride. Write
the equation for this reaction.

a. Assuming constant temperature and pressure,
what volume of xenon should be used to make
3.14 L of XeF6?

b. What volume of fluorine gas should be used to
create 3.14 L of XeF6?
The Ideal Gas Law
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includes, temperature, pressure, volume and
# of moles
single conditions, not P1,P2
PV= nRT
PV=nRT
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P= pressure (must match unit of pressure in R
value)
V= volume (must be in L)
N= # of moles (convert to moles if given grams)
T= temperature (Kelvin)
R= ideal gas law constant
R= .0821 L . atm
mol.K
See various numerical values for R on pg. 384
Ideal Gas Law Practice

Example: What is the pressure in atm
exerted by a .500 mol sample of nitrogen gas
in a 10.0L container at 298K?

Practice: A 2.07 L cylinder contains 2.88 mol
of helium gas at 22° C. What is the pressure
in atm of the gas in the cylinder? (P= 33.7 atm)
Ideal Gas Law Practice

Practice: A tank of hydrogen gas has a volume
of 22.9 L and holds 14.0 mol of the gas at 12° C.
What is the reading on the pressure gauge in
mm Hg?(14.3 atm1.09X104 mmHg)

Practice: A reaction yields 0.00856 mol of
oxygen gas. What volume in mL will the gas
occupy if it is collected at 43 °C and .926 atm?
(.240 L240. mL)
Effusion and diffusion
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lighter molecules move faster
Graham’s Law of Effusion-the rate of effusion and
diffusion is inversely proportional to the square root of
the molecular weights of the gases
Graham’s Law Practice

Example: Nitrogen gas effuses through a
pinhole 1.7 times as fast as another element
under the same conditions. Estimate the
other element’s mass and determine its
probable identity.
Graham’s Law Practice

Practice: Compare the rates of effusion of
hydrogen and oxygen gas at the same
conditions. (Remember, both are diatomic.)