Gases
Mr. Shively
De La Salle High School
Chem. Catalyst 7.1
List a few physical properties of gases that you know from
previous science classes.
General Properties of Gases
Have indefinite shape and
volume
Have mass
Compressible
Pressure
Demonstration
What happened to the can?
Why do you think it happened?
Pressure Defined
Molecules are constantly moving and collide
with each other as well as other molecules
Collisions between gas molecules and walls of
container creates pressure.
forcepresssure
area
Barometer and Units of Pressure
Units of Pressure
1 atmosphere = 1 atm
1 atm = 760 mmHg
1 atm = 760 torr
1 atm = 101,325 Pa
1 atm = 101.325 kPa
Practice Pressure Conversions
A scientist measures three balloons to have the following
pressures. Calculate the pressure of each balloon in
atmospheres.
Balloon 1 = 801 mm Hg
Balloon 2 = 721 torr
Balloon 3 = 102,706 Pa
Manometer
A manometer is a device
used to calculate the
pressure of a gas by
comparing it to
atmospheric pressure
Manometer
Calculate the pressure of
the gas sample in the
manometer to the right in
atmospheres. Assume the
atmospheric pressure to be
769 mm Hg and the height
of the mercury column is
49 mm Hg.
Chem. Catalyst 7.2
What is pressure?
Name the device commonly used to measure atmospheric
pressure.
The current barometric pressure in Concord, CA is 30.12
in Hg. What is the pressure in atmospheres? (1 in = 2.54
cm)
Gas Laws
Gas laws are mathematical relationships between
properties of ideal gases.
1. Boyle’s Law
2. Charles’s Law
3. Gay-Lussac’s Law
4. Combined Gas Law
5. Avogadro’s Law
6. Ideal Gas Law
7. Dalton’s Law
8. Graham’s Law
Boyle’s Law
Pressure and volume are indirectly proportional
1 1 2 2PV PV
“An increase in
volume leads to a
decrease in pressure at
constant temperature.”
Boyle’s Law Practice Problem 1
A balloon with a volume of 1.00 L and a pressure of 1.00
atm is released into the atmosphere. What will the
volume of the balloon be at 3, 000 ft where the pressure
is 0.90 atm. Assume constant temperature.
Charles’s Law
Temperature and volume are
directly proportional
1 2
1 2
V V
T T
“An increase in
temperature leads to
an increase in
volume at constant
pressure.”
Charles’ Law Practice Problem 1
A closed container with a volume of 4.5 L and a
temperature of 28 oC is heated to 45 oC. Calculate the
volume of the closed container at this temperature.
Gay-Lussac’s Law
A quantity of gas in a steel container has a pressure of
760 torr at 25°C. What is the pressure in the container if
the temperature is increased to 50°C?
Combined Gas Law
The combined gas law relates initial conditions of
pressure, temperature and volume and final conditions of
pressure temperature and volume.
What happens to the combined
gas law,
When temperature is constant?
When pressure is constant?
When volume is constant?
Chem. Catalyst 7.3
Two balloons, one red and one blue, have a pressure of 760
torr and a volume of 1.4 L. The balloons both have an initial
temperature of 273 K. The volume of the red balloon is
increased until the pressure of the balloon is 710 torr.
The temperature of the blue balloon is increased to 352
K. Which balloon has a greater volume after the changes
have been made?
Avogadro’s Law
The volume of a gas is directly proportional to the
number of moles of gas at constant temperature and
pressure.
V1 V2
= n1 n2
Avogadro’s Law
A balloon that is not inflated but is full of air has a volume of
275 mL and contains 0.0120 mol of air. As shown in the
figure to the right, a piece of dry ice (solid CO2) weighing
1.00 g is placed in the balloon and the neck is tied. What is
the volume of the balloon after the dry ice has vaporized?
(Assume constant T and P.)
Avogadro’s Law and Combining Volumes
The volume of gas at constant T and P is directly
proportional to the number of moles of gas.
Combining Volumes Practice Problem #1
Nitrogen and hydrogen can be combined according to the
following equation:
N2 + 3 H2 2 NH3
If 3.0 L of nitrogen combine with 3.0 L of hydrogen, how
many L of ammonia are produced? Assume the
temperature and pressure are constant.
Molar Volume of a Gas
Molar volume of a gas
At STP(standard
temperature and
pressure) 1 mole of a gas
has a volume of 22.4 L
Conditions of STP
273 K or O0C
1 atm
Practice Problem
Calculate the number of moles of nitrogen gas and
methane gas (CH4) if each of the following samples are at
STP.
Calculate the mass of gas in each sample.
5.0 L of
N2 gas
5.0 L of
CH4 gas
Chem. Catalyst 7.3
Create a list of all the gas laws you have learned in this unit.
Write the formulas and corresponding names.
Gas Laws Mini Review (1 of 2)
A sample of gas in a closed container has a volume
of 125 mL and a pressure of 450 torr. Calculate the
new pressure if the volume of the gas is increased to
145 mL.
A balloon has a volume of 325 mL at 17oC. What is
the temperature in oC if the volume increases to
392 mL.
An aerosol spray can has gas under pressure of 1.25
atm at 25oC. The can explodes when the pressure
reaches 2.50 atm. At what temperature will this
happen?
390
torr
77 oC
596 K or
323 oC
Gas Laws Mini Review (2 of 2)
In the following table, indicate whether the pressure,
volume or temperature increases or decreases.
A balloon has a volume of 188 L and contains 8.40
moles of gas. How many moles of gas would be
needed to expand the balloon to 275 L? Assume the
same temperature and pressure in the balloon.
Experiment P V T
1
Increases Constant
2 Constant Decreases
3 Decreases constant
3.9 mol
Ideal Gas Law
Relates pressure, volume, number of moles and
temperature (in K) of an ideal gas
R is the ideal gas constant.
This equation can be used to solve for any of the four
variables as long as three are known.
UNITS for PV and T must be in atm, L and K.
=
Ideal Gas Law Practice Problem #1
A model blimp is filled with helium to a pressure of 760
mm Hg. The volume of the blimp is 5.00 L and the
temperature is 24 oC. How many moles of helium are in
the balloon?
Ideal Gas Law Practice Problem #1
A model blimp is filled with helium to a pressure of 760
mm Hg. The volume of the blimp is 5.00 L and the
temperature is 24 oC. How many moles of helium are in
the balloon?
Chem. Catalyst 7.4
What is the equation for the ideal gas law?
Calculate the pressure (in torr) exerted by 0.250 g of O2
in a 250 mL container at 29oC?
590 torr
Ideal Gas Law Practice Problem #2
Calculate the volume of a gas in liters if 1.50 moles of
the gas has a pressure of 0.985 atm at a temperature of
-6oC.
Ideal Gas Law Practice Problem #2
Calculate the volume of a gas in liters if 1.50 moles of
the gas has a pressure of 0.985 atm at a temperature of
-6oC.
Ideal Gas Law Practice Problem 3
Calculate the number of grams of acetylene, C2H2 in a
30.0 L cylinder at a temperature of 20.0oC and a pressure
equal to 9,010 torr.
Ideal Gas Law Practice Problem 3
Calculate the number of grams of acetylene, C2H2 in a
30.0 L cylinder at a temperature of 20.0oC and a pressure
equal to 9,010 torr.
Ideal Gas Law Practice Problem #4
4.2 grams of a gas are added to an evacuated chamber
with a volume 2.0 L. Minutes later the pressure of the
chamber is determined to be 0.98 atm. The temperature
of the gas sample was also determined to be 0 0C. What
is the molar mass of the gas sample?
Ideal Gas Law Practice Problem #4
4.2 grams of a gas are added to an evacuated chamber
with a volume 2.0 L. Minutes later the pressure of the
chamber is determined to be 0.98 atm. The temperature
of the gas sample was also determined to be 0 0C. What
is the molar mass of the gas sample?
Gas Stoichiometry
Mass (g)
Given
Moles
Given
Particles
Given Molar Mass
(g/mol)
6.022x1023
(particles/mol)
Mass (g)
unknown
Moles
unknown
Particles
Unknown
Molar Mass
(g/mol)
6.022x1023
(particles/mol)
Volume of
Gas
1 mol gas = 22.4 L at STP
Volume of
Gas
1 mol gas = 22.4 L at STP
Mole ratio
PV=nRT
PV=nRT
Gas Stoichiometry Practice Problem #1
Calculate the mass of NO2 gas produced if 5.00 L of
nitrogen gas at 1.00 atm of pressure and 0oC react with
excess oxygen gas.
N2 (g) + 2 O2 (g) 2 NO2 (g)
Gas Stoichiometry Practice Problem #1
Calculate the mass of NO2 gas produced if 5.00 L of
nitrogen gas at 1.00 atm of pressure and 0oC react with
excess oxygen gas.
N2 (g) + 2 O2 (g) 2 NO2 (g)
Gas Stoichiometry Practice Problem #2
Quicklime (CaO) is produced by the thermal
decomposition of calcium carbonate (CaCO3). Calculate
the volume of CO2 at STP produced by the
decomposition of 152 g CaCO3 by the reaction below.
CaCO3(s) CaO(s) + CO2(g)
Gas Stoichiometry Practice Problem #2
Quicklime (CaO) is produced by the thermal
decomposition of calcium carbonate (CaCO3). Calculate
the volume of CO2 at STP produced by the
decomposition of 152 g CaCO3 by the reaction below.
CaCO3(s) CaO(s) + CO2(g)
Gas Stoichiometry Practice Problem #3
Limestone is dissolved by CO2 and water according to
the equation
CaCO3 + H2O + CO2 Ca(HCO3)2
What volume of CO2 measured at STP would dissolve
115g of CaCO3?
25.8 L CO2
Gas Stoichiometry Problem #4
Magnesium once used in flashbulbs, burns according to
the equation
2Mg + O2 2 MgO
What mass of Mg combines with 5.80 L of O2 measured
at STP?
12.6 g
Gas Stoichiometry Problem #5
Acetylene (C2H2) is produced from calcium carbide as
shown by the reaction
CaC2 (s) + 2H2O (l) Ca(OH)2 (s) + C2H2 (g)
What volume of acetylene measured at 25°C and 745
torr would be produced from 5.00 g of H2O?
3.47 L
Chem. Catalyst 7.5
At a given temperature and pressure, a 0.33 mol sample of
oxygen gas occupies a volume of 9.0 L. If this sample of gas
reacts with excess hydrogen to form water vapor at the
same conditions of temperature and pressure, what volume
will the water vapor occupy? Explain.
2 H2 (g) + O2 (g) 2 H2O (g)
Dalton’s Law of Partial Pressures
Neon gas (Ne) P = 740 mm Hg
Argon gas (Ar) P = 790 mm Hg
Helium gas (He) P = 740 mm Hg
The total pressure of a gas mixture is equal
to the sum of the partial pressures.
Imagine a cylinder with a fixed
volume is filled with three gases: Ar,
Ne and He. What is the total
pressure of the cylinder?
...total a b cP P P P
Dalton’s Law of Partial Pressures Problem #1
A balloon containing nitrogen and hydrogen gas has a
total pressure of 1.33 atmospheres. Calculate the partial
pressure of nitrogen gas if the partial pressure of
hydrogen gas is 0.45 atm.
Dalton’s Law of Partial Pressures Problem #1
A balloon containing nitrogen and hydrogen gas has a
total pressure of 1.33 atmospheres. Calculate the partial
pressure of nitrogen gas if the partial pressure of
hydrogen gas is 0.45 atm.
Dalton’s Law of Partial Pressures Problem
A cylinder with a total pressure of
5.00 atmospheres contains 4 moles
of He gas and 5 moles of N2 gas.
Calculate the partial pressures of
each gas.
a a totalP P
Partial Pressure Mole Fraction Total Pressure
Dalton’s Law of Partial Pressures Problem 2
A study of the effects of certain gases on plant growth
requires a synthetic atmosphere composed of 1.5 mol
percent CO2, 18.0 mol percent O2, and 80.5 mol percent
Ar. Calculate the partial pressure of O2 in the mixture if
the total pressure of the atmosphere is 745 torr?
Kinetic Molecular Theory
Kinetic Molecular Theory
www.mpcfaculty.net/mark_bishop/KMT.htm
Kinetic Molecular Theory
Gases
Gases have mass and negligible volume.
Gas molecules move in straight lines and collide each
other with elastic collisions.
Speed of molecules is proportional to temperature of gas
sample.
Higher the temperature = faster the movement of gas
molecules
There are no attractive forces or repulsive forces
between gas molecules
Root Mean Square Velocity
Special kind of average of gas particles velocity
Root mean square velocity = urms
R =8.31 J/ mol x K
Gas Diffusion and Effusion
So far we have been able to consider all ideal gases to
behave in a similar fashion.
That is, we have been able to describe all gases as acting
the same way.
Now we will look at two properties of gases that depend
on the identity of the gas.
Diffusion
Effusion
Diffusion and Effusion of Gases
Diffusion Effusion
Rate at which two gases
mix
Gases move from high
concentration to low
concentration
spontaneously
Rate at which a gas
escapes through a pinhole
into a vacuum
Spontaneous process
Effusion and Diffusion of Gases
Rate of diffusion of gas a
Rate of diffusion of gas b
Molar masses of gas a and b
Graham’s Law
Gas Diffusion
NH3 diffuses faster than HCl because it has a smaller molecular mass. This
is why the two gases do not meet in the middle of the tube.
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