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Chapter 11
Gases

11-01
Title
Gas molecules exert pressure
Caption
Pressure is the force exerted by gas molecules as they collide with the surfaces around them. Question: Do you think pressure would increase or decrease if you had more molecules per unit volume?
Notes
Students might explore pressure through the physics definition: P = force/area. The molecules exert a force on the surface when they strike it; the surface has area over which the force is exerted.
Keywords
gas, molecule, pressure
11-02
Title
Pressure explains how you can drink from a straw
Caption
a) When a straw is put into a glass of orange soda, the pressure inside and outside of the straw are the same, so the liquid levels inside and outside of the straw are the same. b) When a person sucks on the straw, the pressure inside the straw is lowered. The greater pressure on the surface of the liquid outside of the straw pushes the liquid up the straw.
Notes
Students could be shown a mercury thermometer, and then asked to take a stand for or against the statement, "A barometer and a straw work according to the same principles."
Keywords
gas, pressure
11-03
Title
A drinking straw's limitations
Caption
Even if you formed a perfect vacuum, atmospheric pressure could only push orange soda to a total height of about 10 m. This is because a column of water 10 m high exerts the same pressure (14.7 lb/in2) as the gas molecules in our atmosphere.
Notes
The caption states that the orange soda can be raised to a maximum height of about 10 m in the straw. Students could be asked to consider how other liquids would behave in the straw: gasoline, mercury, etc.
Keywords
gas, pressure, vacuum
11-04
Title
Assumptions of the kinetic molecular theory
Caption
The kinetic molecular theory is based on the four stated assumptions.
Notes
Here is an opportunity to explore how the assumptions limit how well the kinetic molecular theory predicts the behavior of real gases: Do real gases strictly follow the assumptions? How would deviations from any one of the assumptions affect the observed behavior of a real gas?
Keywords
gas, pressure, kinetic, molecular, theory
11-05
Title
Gases are compressible when we apply a force
Caption
Gases are compressible because there is so much empty space between gas particles.
Notes
Unlike solids and liquids, there is much empty space between the particles in a gas. Applying a force pushes the particles into the empty space, compressing the gas.
Keywords
gas, pressure, kinetic, molecular, theory, compressibility
11-06
Title
Liquids are not very compressible because there is little empty space between particles.
Caption
Liquids are not very compressible when we apply pressure. Some compression does occur, but it is many orders of magnitude less than the compression in a gas subject to the same pressure.
Notes
Unlike gases, there is little empty space between the particles in a liquid.
Keywords
gas, pressure, kinetic, molecular, theory, compressibility, liquid
11-07
Title
Molecules in a gas collectively assume the shape of their container.
Caption
The kinetic molecular theory assumes that there are no attractions between gas molecules. They are therefore free to move wherever there is empty space.
Notes
Question for students: How might you determine that each gas in a mixture of gases will assume the shape of the container?
Keywords
gas, pressure, kinetic, molecular, theory
11-07-01a
Title
Compared to liquids, gases have very low densities
Caption
If all of the water in a 12 oz (350 mL) can of orange soda were converted to gaseous steam (at 1 atmosphere pressure and 100 íC), the steam would occupy a volume equal to 1700 soda cans.
Notes
Students can speculate on where the extra volume came from, when the liquid soda was converted to steam.
Keywords
gas, pressure, kinetic, molecular, theory
11-08
Title
Pressure is directly related to the number of gas particles in a volume
Caption
Since pressure is a result of collisions between gas particles and the surfaces around them, the amount of pressure increases when the number of particles in a given volume increases.
Notes
This makes sense, in terms of the kinetic molecular theory: More molecules mean more collisions; more collisions mean more force exerted on the walls of the jar.
Keywords
gas, pressure, kinetic, molecular, theory, volume
11-09
Title
Pressure imbalance can cause ear pain
Caption
The pain you feel in your ears upon ascending a mountain or ascending in an airplane is caused by an imbalance of pressure between the cavities inside your ear and the outside air.
Notes
Yawning is the body's way of equalizing the internal and external pressure and reducing the pain.
Keywords
gas, pressure, kinetic, molecular, theory, air, ear
11-10
Title
A mercury barometer
Caption
Mercury barometer. Average atmospheric pressure at sea level pushes a column of mercury to a height of 760 mm (29.92 in). Question: What would happen to the height of the mercury column if the external pressure became lower? Higher?
Notes
Students might be asked to match atmospheric pressures with geographic locations: New York City, Denver, Mt. Everest, Death Valley, California.
Keywords
gas, pressure, kinetic, molecular, theory, air, mercury, barometer
11-10-01un
Title
Solution map for converting atm to mm Hg
Caption
The stated factor comes from the definition of the atmosphere, as the average pressure at sea level.
Notes
Students should note that a mm Hg is also called a torr. The solution map would work equally well for the conversion of atm to torr.
Keywords
gas, pressure, air, atmosphere, barometer, mercury, millimeter, torr
11-10-02un
Title
Multistep solution map for converting psi to mm Hg
Caption
The stated factors come from the conversion factor, 1 atm = 14.7 psi (Table 11.1), and from the definition of the atmosphere, as the average pressure at sea level.
Notes
Students should note that a mm Hg is also called a torr. The solution map would work equally well for the conversion of psi to torr.
Keywords
gas, pressure, air, atmosphere, barometer, mercury, millimeter, psi, torr
11-11
Title
Operation of a manual bicycle pump
Caption
The up-stroke fills the cylinder with air that enters through the one-way valve. The down-stroke then sends the air through the attached hose to the bicycle tire.
Notes
The pump works by manipulating the pressure: On the up-stroke, the pressure in the cylinder decreases as the volume increases; on the down-stroke, the pressure increases as the volume decreases. The overall action conforms to Boyle's law, where V is proportional to 1/P.
Keywords
gas, pressure, air, atmosphere, volume, pump, Boyle's law
11-11-01c
Title
J-tubes show the P-V relationship for a sample of gas
Caption
A J-tube, such as the one shown here, can be used to measure the volume of a gas at different pressures. By simply adding mercury to the J-tube, the pressure on the gas sample increases and its volume decreases.
Notes
Students could use the kinetic molecular theory to predict the J-tube's behavior if different gases were trapped in the sealed end of the tube.
Keywords
gas, pressure, volume, J-tube
11-12
Title
A plot of the volume of a gas as a function of pressure
Caption
The inverse relationship is consistent with Boyle's law, where V is proportional to 1/P.
Notes
Students might be asked if the same plot is expected for all gases.
Keywords
gas, pressure, air, atmosphere, volume, Boyle's law
11-13
Title
Increasing the pressure decreases the volume
Caption
As the pressure of a sample of gas increases, its volume decreases.
Notes
Ask students to explain this in terms of P = F/A. The same number of molecules at the same temperature suggest that F is constant. But, with the smaller volume in the rightmost cylinder, the area, A, is smaller. A smaller area yields a higher pressure.
Keywords
gas, pressure, kinetic, molecular, theory, volume, Boyle's law
11-14
Title
Diving and pressure
Caption
For every 10 meters of depth, a diver experiences an additional 1 atm of pressure due to the weight of the water surrounding him. At 20 meters, the diver experiences a total pressure of 3 atm (1 atm from atmospheric pressure plus an additional 2 atm from the weight of the water).
Notes
Have students speculate on what causes the additional pressure as the depth increases: After all, the diver is in a liquid, not a gas!
Keywords
gas, pressure, kinetic, molecular, theory, volume, Boyle's law
11-14-02
Title
Solution map for Boyle's law pressure calculation
Caption
Boyle's law can be expressed in equation form as P1V1 = P2V2. Algebraic rearrangement to get P2 alone in the numerator on one side of the equal sign gives us a solution.
Notes
Note that both volumes must be in the same units, in order for them to cancel.
Keywords
gas, pressure, kinetic, molecular, theory, volume, Boyle's law
11-14-03
Title
Solution map for Boyle's law volume calculation
Caption
Boyle's law can be expressed in equation form as P1V1 = P2V2. Algebraic rearrangement to get V2 alone in the numerator on one side of the equal sign gives us a solution.
Notes
Note that both volumes must be in the same units, in order for them to cancel.
Keywords
gas, pressure, kinetic, molecular, theory, volume, Boyle's law
11-14-04e
Title
Snorkels
Caption
Cartoon characters often use reeds to breathe air from the surface, even when they are at depth. This would not work because the pressure at depth would push air out of their lungs, preventing them from breathing.
Notes
Have students speculate on why a reed (or a snorkel) would work if the diver were at a very shallow depth.
Keywords
gas, pressure, kinetic, molecular, theory, volume, Boyle's law
11-15
Title
Which balloon would get bigger?
Caption
If one end of a long tube with balloons tied on both ends were submerged into water, in which direction would air flow?
Notes
Could this device be adapted for use as a depth gauge? Why or why not?
Keywords
gas, pressure, kinetic, molecular, theory, volume, Boyle's law
11-16
Title
A plot of the volume of a gas as a function of temperature.
Caption
The volume of a gas increases linearly with increasing temperature. Question: How does this graph demonstrate that -273 íC is the coldest possible temperature?
Notes
The direct relationship is consistent with Charles's law, where V is proportional to T. Students might be asked if the same plot is expected for all gases.
Keywords
gas, temperature, kinetic, molecular, theory, volume, Charles's law, absolute zero
11-17
Title
Temperature effect on volume
Caption
If a balloon is moved from an ice water bath into a boiling water bath, its volume increases because as the molecules move faster (due to increased temperature) they collectively occupy more volume.
Notes
Students might be asked to speculate on what would happen if the balloon were moved to a liquid nitrogen bath.
Keywords
gas, temperature, kinetic, molecular, theory, volume, Charles's law
11-17-02un
Title
Solution map for Charles's law volume calculation
Caption
Charles's law can be expressed in equation form as V1/T1 = V2/T2. Algebraic rearrangement to get V2 alone in the numerator on one side of the equal sign gives us a solution.
Notes
Note that both temperatures must be in kelvin, in order for them to cancel, and for there to be no possibility of calculating a negative volume.
Keywords
gas, temperature, kinetic, molecular, theory, volume, Charles's law
11-17-03un
Title
Solution map for Charles's law temperature calculation
Caption
Charles's law can be expressed in equation form as V1/T1 = V2/T2. Algebraic rearrangement to get T1 alone in the numerator on one side of the equal sign gives us a solution.
Notes
Note that the temperature must be in kelvin, in order for there to be no possibility of calculating a negative volume. Also, the volumes must be in the same units for the units to cancel.
Keywords
gas, temperature, kinetic, molecular, theory, volume, Charles's law
11-17-04un
Title
Solution map for combined gas law pressure calculation
Caption
The combined gas law can be expressed in equation form as P1V1/T1 = P2V2/T2. Algebraic rearrangement to get P2 alone in the numerator on one side of the equal sign gives us a solution.
Notes
Note that the temperatures must be in kelvin, in order for them to cancel, and for there to be no possibility of calculating a negative pressure. Also, the volumes must be in the same units for the units to cancel.
Keywords
gas, temperature, kinetic, molecular, theory, volume, pressure, combined gas law
11-17-05un
Title
Solution map for combined gas law volume calculation
Caption
The combined gas law can be expressed in equation form as P1V1/T1 = P2V2/T2. Algebraic rearrangement to get V2 alone in the numerator on one side of the equal sign gives us a solution.
Notes
Note that the temperatures must be in kelvin, in order for them to cancel, and for there to be no possibility of calculating a negative volume. Also, the pressures must be in the same units for the units to cancel.
Keywords
gas, temperature, kinetic, molecular, theory, volume, pressure, combined gas law
11-18
Title
The volume of a gas sample increases linearly with the number of moles in the sample
Caption
Avogadro's law states that the volume of a gas increases linearly with increasing number of moles, provided the temperature and pressure are kept constant.
Notes
The direct relationship is consistent with Avogadro's law, where V is proportional to n. Students might be asked if the same plot is expected for all gases.
Keywords
gas, temperature, kinetic, molecular, theory, volume, Charles's law, absolute zero
11-19-01un
Title
Solution map for Avogadro's law moles calculation
Caption
Avogadro's law can be expressed in equation form as V1/n1 = V2/n2. Algebraic rearrangement to get n2 alone in the numerator on one side of the equal sign gives us a solution.
Notes
Note that the volumes must be in the same units for the units to cancel.
Keywords
gas, kinetic, molecular, theory, volume, mole, Avogadro's law
11-19-02un
Title
Solution map for ideal gas law pressure calculation
Caption
The ideal gas law can be expressed in equation form as PV = nRT. Algebraic rearrangement to get P alone in the numerator on one side of the equal sign gives us a solution.
Notes
The ideal gas law contains within it Boyle's law, Charles's law, and Avogadro's law.
Keywords
gas, kinetic, molecular, theory, volume, mole, pressure, temperature, Boyle's law, Charles's law, Avogadro's law, ideal gas law
11-19-03un
Title
Solution map for ideal gas law volume calculation
Caption
The ideal gas law can be expressed in equation form as PV = nRT. Algebraic rearrangement to get V alone in the numerator on one side of the equal sign gives us a solution.
Notes
The ideal gas law contains within it Boyle's law, Charles's law, and Avogadro's law.
Keywords
gas, kinetic, molecular, theory, volume, mole, pressure, temperature, Boyle's law, Charles's law, Avogadro's law, ideal gas law
11-19-04un
Title
Solution map for ideal gas law mole calculation
Caption
The ideal gas law can be expressed in equation form as PV = nRT. Algebraic rearrangement to get n alone in the numerator on one side of the equal sign gives us a solution.
Notes
The ideal gas law contains within it Boyle's law, Charles's law, and Avogadro's law.
Keywords
gas, kinetic, molecular, theory, volume, mole, pressure, temperature, Boyle's law, Charles's law, Avogadro's law, ideal gas law
11-19-06un
Title
Solution map for molar mass calculation
Caption
The molar mass calcualtion is not necessarily a gas law calculation, but is often a followup calculation to ideal gas law calculations for moles.
Notes
The ideal gas law offers a way to experimentallly determine the molar mass of a gaseous chemical compound. The ideal gas law provides a measure of the moles; direct measurement of the mass provides the other variable.
Keywords
mole, ideal gas law, molar mass
11-20
Title
A real gas behaves most like an ideal gas when these conditions apply
Caption
High temperatures and low pressures cause the molecules to behave most similarly to the behavior predicted by the ideal gas law and the kinetic molecular theory.
Notes
To the students: Why were volume and moles not explicitly addressed in this image?
Keywords
gas, kinetic, molecular, theory, volume, mole, pressure, temperature, ideal gas
11-21
Title
A real gas behaves less like an ideal gas when these conditions apply
Caption
Low temperatures and high pressures cause the molecules to behave most similarly to the behavior predicted by the ideal gas law and the kinetic molecular theory.
Notes
To the students: Why were volume and moles not explicitly addressed in this image?
Keywords
gas, kinetic, molecular, theory, volume, mole, pressure, temperature, ideal gas, non-ideal gas
11-22
Title
Gas mixtures: the partial pressure concept
Caption
A gas mixture at a total pressure of 1.0 atm consisting of 80% helium and 20% neon will have a helium partial pressure of 0.80 atm and a neon partial pressure of 0.20 atm.
Notes
The partial pressure calculations used here are valid for ideal gases. The specific example addresses the use of helium/oxygen mixtures by divers.
Keywords
gas, kinetic, molecular, theory, volume, mole, pressure, temperature, ideal gas, partial pressure
11-23
Title
The partial pressure of oxygen increases when overall pressure increases
Caption
When breathing compressed air, there is a larger partial pressure of oxygen in the lungs. A large oxygen partial pressure in the lungs results in a larger amount of oxygen in bodily tissues. When the oxygen partial pressure increases beyond 1.4 atm, oxygen toxicity results.
Notes
Students might be asked to predict the partial pressure of oxygen if the pressure is lowered, instead of raised.
Keywords
gas, kinetic, molecular, theory, volume, mole, pressure, temperature, ideal gas, partial pressure, oxygen
11-24
Title
Physiological effects of oxygen partial pressure
Caption
Oxygen partial pressure must lie between 0.1 and 1.4 atm to avoid significant health effects.
Notes
These limits present significant problems to extreme athletes, aviators, and others venturing into extraordinary environments.
Keywords
gas, kinetic, molecular, theory, volume, mole, pressure, temperature, ideal gas, partial pressure, oxygen
11-25
Title
A chemical reaction to generate hydrogen gas
Caption
When a gas from a chemical reaction is collected through water, water molecules become mixed with the gas molecules. The pressure of water vapor in the final mixture is the vapor pressure of water at the temperature that the gas is collected.
Notes
The vapor pressure of water can be introduced through a far more familiar phenomenon: When everyday items (clothes, paper, hands, etc.) come in contact with water, they get wet. Gases are made of matter, just like the everyday items; it should be no surprise that gases get "wet" when they come in contact with water. We call this wetness humidity, and measure it as the vapor pressure of water.
Keywords
gas, pressure, temperature, vapor, partial pressure, water, hydrogen
11-26
Title
Vapor pressure vs. temperature
Caption
The vapor pressure of water as a function of temperature. Vapor pressure increases with increasing temperature, because the higher temperatures cause more water molecules to evaporate.
Notes
Challenge students to think about how general this graph is: Will all gases produce a higher vapor pressure as the temperature increases? Why or why not?
Keywords
gas, pressure, temperature, vapor, partial pressure, water, hydrogen
11-26-01un
Title
Multistep solution map for determining moles ammonia produced from a sample of hydrogen
Caption
The ideal gas law can be expressed in equation form as PV = nRT. Algebraic rearrangement to get n alone in the numerator on one side of the equal sign gives us a solution for the moles hydrogen in a measured sample. The stated mole-mole factor comes from the balanced equation for the reaction that creates the ammonia: 3 H2 + N2 --> 2 NH3
Notes
Students should see that this problem is really two problems in one: an ideal gas law problem followed by a mole concept problem.
Keywords
gas, volume, mole, pressure, temperature, ideal gas law, hydrogen, ammonia, nitrogen
11-26-02un
Title
Multistep solution map for determining moles oxygen produced from a sample of KClO3
Caption
The stated factors use data from the periodic table and the balanced reaction equation to make the conversion from mass of KClO3 to moles of O2. The mass of one mole of KClO3 is calculated by adding the masses of potassium, chlorine, and oxygen, according to the chemical formula. The 3:2 factor arises from the balanced equation.
Notes
The factors can be adapted to any mass-moles conversion. The calculation is intended as a prelude to an ideal gas law calculation that converts the moles of oxygen to volume in liters, at a stated temperature and pressure. The stated mole-mole factor comes from the balanced equation for the reaction that creates the oxygen: 2 KClO3 --> 2 KCl + 3 O2
Keywords
gas, volume, mole, chemical formula, periodic table, balanced equation, ideal gas law, oxygen, potassium chlorate
11-26-03h
Title
One mole of any gas at standard temperature and pressure (STP) occupies 22.4 L
Caption
All gases occupy 22.4 L at STP, according to the kinetic molecular theory, because the particles are small compared to the overall volume. However, the masses will differ.
Notes
Students might research why the STP is a "special" condition for gases.
Keywords
gas, kinetic, molecular, theory, volume, mole, pressure, temperature, ideal gas, STP, liter
11-26-04un
Title
Solution map for determining volume of oxygen using the ideal gas law
Caption
The ideal gas law can be expressed in equation form as PV = nRT. Algebraic rearrangement to get V alone in the numerator on one side of the equal sign gives us a solution.
Notes
This is an ideal gas law calculation that converts the moles of oxygen to volume in liters, at a stated temperature and pressure. The balanced equation for the reaction that creates the oxygen is: 2 KClO3 --> 2 KCl + 3 O2
Keywords
gas, periodic table, ideal gas law, oxygen, volume, mole, pressure, temperature
11-26-05un
Title
Multistep solution map for determining liters CO2 produced from a sample of CaCO3
Caption
The stated factors use the definition of the STP volume and the balanced reaction equation to make the conversion from moles of CaCO3 to liters of CO2. The 1:1 factor arises from the balanced equation.
Notes
The factors can be adapted to any moles-liters conversion. The stated mole-mole factor comes from the balanced equation for the reaction that creates the carbon dioxide: CaCO3 --> CaO + CO2
Keywords
gas, volume, mole, chemical formula, periodic table, balanced equation, ideal gas law, carbon dioxide, calcium carbonate
11-26-07un
Title
Multistep solution map for converting in Hg to torr
Caption
The stated factors come from the conversion factor, 1 atm = 29.92 in Hg, and from the definition of the atmosphere, as the average pressure at sea level.
Notes
Students should note that "torr" is also called "mm Hg." The solution map would work equally well for the conversion of in Hg to mm Hg. The same problem could be worked with one factor, if the student knew that 1 inch = 25.4 mm.
Keywords
gas, pressure, air, atmosphere, barometer, mercury, inch, torr
11-26-16un
Title
Multistep solution map for determining mass of H2O produced from a sample of H2
Caption
The stated factors use the definition of the STP volume, the balanced reaction equation, and the molar mass of water to make the conversion from liters of H2O to grams of H2O. The 2:2 factor arises from the balanced equation.
Notes
The factors can be adapted to any similar liters-grams conversion. The stated mole-mole factor comes from the balanced equation for the reaction that creates the water: 2 H2 + O2 --> 2 H2O
Keywords
gas, volume, mole, chemical formula, periodic table, balanced equation, ideal gas law, hydrogen, oxygen, water

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