Chapter 8: Gases
How Do Gases Behave?
The behavior of gases can be described by the kinetic molecular theory of ideal gases.
•  Gases consist of submicroscopic particles moving about randomly with no attractive forces for
one another.
•  Gases particles move in a straight line until a collision occurs (either with another particle or with
the side of a container). Upon collision, the particle will change its direction and continue moving
in a straight line until another collision occurs.
•  Gases are mostly empty space. This explains why gases can be compressed and expanded
greatly.
•  The average kinetic energy (KE) of gas particles is directly proportional to temperature (in K). The
higher the T, the higher the KE and the faster the particles move about. The lower the T, the
lower the KE and the slower the particles move about.
•  Collisions of gas particles are elastic, meaning that the total KE is unchanged for a sample of gas.
•  The collisions of a gas with the walls of its container result in pressure (a force per unit of area).
Common units of pressure include atmospheres (atm), millimeters of mercury (mmHg) and
pounds per square inch (psi).
•  Real gases do not follow all of these behaviors. Real gases deviate most from ideal behavior as
pressure is increased and/or temperature is decreased.
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Chapter 8: Gases
How Do Gases Behave?
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•
Boyle’s Law: the volume of a fixed amount of gas at constant temperature is inversely proportional to its pressure.
•
Charles’s Law: the volume of a fixed amount of gas at constant pressure is directly proportional to its Kelvin
temperature.
•
Gay-Lussac’s Law: the pressure of a fixed amount of gas at a constant volume is directly proportional to its Kelvin
temerature.
•
Avogadro’s Law: the volume of a gas is directly proportional to its molar amount at a constant pressure and
temperature; one mole of a gas at STP is always 22.4 L (the molar volume of all gases).
Chapter 8: Gases
How Do Gases Behave?
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Chapter 8: Gases
Ch 8 Problems-Gases
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1.
How does the volume of a gas change if the temperature is increased while the pressure and moles are
held constant?
2.
How does the volume of a gas change if the pressure is increased while the temperature and number of
moles are held constant?
3.
Convert 0.820 atm into mmHg. HINT: 760 mmHg = 1 atm
4.
A balloon indoors at a temperature of 27°C has a volume of 2.00 L. What is the new volume if the
balloon is taken outside where the temperature is -23°C? Assume no change in pressure and moles.
5.
What volume is occupied by 4.11 g of methane gas (CH4) at STP?
Chapter 8: Gases
A sample of ideal gas is in a sealed container. The pressure of the gas is 315 torr, and the temperature is 3 degree
C. If the temperature changes to 57 degree C with no change in volume or amount of gas, what is the new
pressure, , of the gas inside the container?
Using the same sample of gas ( P1= 315 torr , T1 = 3 degree C), we wish to change the pressure to 3150 torr with
no accompanying change in volume or amount of gas. What temperature T2 , in Celsius, is needed to reach this
pressure?
The pressure in car tires is often measured in pounds per square inch (lb/in^2), with the recommended pressure
being in the range of 25 to 45 lb/in^2 . Suppose a tire has a pressure of 42.5 . Convert 42.5 lb/in^2 to its
equivalent in atmospheres.
High-pressure liquid chromatography (HPLC) is a method used in chemistry and biochemistry to purify
chemical substances. The pressures used in this procedure range from around 500 kilopascals (500,000 Pa) to
about 60,000 kPa (60,000,000 Pa ). It is often convenient to know the pressure in torr. If an HPLC procedure is
running at a pressure of 3.00×108 Pa, what is its running pressure in torr?
A sample of gas in a balloon has an initial temperature of 8 degree Cand a volume of 1450 L. If the temperature
changes to 78 degree C, and there is no change of pressure or amount of gas, what is the new volume, V2, of the
gas?
What Celsius temperature, T2, is required to change the volume of the gas sample in Part A (T1 = 8 degree C,
V1= 1450 L ) to a volume of 2900 L ? Assume no change in pressure or the amount of gas in the balloon.
A very flexible helium-filled balloon is released from the ground into the air at 20 degree C. . The initial volume
of the balloon is 5.00 L , and the pressure is 760 mmHg. . The balloon ascends to an altitude of 20 km, where the
pressure is 76.0 mmHg and the temperature is -50. . What is the new volume, V2, of the balloon in liters,
assuming it doesn't break or leak?
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Chapter 8: Gases
Consider 4.70 L of a gas at 365 mmHg and 20 degree C. . If the container is compressed to 2.30 L and the
temperature is increased to 37 degree C, what is the new pressure, P2, inside the container? Assume no change
in the amount of gas inside the cylinder.
Description: Calculate final volume given the initial number of moles and the number of moles of gas that leak out of
the container. Determine final moles given initial and final volume.
A cylinder, with a piston pressing down with a constant pressure, is filled with 2.20 moles of a gas (n1), and its
volume is 40.0 L (V1). If 0.500 mole of gas leak out, and the pressure and temperature remain the same, what is
the final volume of the gas inside the cylinder?
A sample of gas in a cylinder as in the example in Part A has an initial volume of 56.0 L , and you have
determined that it contains 1.20 mol of gas. The next day you notice that some of the gas has leaked out. The
pressure and temperature remain the same, but the volume has changed to 14.0 L . How many moles of gas (N2)
remain in the cylinder?
Which description best fits a gas?
definite volume; shape of container; weak intermolecular attractions
volume and shape of container; strong intermolecular attractions
volume and shape of container; no intermolecular attractions
definite volume; shape of container; moderate intermolecular attractions
definite shape and volume; strong intermolecular attractions
Which description best fits a liquid?
definite volume; shape of container; moderate intermolecular attractions
volume and shape of container; no intermolecular attractions
definite volume; shape of container; no intermolecular attractions
volume and shape of container; strong intermolecular attractions
definite shape and volume; strong intermolecular attractions
Which description best fits a solid?
definite shape and volume; strong intermolecular attractions
volume and shape of container; strong intermolecular attractions
definite volume; shape of container; moderate intermolecular attractions
volume and shape of container; no intermolecular attractions
definite volume; shape of container; no intermolecular attractions
Which transformation is evaporation?
solid → gas
liquid → solid
gas → solid
solid → liquid
liquid → gas
Which transformation is condensation?
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Chapter 8: Gases
solid → liquid
solid → gas
liquid → solid
gas → liquid
liquid → gas
Which transformation is freezing?
solid → gas
solid → liquid
liquid → gas
gas → liquid
liquid → solid
Which transformation is melting?
solid → liquid
liquid → gas
solid → gas
liquid → solid
gas → liquid
Which of the assumptions of the kinetic-molecular theory best explains the observation that a balloon collapses
when exposed to liquid nitrogen (which is much colder than a cold winter day!!)?
Collisions with the walls of the container or with other molecules are elastic.
The velocity of gas molecules is proportional to their Kelvin temperature.
The amount of space occupied by a gas is much greater than the space occupied by the actual gas molecules.
Gas molecules move at random with no attractive forces between them.
In collisions with the walls of the container or with other molecules, energy is conserved.
Most gases show variations from ideality at ________ pressure and ________ temperature.
moderate; moderate
low; high
low; low
high; high
high; low
Volume and pressure are ________ proportional.
inversely
directly
all of the above
none of the above
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Chapter 8: Gases
What would be the new pressure if a 400 mL gas sample at 380 mm Hg is expanded to 800 mL with no change in
temperature?
760 mm Hg
190 mm Hg
570 mm Hg
380 mm Hg
950 mm Hg
How will the volume of a fixed sample of gas change if its pressure is doubled and the Kelvin temperature is
doubled?
It will not change.
The change cannot be determined without more specific information.
It will increase by a factor of 4.
It will decrease by a factor of 2.
It will double.
Which of the following is the definition of standard temperature and pressure?
273 K and 655 mm Hg
273°C and 760 torr
0 K and 1 atm
273 K and 760 mm Hg
How many moles of gas are present in a 10.0 liter sample at STP?
224 moles
0.446 moles
22.4 moles
10.0 moles
2.24 moles
Consider a sample of helium and a sample of neon, both at 30.0°C and 1.5 atm. Both samples have a volume of
5.0 liters. Which statement concerning these samples is not true?
Each sample contains the same number of moles of gas.
Each sample contains the same number of atoms of gas.
The density of the neon is greater than the density of the helium.
Each sample weighs the same amount.
none of the above
For the following reaction, if 11.2 L of nitrogen are reacted to form NH3 at STP, How many liters of hydrogen
will be required to completely consume all of the nitrogen?
N2 + 3H2 → 2NH3
33.6 L
11.2 L
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Chapter 8: Gases
7.4 L
16.8 L
6.1 L
How many grams of N2 are contained in an 11.2 liter sample at STP?
6.02 × 1023
14.0
28.0
11.2
7.00
A sample of CO2 gas at 100.°C has a volume of 250. mL at 760. mm Hg. How many moles of CO2 are present?
0.00816 mol
0.0304 mol
6.20 mol
23.14 mol
8.16 mol
What is the pressure in a 1.00 liter container of methane, CH4, that contains 40.0 g of the gas at 25.0°C?
3,920 atm
82.1 atm
61.0 atm
979 atm
5.13 atm
Calculate the volume of a 3.50 mol sample of carbon dioxide at 30.0°C and 0.900 atm.
421 L
9.58 L
8.54 × 10-4 L
96.7 L
2.20 L
If a 0.614 g sample of a gas maintains a pressure of 238 mm Hg when contained in a 1.0 L flask at 0.0°C, what is
the gas's molecular weight?
5 g/mol
120 g/mol
44 g/mol
22 g/mol
8.8 g/mol
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