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```									Chapter 11

GASES
If force is held constant as surface area decreases,
pressure
a.remains constant.
b.decreases.
c.increases.
d.increases or decreases, depending on the volume
change.
Why does a can collapse when a vacuum pump
removes air from the can?
a.The inside and outside forces balance out and crush
the can.
b.The unbalanced outside force from atmospheric
pressure crushes the can.
c.The atmosphere exerts pressure on the inside of the
can and crushes it.
d.The vacuum pump creates a force that crushes the
can.
If the height of mercury in a barometer at 0ºC is less
than 760 mm Hg, then
a.the atmospheric pressure is less than standard
atmospheric pressure.
b.the atmospheric pressure is greater than standard
atmospheric pressure.
c.the atmospheric pressure is equal to standard
atmospheric pressure.
d.the atmospheric pressure cannot be determined.
Convert the pressure 0.840 atm to mm Hg.
a.365 mm Hg
b.437 mm Hg
c.638 mm Hg
d.780 mm Hg
Convert the pressure 1.30 atm to kPa.
a.2 kPa
b.115 kPa
c.132 kPa
d.245 kPa
Standard temperature is exactly
a.100ºC.
b.273ºC.
c.0ºC.
d.0 K.
To correct for the partial pressure of water vapor in a
gas collection bottle, the vapor pressure of H2O at
the collecting temperature is generally
a.subtracted from the partial pressure of the collected
gas.
b.added to the pressure of the collected gas.
c.subtracted from the atmospheric pressure.
Three samples of gas each exert 740. mm Hg in
separate 2 L containers. What pressure do they exert
if they are all placed in a single 2 L container?
a.247 mm Hg
b.740 mm Hg
c.1.48 x 103 mm Hg
d.2.22 x 103 mm Hg
The volume of a gas is 400.0 mL when the pressure is
1.00 atm. At the same temperature, what is the
pressure at which the volume of the gas is 2.0 L?
a.0.5 atm
b.5.0 atm
c.0.20 atm
d.800 atm
If the temperature of a fixed quantity of gas decreases
and the pressure remains unchanged,
a.its volume increases.
b.its volume is unchanged.
c.its volume decreases.
d.its density decreases.
The volume of a gas is 5.0 L when the temperature is
5.0ºC. If the temperature is increased to 10.0ºC
without changing the pressure, what is the new
volume?
a.2.5 L
b.4.8 L
c.5.1 L
d.10.0 L
Why could the pressure of a sample of gas at a
constant volume fall 75 mm Hg?
a.The container exploded.
b.The temperature increased.
c.The temperature decreased.
d.The volume increased.
Why does the air pressure inside the tires of a car
increase when the car is driven?
a.Some of the air has leaked out.
b.The air particles collide with the tire after the car is
in motion.
c.The air particles inside the tire increase their speed
because their temperature rises.
d.The atmosphere compresses the tire.
On a cold winter morning when the temperature is –
13ºC, the air pressure in an automobile tire is 1.5
atm. If the volume does not change, what is the
pressure after the tire has warmed to 15ºC?
a.–1.5 atm
b.1.7 atm
c.3.0 atm
d.19.5 atm
The volume of a gas collected when the temperature is
11.0ºC and the pressure is 710 mm Hg measures 14.8
mL. What is the calculated volume of the gas at
20.0ºC and 740 mm Hg?
a.7.8 mL
b.13.7 mL
c.14.6 mL
d.15 mL
Gas Volumes

 Gay-Lussac’s Law of Combining Volumes of Gases
 At constant temperature and pressure, the volumes of gaseous
reactants and products can be expresses as ratios of small
whole numbers

hydrogen gas + oxygen gas  water vapor
2 L (2 volumes) 1 L (1 volume) 2 L (2 volumes)

 Equal volumes of gases at the same temperature
and pressure contain equal numbers of molecules
   The equation for this relationship is shown below, where
V is the volume, k is a constant, and n is the amount of
moles of the gas.

V = kn

 Dalton had guessed that the formula for water
was HO, but Avogadro’s reasoning established that
water must contain twice as many H atoms as O
atoms because of the volume ratios in which the
gases combine:

hydrogen gas + oxygen gas  water vapor
2 L (2 volumes) 1 L (1 volume)   2 L (2 volumes)
Gay-Lussac recognized that at constant temperature
and pressure, the volumes of gaseous reactants and
products
a.always equal 1 L.
c.equal R.
d.can be expressed as ratios of small whole numbers.
The law of combining volumes applies only to gas
volumes
a.measured at constant temperature and pressure.
b.that equal 1 L.
c.that equal 22.4 L.
d.measured at STP.
If 0.5 L of O2(g) reacts with H2 to produce 1 L of
H2O(g), what is the volume of H2O(g) obtained from
1 L of O2(g)?
a.0.5 L
b.1.5 L
c.2 L
d.2.5 L
Molar Volume of a Gas

 Recall that one mole of a substance contains a
number of particles equal to Avogadro’s constant
(6.022  1023).

 According to Avogadro’s law, one mole of any gas
will occupy the same volume as one mole of any
other gas at the same conditions, despite mass
differences.
Molar Volume of a Gas

 The volume occupied by one mole of
gas at STP is known as the

standard molar volume of a gas

which is 22.414 10 L
(rounded to 22.4 L).
Gas Stoichiometry

 The coefficients in chemical equations of gas
reactions reflect not only molar ratios, but also
volume ratios (assuming conditions remain the
same).
Example-reaction of carbon dioxide formation:
2CO(g)   +   O2(g)      2CO2(g)
In the reaction represented by the equation N2(g) +
2O2(g) -> 2NO2(g), what is the volume ratio of N2 to
NO2?
a.1:1
b.1:2
c.2:1
d.2:5
The equation for the production of methane is C +
2H2(g) -> CH4(g). How many liters of hydrogen are
needed to produce 20. L of methane?
a.2.0 L
b.20. L
c.22.4 L
d.40. L
What is the number of moles of H2 produced when 23
g of sodium react with water according to the
equation 2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)?
a.0.50 mol
b.1.0 mol
c.2.0 mol
d.4.0 mol
If the temperature of a container of gas remains
constant, how could the pressure of the gas increase?
a.The mass of the gas molecules increase.
b.The diffusion of the gas molecules increases.
c.The size of the container increases.
d.The number of gas molecules in the container
increases.
If gas A has a molar mass greater than that of gas B
and samples of each gas at identical temperatures
and pressures contain equal numbers of molecules,
then
a.the volumes of gas A and gas B are equal.
b.the volume of gas A is greater than that of gas B.
c.the volume of gas B is greater than that of gas A.
d.their volumes are proportional to their molar
masses.
At STP, the standard molar volume of a gas of known
volume can be used to calculate the
a.number of moles of gas.
b.rate of diffusion.
c.gram-molecular weight.
d.gram-molecular volume.
Ideal Gas Law

 It is stated as shown below, where R is a constant:
PV = nRT
 In the equation representing the ideal gas law, the
constant R is known as the ideal gas constant.

   Its value depends on the units chosen for pressure,
volume, and temperature in the rest of the equation.
Ideal Gas Constants
Sample Problem I

 What is the pressure in atmospheres
exerted by a 0.500 mol sample of
nitrogen gas in a 10.0 L container at
298 K?
Sample Problem I
The ideal gas law is equivalent to Boyle's law when
a.the number of moles and the pressure are constant.
b.R equals zero.
c.the pressure is 1 atm.
d.the number of moles and the temperature are
constant.
A 1.00 L sample of a gas has a mass of 1.92 g at STP.
What is the molar mass of the gas?
a.1.92 g/mol
b.19.2 g/mol
c.22.4 g/mol
d.43.0 g/mol
Iron(IV) oxide, FeO2, is produced by the reaction Fe +
O2 ® FeO2 (87.8 g/mol). How many grams of FeO2
can be produced from 50.0 L of O2 at STP?
a.19.5 g
b.37.8 g
c.50 g
d.196. g
Calculate the approximate volume of a 0.600 mol
sample of gas at 15.0°C and a pressure of 1.10 atm.
a.12.9 L
b.22.4 L
c.24.6 L
d.139 L
What is the pressure exerted by 1.2 mol of a gas with a
temperature of 20.ºC and a volume of 9.5 L?
a.0.030 atm
b.1.0 atm
c.3.0 atm
d.30. atm
Diffusion and Effusion
 The gradual mixing of two or more gases due to their
spontaneous, random motion is known as diffusion.

 Effusion is the process whereby the molecules of a
gas confined in a container randomly pass through a
tiny opening in the container.
Graham’s Law of Effusion

 Rates of effusion and diffusion depend on the
relative velocities of gas molecules. The velocity of a
gas varies inversely with the square root of its molar
mass.

   Recall that the average kinetic energy of the molecules in any
gas depends only the temperature and equals
Graham’s Law of Effusion
Graham’s Law
Sample Problem

 Compare the rates of effusion of
hydrogen and oxygen at the same
temperature and pressure.
Sample Problem

Hydrogen effuses 3.98 times faster than oxygen.
If a gas with an odor is released in a room, it can
quickly be detected across the room because it
a.diffuses.
b.is dense.
c.is compressed.
d.condenses.
What is the process by which molecules of a gas
randomly encounter and pass through a small
opening in a container?
a.diffusion
b.vaporization
c.distillation
d.effusion
According to Graham's law, two gases at the same
temperature and pressure will have different rates of
effusion because they have different
a.volumes.
b.molar masses.
c.kinetic energies.
d.condensation points.
How many times greater is the rate of effusion of
molecular fluorine than that of molecular bromine at
the same temperature and pressure?
a.2.051
b.3.062
c.4.450
d.7.280
A sample of hydrogen gas diffuses 3.8 times faster
than an unknown gas diffuses. What is the molar
mass of the unknown gas?
a.4.0 g/mol
b.7.6 g/mol
c.22 g/mol
d.29 g/mol

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