Gases
1

Describe a gas sample. Describe the position
and motion of atoms/molecules in a sample.
• Gases assume the volume and shape of their
containers.
• Gases are the most compressible state of matter.
• Gases will mix evenly and completely when
confined to the same container.
• Gases have much lower densities than liquids and
solids.
2
3
Pressure
= force/area
= N/m2
1 atm = 760 torr =
760 mm Hg = 101.3 kPa
Pa
10 miles
4 miles
Sea level
0.2 atm
0.5 atm
1 atm
4
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Atmospheric Pressure: pressure
exerted by Earth’s atmosphere;
measured by a barometer
Standard atmospheric
pressure: column of
mercury is 760 mm high
at 0oC at sea level.
What is 475 mmHg in atm?
5

Which image shows Patm > Pgas?


What causes the observed ideal gas
behavior?
Model of gases is Kinetic Molecular Theory
◦ 1. A gas consists of tiny particles, either atoms
or molecules, moving about at random.
 low density and compressible
◦ 2. Volume of particles is negligible compared to
the total volume of the gas; most of the volume
is empty space
 Xe at STP, only 0.025% of the volume is
occupied by the atoms
7
◦ 3. Gas molecules move independently of one
another
 no intermolecular forces to attract
atoms/molecules
◦ 4. Pressure arises from collisions of atoms/
molecules with walls of containers
 no net energy loss from collisions
 pressure is proportional to number of moles
◦ 5. Average kinetic energy (KE) is proportional to
the Kelvin temperature (T)
Gas Simulation
8

Ideal gas: pressure-volume-temperature behavior
can be accounted for by the ideal gas equation
◦ Many gases behave ideally at 0oC and 1 atm

STP = Standard temperature (0oC = 273.15K) and
pressure (1 atm)
◦ Convert variables to Kelvin and atm to correctly solve
problems
 K = oC + 273.15; 1 atm = 760 torr

Ideal gases behave according to the Ideal Gas Law:
PV = nRT
http://mc2.cchem.berkeley.edu/Java/molecules/index.html


We can calculate volume of 1 mole of gas at
0oC and 1 atm. How many molecules (or
atoms) will be in this sample?
1 mol = 22.414 L


What is the volume of 0.5068 g of nitrogen
gas at STP?
How many grams of neon gas will occupy a
volume of 1.0 L at STP?


What happens if the pressure on the outside
of a balloon is increased? What happens to
the atoms inside the balloon?
What causes the can to be crushed?
Can crush
Crushing can be
scaled up a little!
Bicycle Pump
Boyle’s Law
http://preparatorychemistry.com/Bishop_Boyles_frames.htm
14

Why does a bag of potato chips expand as
you drive from Phoenix to Flagstaff?
If a closed bag holds 0.568 L of gas at 0.989
atm, what is the volume of the bag if the
pressure is decreased to 0.822 atm?
P1V1 = P2V2

V2 = 0.683 L



What happens if the balloon is slowly heated?
What will happen to the atoms inside?

Charles and Gay-Lussac
Balloons in LN2
17

If a balloon holds 3.97 L of gas at 2.0oC, what
will the volume be if the temperature of the
gas increases to 90.0oC?
V1/T1 = V2/T2

V2 = 5.24 L


What happens as more atoms are added to
the balloon? Where have you experienced
this before?

Avogadro’s Law
Figure 9.9
20
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
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If we have a closed container that holds 1.08
moles of gas with a volume of 16.7 L. What
will be the new volume of the container if
0.65 more moles of gas are injected?
V1/n1 = V2/n2
V2 = 26.8 L

Boyle’s Law (T, n constant)
◦ PV = constant

V1/T1 = V2/T2
Avogadro’s Law (P, T constant)
◦ V / n = constant

P1V1 = P2V2
Charles’ Law (P, n constant)
◦ V / T = constant

Pressure in
basketball
V1/n1 = V2/n2
Combined Gas Law (n constant)
◦ P1V1 / T1 = P2V2 / T2
Gas Simulation
http://mc2.cchem.berkeley.edu/Java/molecules/index.html
22

Draw the level of the piston in (a) and (b)
under the specified conditions.
Dec Vol
Cons Atoms

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increase the pressure of a container at
constant temperature?
Inc Temp
Cons Vol
increase temperature at constant pressure?
increase the temperature at a constant
volume?
Increase number of gas particles?
Cons Vol
Double Atoms
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/gasesv6.swf
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A small bubble rises from the bottom of a lake, (T
= 8.0oC, P = 6.41 atm, V = 2.1 mL) to the water’s
surface (T = 25.0oC, P = 1.02 atm). What is the
final volume of the bubble?
If a closed cylinder holds 50.0 L of O2 gas at 18.5
atm and 21.0oC, what volume will the gas occupy if
the temperature is maintained while the pressure is
reduced to 1.00 atm?
The volume of a nitrogen cylinder is 11.28 L. What
mass of nitrogen gas is in the cylinder if the
pressure is 775 torr and the temperature is 5.3oC?
14 mL; 925 L; 14 g N2

There are 2 methods for solving these
problems.
If 2 variables in Ideal Gas Law are unknown, use
stoichiometry first to find moles of gas (n) to plug
into equation.
◦ If only 1 variable in Ideal Gas Law is unknown,
solve it first, then use moles of gas (n) in
stoichiometry.
What is the volume of CO2 produced at 370 C and
1.00 atm when 5.60 g of glucose are used up in
the reaction:
◦

C6H12O6 (s) + O2 (g) -->
CO2 (g) + H2O (l)
26
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•
N2(g) + H2(g) 
NH3(g)
How many grams of ammonia can be made with
689 L of hydrogen and excess nitrogen at 350oC
and 7.80 atm?
Answer:
How many L of O2 are needed to react 28.0 g NH3
at 24°C and 0.950 atm?
NH3(g) + O2(g)  NO(g) + H2O(g)
Answer:
airbags
27
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
What mass of solid sodium azide (NaN3) is
needed to generate 75.2 L of nitrogen gas
(and solid sodium) at 100.0oC and 1.00 atm?
(Hint: Start with a balanced chemical
equation!)

Gas densities (d), units of g/L
◦ Use MM, P, R, and T to solve

Molar mass (MM), units of g/mol
◦ Use d, P, R, and T to solve

Use units to arrange variables to solve for
density and molar mass
29

Calculate the density of bromine gas at
50.0oC and 785.0 torr.
Pouring Br2
30
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An unknown diatomic gas has a density of
3.164 g/L at STP. What is the identity of the
gas?
Answer: Cl2
31
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Dalton’s Law of Partial Pressures: In a
mixture of gases, each exerts a partial
pressure the same as it would exert alone.
32
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
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We can only measure the total pressure of a
system
Knowing what fraction of moles belong to
each gas, we can calculate their partial
pressures.
Mole fraction:
◦ XA = nA / (nA + nB)

(moles of A ÷ total moles)
Calculating partial pressure:
◦ PA = XA * PT
(mole fraction * total pressure)


A mixture of gases contains 0.102 mol of CO,
0.598 mol of CO2, and 0.679 mol of O2.
Calculate the mol fraction and partial
pressures of the gases if the total pressure is
1.50 atm at room temperature.
Answers:
◦ XCO = 0.0739; PCO = 0.111 atm
◦ XCO2 = 0.434; PCO2 = 0.651 atm
◦ XO2 = 0.492; PO2 = 0.738 atm

Diffusion: gradual mixing of one gas with
another (applet link)
◦ Lighter gas diffuses more quickly than heavier gas

Effusion: escape of a gas through a pinhole
(used to separate different mass species)
effusion
37
Avg KE of gas

Molecular Speeds depend on Temperature:
38

Which molecule will escape from a leaky
balloon fastest?
39
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Which gas in each pair below will effuse
faster?
CH4
and
N2
O2
and
CO2
CO
and
NH3
Cl2
and
H2
Kr
and
SO2