Chapter 5 – AP Chemistry
Gases
Substances that Exist as Gases
at 1atm and 25 C
 Elements
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H2
N2
O2
F2
Cl2
He
Ne
Ar
Kr
Xe
Rn
 Compounds
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HF
HCl
HBr
HI
CO
CO2
NH3
NO
NO2
N2O
SO2
All Gases have the following
Physical Properties
 Assume the volume and shape of their
containers
 Are the most compressible of the states of
matter
 Will mix evenly and completely when
confined to the same container
 Have much lower densities than liquids and
solids
Pressure of a Gas
 Gases exert pressure on any surface with
which they come into contact
– Example of atmospheric pressure
 The ability to drink liquid through a straw
– Sucking air out of the straw reduces the pressure inside the
straw
– Therefore the greater atmospheric pressure on the liquid
pushes it up into the straw to replace the air that has been
sucked out
Atmospheric Pressure
The pressure exerted by Earth’s atmosphere
 Atoms and molecules are subjected to
earth’s gravitational pull
– Therefore, the atmosphere is much denser near
the surface of the earth
– The denser the air is the greater the pressure
Air
 Gases are fluid
– Pressure exerted on an object in a fluid comes from all
directions
 Air pressure at the molecular level results from
collisions between air molecules and any surface
with which they come into contact with
 The magnitude of pressure depends on how often
and how strongly the molecules impact the surface
Gas Laws
 Boyle’s Law
– P1V1 = P2V2
 Charles’s Law
– V1/T1 = V2/T2
 Gay – Lussac’s Law
– P1/T1 = P2/T2
 Combined Gas Law
– P1V1/T1 = P2V2/T2
Combined Gas Law
 A small bubble rises from the bottom of a
lake, where the temperature and pressure
are 8 degrees Celsius and 6.4atm, to the
water’s surface, where the temperature is 25
degrees Celsius and pressure is 1.0 atm.
Calculate the final volume of the bubble if its
initial volume was 2.1ml.
Ideal Gas Law
 PV=nRT
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R = .0821L*atm/K*mol
Volume in L
Pressure in atm
n in moles
Temperature in Kelvin
 Sulfur hexafluoride is a colorless, odorless, very
unreactive gas. Calculate the pressure exerted by
84g of the gas in a steel vessel of a volume 6.09L
and 55 degrees Celcius.
Ideal Gas Law
 Calculate the volume occupied by 14.2g of
NH3 at STP. (Hint 22.4L = 1mol)
Gas Stoichiometry
 Same a basic reaction chemistry
– Please note: 1mole = 22.4L can only be used
when the chemical reaction is at STP
– If the reaction is at STP, after moles of the
unknown are calculated, then the moles can be
converted to liters
– If the reaction is NOT at STP then you must use
ideal gas law to solve for volume
Gas Stoich Problems
 Sodium Azide (NaN3) is used in some
automobile air bags. The impact of a
collision triggers the decomposition of NaN3.
The nitrogen gas produced quickly inflates
the bag between the driver and the
windshield and dashboard. Calculate the
volume of N2 generated at 85 degrees
Celsius and 812mmHg by decomposition of
50g NaN3.
Gas Stoich
The breakdown on glucose is below. Calculate
the volume of CO2 produced at 37 degrees
and .5atm when 5.6g of glucose is used to
completion in this reaction
C6H12O6 + O2 → CO2 + H2O
Dalton’s Law of Partial Pressures
 Dalton's law of partial pressures states
that the total pressure exerted by a gaseous
mixture is equal to the sum of the partial
pressures of each individual component in a
gas mixture.
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Dalton’s Law of Partial Pressure
 Calculation of the moles of each gas based on the partial
pressures
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For example two gases in a container (nitrogen and oxygen)
P = Pnitrogen + Poxygen
Then you can use ideal gas law to calculate the moles of each gas
Pnitrogen V = nnitrogen R T
– Poxygen V = noxygen R T
 Please note you can always use the equations backwards.
Start with moles of each to find partial pressures, then find
the total pressure
Dalton’s Law Problems
Quiz Problem
 Oxygen gas is generated by the
decomposition of potassium chlorate in wate
vapor chamber. The volume of oxygen gas
collected at 26 degrees Celsius is 752ml
and the atmospheric pressure is
1841mmHg. The pressure of the water
vapor at 26 degrees Celsius is 25.2 mmHg.
Calculate the mass of oxygen obtained.
Dalton’s Law of Partial Pressures
Mole Fraction
 The mole fraction that express the ratio of
the number of moles of one component to
the number of moles of all components
present
 By using mole fraction you can calculate the
partial pressure of each gas in a system
 Pi = XiPT
 Xi -Is the percent of each molar amount of
each gas (part over whole)
Dalton’s Law Problems
 A mixture of gases contain 3.85 moles of
Ne, 0.92 moles of argon and 2.59 moles of
xenon. Calulate the partial pressure of each
gas if the total pressure is 2.50atm.
Kinetic Molecular Theory of Gases
 A gas consists of a collection of small particles traveling in
straight-line motion and obeying Newton's Laws.
 The molecules in a gas occupy no volume (that is, they are
points).
 Collisions between molecules are perfectly elastic (that is,
no energy is gained or lost during the collision).
 There are no attractive or repulsive forces between the
molecules.
 The average kinetic energy of a molecule is 3kT/2. (T is the
absolute temperature and k is the Boltzmann constant.)
Root Mean Squared
 To calculate how fast a molecule moves at any
given temperature
R = 8.314J/K*mol
M = (molar mass in Kg/mol)
U = calculated in meters per second
Root Mean Squared Problem
 Calculate the root-mean squared speeds of
helium atoms and nitrogen molecules in m/s
at 25 degrees Celsius
Gas Diffusion and Effusion
 Diffusion – the gradual mixing of molecules
of one gas with molecules of another by
virtue of their kinetic properties
 Effusion – the process by which a gas under
pressure escapes from one compartment of
a container to another by passing through a
small opening
Graham’s law of Diffision
 A flammable gas made up only of
carbon and hydrogen is found to
effuse through a porous barrier in
3.5min. Under the same
conditions of temperature and
pressure, it takes an equal
volume of chlorine gas 7.34 min
to effuse through the same
barrier. Calculate the molar mass
of the unknown gas and suggest
what this gas might be.
Deviation from Ideal Behavior
Van der Waals
AP Questions
 2 H2O2(aq) → 2 H2O(l) + O2(g)
 The mass of an aqueous solution of H2O2 is 6.951 g. The H2O2 in the
solution decomposes completely according to the reaction represented
above. The O2(g) produced is collected in an inverted graduated tube
over water at 23.4°C and has a volume of 182.4 mL when the water
levels inside and outside of the tube are the same. The atmospheric
pressure in the lab is 762.6 torr, and the equilibrium vapor pressure of
water at 23.4°C is 21.6 torr.
 (a) Calculate the partial pressure, in torr, of O2(g) in the gascollection tube.
 (b) Calculate the number of moles of O2(g) produced in the reaction.
 (c) Calculate the mass, in grams, of H2O2 that decomposed.
 (d) Calculate the percent of H2O2 , by mass, in the original 6.951 g
aqueous sample.
AP Questions
A rigid 5.00 L cylinder contains 24.5 g of N2(g) and 28.0 g of O2(g)
 (a) Calculate the total pressure, in atm, of the gas mixture in the cylinder at
298 K.
 (b) The temperature of the gas mixture in the cylinder is decreased to 280 K.
Calculate each of the following.
 (i) The mole fraction of N2(g) in the cylinder.
 (ii) The partial pressure, in atm, of N2(g) in the cylinder.
 (c) If the cylinder develops a pinhole-sized leak and some of the gaseous
mixture escapes, would the ratio in the cylinder increase, decrease, or remain
the same? Justify your answer.
A different rigid 5.00 L cylinder contains 0.176 mol of NO(g) at 298 K. A 0.176 mol
sample of O2(g) is added to the cylinder, where a reaction occurs to produce
NO2(g).
 (d) Write the balanced equation for the reaction.
 (e) Calculate the total pressure, in atm, in the cylinder at 298 K after the
reaction is complete.
AP Questions
 A mixture of H2(g), O2(g), and 2 millilitres of H2O(l) is present in a 0.500
litre rigid container at 25°C. The number of moles of H2 and the number
of moles of O2 are equal. The total pressure is 1,146 millimetres
mercury. (The equilibrium vapor pressure of pure water at 25°C is 24
millimetres mercury.)
 The mixture is sparked, and H2 and O2 react until one reactant is
completely consumed.
 (a) Identify the reactant remaining and calculate the number of moles
of the reactant remaining.
 (b) Calculate the total pressure in the container at the conclusion of
the reaction if the final temperature is 90°C. (The equilibrium vapor
pressure of water at 90°C is 526 millimetres mercury.)
 (c) Calculate the number of moles of water present as vapor in the
container at 90°C.