AP Chemistry Midterm Review: Gases
1 atm = 760 mmHg = 760 torr
Ideal Gases are those in which come close to a low pressure at a high temperature. Gases begin to
deviate from an “ideal” state when they are exposed to a low temperature and exhibit a high pressure.
Gas Laws:
The ideal gas law relates the variables of pressure, volume, temperature, and number of moles of gas
within a closed system.
The ideal gas law takes the form:
PV = nRt______________________________________________________________________________
P = Pressure of the confined gas in atmospheres
R = Gas Constant, 0.0821 L·atm/mol·K
V = Volume of the confined gas, in liters
T = Temperature in Kelvin
n = Number of moles of gas________________
Boyles Law- The pressure exerted by a gas held at a constant temperature varies
inversely with the volume of the gas.
Gay-Lussac’s Law- If you increase the temperature of a container with fixed volume,
the pressure inside the container will increase.
Charles Law- When the temperature of a gas increases, the volume increases.
Combined Gas Law- If the number of moles of gas are constant (No gas can get in
or out) then we can combine the previous three gas laws.
Calculating the Root Mean Square Speed of a Gas
Calculating Kinetic Energy:
Dalton’s Law of Partial Pressures:
Mole Fraction (X1)= Moles A/ Total Moles
To calculate the actual partial pressure (In atm):
P1= Total Pressure X (XA
COMMON MISTAKES TO AVOID:
1. When using any gas laws, be sure you are dealing with gases!
2. In any laws be sure to express temperature in kevlin units.
3. Make sure your answer is reasonable.
4. Make sure your units cancel.
5. Be sure to use the correct molecular mass for those gases that exist as diatomic
molecules. (BrINClHOF)
6. If value 22.4 L/mol is to be used, make sure that it is applied to a GAS at STP.
AP Chemistry Midterm Review: Gases
Recommended Free-Response questions to be completed for personal study:
1.
A hydrogen gas sample is collected over water. The volume of the sample was 190.0 mL at 26°C, and the
pressure in the room was 754 mm Hg. The vapor pressure of water at 26°C is 25.2 mm Hg.
a.
b.
c.
d.
Calculate the number of moles of hydrogen in the sample.
Calculate how many molecules of water vapor are present in the sample.
Determine the density (in g/L) of the gas mixture.
Determine the mole fraction of water.
A sample containing 2/3 mole of potassium chlorate, KClO3, is heated until it decomposes to potassium chloride and
oxygen gas. The oxygen is collected in an inverted bottle through the displacement of water. Answer the following
questions using this information.
2.
3.
4.
5.
Write a balanced chemical equation for the reaction.
How many moles of oxygen gas are produced?
The temperature and pressure of the sample are adjusted to STP. The volume of the sample is found to be
slightly greater than 22.4 liters. Explain.
An excess of sulfur is burned in the oxygen. Write a balanced chemical equation and calculate the number
of moles of gas formed.
Recommended Multiple-Choice questions to be completed for personal study:
 A sample of argon gas is sealed in a container. The volume of
the container is doubled. If the pressure remains constant, what
happens to the absolute temperature?
A.
B.
C.
D.
E.
It does not change.
It is halved.
It is doubled.
It is squared.
It cannot be predicted.
 A sealed, rigid container is filled with three ideal gases: A, B,
and C. The partial pressure of each gas is known. The temperature
and volume of the system are known. What additional information
is needed to determine the masses of the gases in the container?
A.
B.
C.
D.
E.
the average distance traveled between molecular
collisions
the intermolecular forces
the volume of the gas molecules
the total pressure
the molar masses of the gases
 Two balloons are at the same temperature and pressure. One
contains 14 g of nitrogen and the other contains 20.0 g of argon.
Pick the false statement from the following list.
A.
The density of the nitrogen sample is less than the
density of the argon sample.
B.
C.
D.
E.
The average speed of the nitrogen molecules is the
same as the average speed of the argon molecules.
The average kinetic energy of the nitrogen molecules is
the same as the average kinetic energy of the argon
molecules.
The volume of the nitrogen container is the same as the
volume of the argon container.
The number of molecules in the nitrogen container is
the same as the number of atoms in the argon container.
 An experiment to determine the molecular mass of a gas begins
by heating a solid to produce a gaseous product. The gas passes
through a tube and displaces water in an inverted, water-filled
bottle. Which of the following necessary items may be determined
after the experiment is completed?
A.
B.
C.
D.
E.
vapor pressure of water
temperature of the displaced water
barometric pressure in the room
mass of the solid used
volume of the displaced water
 The true volume of a particular real gas is larger than that
calculated from the ideal gas equation. This occurs because the
ideal gas equation does NOT correct for:
A.
B.
C.
D.
the attraction between the molecules
the shape of the molecules
the volume of the molecules
the mass of the molecules
AP Chemistry Midterm Review: Gases
E.
the speed the molecules are moving
 Aluminum metal reacts with HCl to produce aluminum chloride
and hydrogen gas. How many grams of aluminum metal must be
added to an excess of HCl to produce 33.6 L of hydrogen gas, if
the gas is at STP?
A.
B.
C.
D.
E.
18.0 g
35.0 g
27.0 g
4.50 g
9.00 g
 A reaction produces a gaseous mixture of carbon dioxide,
carbon monoxide, and water vapor. After one reaction, the mixture
was analyzed and found to contain 0.60 mol of carbon dioxide,
0.30 mol of carbon monoxide, and 0.10 mol of water vapor. If the
total pressure of the mixture was 0.80 atm, what was the partial
pressure of the carbon monoxide?
A.
B.
C.
D.
E.
B.
C.
D.
E.
F.
760 mm Hg
41 mm Hg
715 mm Hg
797 mm Hg
756 mm Hg
 A sample of oxygen gas with a volume of 8.00 L at 127°C and
775 mm Hg is heated until it expands to a volume of 20.00 L.
Determine the final temperature of the oxygen gas, if the pressure
remains constant.
A.
B.
C.
D.
E.
727°C
318°C
1000°C
160°C
45°C
 The average kinetic energy of nitrogen molecules changes by
what factor when the temperature is increased from 30°C to 60°C?
0.080 atm
0.34 atm
0.13 atm
0.24 atm
0.48 atm
 A sample of methane gas was collected over water at 35°C. The
sample was found to have a total pressure of 756 mm Hg.
Determine the partial pressure of the methane gas in the sample
(vapor pressure of water at 35°C is 41 mm Hg).
_____________________________________________________________________________________
Free-Response Answers
1A.0.00742 mol H2
1B. 1.55 × 1020 molecules
1C. 0.103 g/L
1D. 0.0335
2A. 2 KClO3(s) → 2 KCl(s) + 3 O2(g)
2B. 2/3 mole KClO3) (3 moles O2/2 moles KClO3) = 1 mole O2
2C. At STP the volume of 1 mole of O2 should be 22.4 liters. The volume is greater because oxygen was
not the only gas in the sample. Water vapor was present. The presence of the additional gas leads to a
larger volume.
2D. 2 S(s) + 3 O2(g) → 2 SO3(g), (1 mole O2) (2 mole SO3/3 mole O2) = 2/3 mole SO2
2E. SO2 + H2O → H2SO3
Multiple Choice Answers:
1.C 2.E 3.B 4.A 5.C 6.C 7.D 8.C 9.A 10.A
AP Chemistry Midterm Review: Gases
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