Problems
You have 25 minutes to do the following questions. You may not use a calculator. You
may use the periodic table at the back of the book. For each question, circle the letter
of your choice.
1. 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.
It does not change.
b. It is halved.
c.
It is doubled.
d. It is squared.
e.
It cannot be predicted.
2. 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. the average distance traveled between molecular collisions
b. the intermolecular forces
c.
the volume of the gas molecules
d. the total pressure
e.
the molar masses of the gases
3. 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. The average speed of the nitrogen molecules is the same as the average speed of the argon
molecules.
c.
The average kinetic energy of the nitrogen molecules is the same as the average kinetic energy of
the argon molecules.
d. The volume of the nitrogen container is the same as the volume of the argon container.
e.
The number of molecules in the nitrogen container is the same as the number of atoms in the
argon container.
4. 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.
vapor pressure of water
b. temperature of the displaced water
c.
barometric pressure in the room
d. mass of the solid used
e.
volume of the displaced water
5. 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.
the attraction between the molecules
b. the shape of the molecules
c.
the volume of the molecules
d. the mass of the molecules
e.
the speed the molecules are moving
6. 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.
18.0 g
b. 35.0 g
c.
27.0 g
d. 4.50 g
e.
9.00 g
7. 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.
0.080 atm
b. 0.34 atm
c.
0.13 atm
d. 0.24 atm
e.
0.48 atm
8. 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).
a.
760 mm Hg
b. 41 mm Hg
c.
715 mm Hg
d. 797 mm Hg
e.
756 mm Hg
9. 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.
727°C
b. 318°C
c.
1000°C
d. 160°C
e.
45°C
10. The average kinetic energy of nitrogen molecules changes by what factor when the temperature is
increased from 30°C to 60°C?
11. A 1.15-mol sample of carbon monoxide gas has a temperature of 27°C and a pressure of 0.300 atm. If
the temperature is lowered to 17°C, at constant volume, what would be the new pressure?
a.
0.290 atm
b. 0.519 atm
c.
0.206 atm
d. 0.338 atm
e.
0.310 atm
12. An ideal gas sample weighing 1.28 grams at 127°C and 1.00 atm has a volume of 0.250 L. Determine
the molar mass of the gas.
a.
322 g/mol
b. 168 g/mol
c.
0.00621 g/mol
d. 80.5 g/mol
e.
49.4 g/mol
13. Increasing the temperature of an ideal gas from 50°C to 75°C at constant volume will cause which of
the following to increase for the gas?
a.
the average distance between the molecules
b. the average speed of the molecules
c.
the average molecular mass of the gas
d. III only
e.
I only
f.
II only
g. II and III
h. I and II
14. If a sample of CH4 effuses at a rate of 9.0 mol per hour at 35°C, which of the gases below will effuse at
approximately twice the rate under the same conditions?
a.
CO
b. He
c. O2
d. F2
e. SiH4
15. A steel tank containing argon gas has additional argon gas pumped into it at constant temperature.
Which of the following is true for the gas in the tank?
a.
There is no change in the number of gas atoms.
b. There is an increase in the volume of the gas.
c.
There is a decrease in the pressure exerted by the gas.
d. The gas atoms travel with the same average speed.
e.
The gas atoms are separated by a greater average distance.
16. 2 N2O5(s) → 4NO2(g) O2(g)
A 2 L evacuated flask has a 0.2 mol sample of N2O5(s) sealed inside it. The flask is heated to
decompose the solid and cooled to 300 K. The N2O5(s) is completely decomposed according to the
balanced equation above. What is the nearest value to the final total pressure of the gases in the flask?
(The value of the gas constant, R, is 0.082 L atm mol–1 K–1.)
a. 0.6 atm
b. 6 atm
c.
0.05 atm
d. 1.2 atm
e.
3 atm
17. A glass container is filled, at room temperature, with equal numbers of moles of H2(g), O2(g), and
NO2(g). The gases slowly leak out through a pinhole. After some of the gas has effused, which of the
following is true of the relative values for the partial pressures of the gases remaining in the container?
a. H2 < NO2 < O2
b. NO2 < H2 < O2
c. H2 = NO2 = O2
d. O2 < NO2 < H2
e. H2 < O2 < NO2
18. Choose the gas that probably shows the greatest deviation from ideal gas behavior.
a.
He
b.
c.
d.
e.
O2
SF4
SiH4
Ar
19. Determine the formula for a gaseous silane (SinH2n+2) if it has a density of 5.47 g per L at 0°C and 1.00
atm.
a. SiH4
b. Si2H6
c. Si3H8
d. Si4H10
e. Si5H12
20. Which of the following best explains why a hotair balloon rises?
a.
The heating of the air causes the pressure inside the balloon to increase.
b. The cool outside air pushes the balloon higher.
c.
The temperature difference between the inside and outside air causes convection currents.
d. Hot air has a lower density than cold air.
e.
Cooler air diffuses more slowly than the warmer air.
Answers and explanation
Answers and Explanations
1. C—This question relates to the combined gas law: P1V1/T1 = P2V2/T2. Since the pressure remains
constant, the pressures may be removed from the combined gas law to produce Charles's
law: V1/T1 = V2/T2. This equation may be rearranged to: T2 = V2T1/V1. The doubling of the volume
means V2 = 2 V1. On substituting: T2 = 2V1T1/V1; giving T2 = 2T1. The identity of the gas is irrelevant in
this problem.
2. E—This problem depends on the ideal gas equation: PV = nRT. R, V, and T are known, and by using the
partial pressure for a gas, the number of moles of that gas may be determined. To convert from moles to
mass, the molar mass of the gas is needed.
3. B—Since T and P are known, and since the moles (n) can be determined from the masses given, this
question could use the ideal gas equation. The number of moles of each gas is 0.50. Equal moles of
gases, at the same T and P, have equal volumes. Equal volume eliminates answer choice D. Equal
volume also means that the greater mass has the greater density, eliminating choice A. Equal moles
means that the numbers of molecules and atoms are equal, eliminating choice E. The average kinetic
energy of a gas depends on the temperature. If the temperatures are the same, then the average kinetic
energy is the same, eliminating C. Finally, at the same temperature, heavier gases travel slower than
lighter gases. Nitrogen is lighter than argon, so it travels at a faster average speed, making B the correct
answer. You may find this reasoning process beneficial on any question in which you do not
immediately know the answer.
4. A—This experiment requires the ideal gas equation. The mass of the solid is needed (to convert to
moles); this eliminates answer choice D. The volume, temperature, and pressure must also be measured
during the experiment, eliminating choices B, C, and E. The measured pressure is the total pressure.
Eventually the total pressure must be converted to the partial pressure of the gas using Dalton's law. The
total pressure is the sum of the pressure of the gas plus the vapor pressure of water. The vapor pressure
of water can be looked up in a table when the calculations are performed (only the temperature is
needed to find the vapor pressure in a table). Answer A is correct.
5. C—Real gases are different from ideal gases because of two basic factors (see the van der Waals
equation): molecules have a volume, and molecules attract each other. The molecules' volume is
subtracted from the observed volume for a real gas (giving a smaller volume), and the pressure has a
term added to compensate for the attraction of the molecules (correcting for a smaller pressure). Since
these are the only two directly related factors, answers B, D, and E are eliminated. The question is
asking about volume; thus, the answer is C. You should be careful of "NOT" questions such as this one.
6. C—A balanced chemical equation is needed:
2 Al + 6 HCl → 2 AlCl3 + 3 H2
The reaction produced 33.6L/22.4 L or 1.50 mol at STP. To produce this quantity of hydrogen, (2 mol
Al/3 mol H2) × 1.50 moles H2 = 1.00 mol of Al is needed. The atomic weight of Al is 27.0; thus, 27.0 g
of Al are required.
7. D—The partial pressure of any gas is equal to its mole fraction times the total pressure. The mole
fraction of carbon monoxide is [0.30/(0.60 + 0.30 + 0.10)] = 0.30, and the partial pressure of CO is 0.30
× 0.80 atm = 0.24 atm.
8. C—Using Dalton's law (PTotal = PA + PB + …), the partial pressure may be found by: 756 mm Hg –41
mm Hg = 715 mm Hg.
9. A—The answer may be found using the combined gas law. Removing the constant pressure leaves
Charles's law: V1/T1 = V2/T2. This is rearranged to: T2 = V2T1/V1 = (20.00 L × 400. K)/(8.00 L) = 1000 K
(= 727°C). The other answers result from common errors in this problem.
10. A—The average kinetic energy of the molecules depends on the temperature. The correct answer
involves a temperature difference (333 K – 303 K). Do not forget that ALL gas law calculations require
Kelvin temperatures.
11. A—You can begin by removing the volume (constant) from the combined gas law to produce GayLussac's law = P1/T1 = P2/T2. This equation may be rearranged to: P2 =P1T2/T1 = (0.300 atm × 290.
K)/(300. K) = 0.290 atm. The moles are not important since they do not change. Some of the other
answers result from common errors.
12. B—The molar mass may be obtained by dividing the grams by the number of moles (calculated from
the ideal gas equation). Do not forget to convert the temperature to kelvin.
13. C—Choice I requires an increase in volume. Choice II requires an increase in temperature. Choice III
requires a change in the composition of the gas.
14. B—Lighter gases effuse faster. The only gas among the choices that is lighter than methane is helium.
To calculate the molar mass, you would begin with the molar mass of methane and divide by the rate
difference squared:
15. D—A steel tank will have a constant volume, and the problem states that the temperature is constant.
Adding gas to the tank will increase the number of moles (molecules) of the gas and the pressure
(forcing the molecules closer together). A constant temperature means there will be a constant average
speed.
16. B—The pressure is calculated using the ideal gas equation. A common mistake is forgetting that 5 mol
of gas are produced for every 2 mol of solid reactant. The ideal gas equation is rearranged
to P = nRT/V = ( )(0.2 mol) (0.082 L atm mol–1 K–1). (300. K)/(2 L) = 6 atm.
17. E—The lighter the gas, the faster it effuses (escapes). Equal moles of gases in the same container would
give equal initial partial pressures. The partial pressures would be reduced relative to the masses of the
molecules, with the lightest gas being reduced the most.
18. C—Deviations from ideal behavior depend on the size and the intermolecular forces between the
molecules. The greatest deviation would be for a large polar molecule. Sulfur tetrafluoride is the largest
molecule, and it is the only polar molecule listed.
19. D—The molar mass of gas must be determined. The simplest method to find the molar mass is: (5.47
g/L) × (22.4 L/mol) = 123 g/mol (simple factor label). The molar mass may also be determined by
dividing the mass of the gas by the moles (using 22.4 L/mol for a gas at STP and using 1 L). If you did
not recognize the conditions as STP, you could find the moles from the ideal gas equation. The correct
answer is the gaswith the molar mass closest to 123 g/mol.
20. D—The hot-air balloon rises because it has a lower density than air. Less dense objects will float on
more dense objects. In other words "lighter" objects will float on "heavy" objects.