The Advanced Placement Examination in Chemistry AP Exam Questions.pdf · The Advanced Placement...

Gases

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The Advanced Placement Examination in Chemistry

Part I – Multiple Choice Questions Part II – Free Response Questions

Selected Questions from1970 to 2010

Gases Part I

1984

21. When a sample of oxygen gas in a closed container of constant volume is heated until its absolute

temperature is doubled, which of the following is also doubled?

(A) The density of the gas

(B) The pressure of the gas

(C) The average velocity of the gas molecules

(D) The number of molecules per cm3

(E) The potential energy of the molecules

23. The density of an unknown gas is 4.20 grams per liter at 3.00 atmospheres pressure and 127°C.

What is the molecular weight of this gas?

(R = 0.0821 liter–atm/mole–K)

(A) 14.6

(B) 46.0

(C) 88.0

(D) 94.1

(E) 138

39. Equal masses of three different ideal gases, X, Y, and Z, are mixed in a sealed rigid container. If the

temperature of the system remains constant, which of the following statements about the partial

pressure of gas X is correct?

(A) It is equal to 1/3 the total pressure

(B) It depends on the intermolecular forces of attraction between molecules of X, Y, and Z.

(C) It depends on the relative molecular masses of X, Y, and Z.

(D) It depends on the average distance traveled between molecular collisions.

(E) It can be calculated with knowledge only of the volume of the container.

50. Two flexible containers for gases are at the same temperature and pressure. One holds 0.50 gram of

hydrogen and the other holds 8.0 grams of oxygen. Which of the following statements regarding

these gas samples is FALSE?

(A) The volume of the hydrogen container is the same as the volume of the oxygen container.

(B) The number of molecules in the hydrogen container is the same as the number of molecules in

the oxygen container.

(C) The density of the hydrogen sample is less than that of the oxygen sample.

(D) The average kinetic energy of the hydrogen molecules is the same as the average kinetic energy

of the oxygen molecules.

(E) The average speed of the hydrogen molecules is the same as the average speed of the oxygen

molecules.

72. A compound is heated to produce a gas whose molecular weight is to be determined. The gas is

collected by displacing water in a water–filled flask inverted in a trough of water. Which of the

following is necessary to calculate the molecular weight of the gas, but does NOT need to be

measured during the experiment?

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(A) Mass of the compound used in the experiment

(B) Temperature of the water in the trough

(C) Vapor pressure of the water

(D) Barometric pressure

(E) Volume of water displaced from the flask

78. When the actual gas volume is greater then the volume predicted by the ideal gas law, the

explanation lies in the fact that the ideal gas law does NOT include a factor for molecular.

(A) volume

(B) mass

(C) velocity

(D) attractions

(E) shape

85. A sample of 9.00 grams of aluminum metal is added to an excess of hydrochloric acid. The volume

of hydrogen gas produced at standard temperature and pressure is

(A) 22.4 L

(B) 11.2 L

(C) 7.46 L

(D) 5.6 L

(E) 3.74 L

1989

16. A gaseous mixture containing 7.0 moles of nitrogen, 2.5 moles of oxygen, and 0.50 mole of helium

exerts a total pressure of 0.90 atmosphere. What is the partial pressure of the nitrogen?

(A) 0.13 atm

(B) 0.27 atm

(C) 0.63 atm

(D) 0.90 atm

(E) 6.3 atm

30. Hydrogen gas is collected over water at 24°C. The total pressure of the sample is 755 millimeters of

mercury. At 24°C, the vapor pressure of water is 22 millimeters of mercury. What is the partial

pressure of the hydrogen gas?

(A) 22 mm Hg

(B) 733 mm Hg

(C) 755 mm Hg

(D) 760 mm Hg

(E) 777 mm Hg

32. A 2.00–liter sample of nitrogen gas at 27°C and 600. millimeters of mercury is heated until it

occupies a volume of 5.00 liters. If the pressure remains unchanged, the final temperature of the gas

is

(A) 68°C

(B) 120°C

(C) 477°C

(D) 677°C

(E) 950°C

40. 2 K + 2 H2O 2K+ + 2 OH

– + H2

When 0.400 mole of potassium reacts with excess water at standard temperature and pressure as

shown in the equation above, the volume of hydrogen gas produced is

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(A) 1.12 L

(B) 2.24 L

(C) 3.36 L

(D) 4.48 L

(E) 6.72 L

62. As the temperature is raised from 20°C to 40°C, the average kinetic energy of neon atoms changes

by a factor of

(A) 2

1

(B) 293

313

(C) 293

313

(D) 2

(E) 4

1994

24. A sample of 0.0100 mole of oxygen gas is confined at 37°C and 0.216 atmosphere. What would be

the pressure of this sample at 15°C and the same volume?

(A) 0.0876 atm

(B) 0.175 atm

(C) 0.201 atm

(D) 0.233 atm

(E) 0.533 atm

33. A hydrocarbon gas with an empirical formula CH2 has a density of 1.88 grams per liter at 0°C and

1.00 atmosphere. A possible formula for the hydrocarbon is

(A) CH2

(B) C2H4

(C) C3H6

(D) C4H8

(E) C5H10

37. A sample of 3.30 grams of an ideal gas at 150.0°C and 1.25 atmospheres pressure has a volume of

2.00 liters. What is the molar mass of the gas? The gas constant, R, is 0.0821 (L.atm)/(mol

.K).

(A) 0.0218 g mol–1

(B) 16.2 g mol–1

(C) 37.0 g mol–1

(D) 45.8 g mol–1

(E) 71.6 g mol–1

39. Samples of F2 gas and Xe gas are mixed in a container of fixed volume. The initial partial pressure

of the F2 gas is 8.0 atmospheres and that of the Xe gas is 1.7 atmospheres. When all of the Xe gas

reacted, forming a solid compound, the pressure of the unreacted F2 gas was 4.6 atmospheres. The

temperature remained constant. What is the formula of the compound?

(A) XeF

(B) XeF3

(C) XeF4

(D) XeF6

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(E) XeF8

40. The system shown above is at equilibrium at 28°C. At this temperature, the vapor pressure of water

is 28 millimeters of mercury. The partial pressure of O2(g) in the system is

(A) 28 mm Hg

(B) 56 mm Hg

(C) 133 mm Hg

(D) 161 mm Hg

(E) 189 mm Hg

45. A sample of an ideal gas is cooled from 50.0°C to 25.0°C in a sealed container of constant volume.

Which of the following values for the gas will decrease?

I. The average molecular mass of the gas

II. The average distance between the molecules

III. The average speed of the molecules

(A) I only

(B) II only

(C) III only

(D) I and III

(E) II and III

64. At 25°C, a sample of NH3 (molar mass 17 grams) effuses at the rate of 0.050 mole per minute.

Under the same conditions, which of the following gases effuses at approximately one–half that

rate?

(A) O2 (molar mass 32 grams)

(B) He (molar mass 4.0 grams)

(C) CO2 (molar mass 44 grams)

(D) Cl2 (molar mass 71 grams)

(E) CH4 (molar mass 16 grams)

1999

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23. A hot-air balloon, shown above, rises. Which of the following is the best explanation for this

observation?

(A) The pressure on the walls of the balloon increases with increasing temperature.

(B) The difference in temperature between the air inside and outside the balloon produces

convection currents.

(C) The cooler air outside the balloon pushes in on the walls of the balloon.

(D) The rate of diffusion of cooler air is less than that of warmer air.

(E) The air density inside the balloon is less than that of the surrounding air.

44. A rigid metal tank contains oxygen gas. Which of the following applies to the gas in the tank when

additional oxygen is added at constant temperature?

(A) The volume of the gas increases.

(B) The pressure of the gas decreases.

(C) The average speed of the gas molecules remains the same.

(D) The total number of gas molecules remains the same.

(E) The average distance between the gas molecules increases.

W(g) + X(g) Y(g) + Z(g)

53. Gases W and X react in a closed, rigid vessel to form gases Y and Z according to the equation above.

The initial pressure of W(g) is 1.20 atm and that of X(g) is 1.60 atm. No Y(g) or Z(g) is initially

present. The experiment is carried out at constant temperature. What is the partial pressure of Z(g)

when the partial pressure of W(g) has decreased to 1 .0 atm?

(A) 0.20 atm

(B) 0.40 atm

(C) 1.0 atm

(D) 1.2 atm

(E) 1.4 atm

NH4NO3(s) N2O(g) + 2 H2O(g)

60. A 0.03 mol sample of NH4NO3(s) is placed in a 1 L evacuated flask, which is then sealed and

heated. The NH4NO3(s) decomposes completely according to the balanced equation above. The total

pressure in the flask measured at 400 K is closest to which of the following? (The value of the gas

constant, R, is 0.082 L atm mol-1

K-1

.)

(A) 3 atm

(B) 1 atm

(C) 0.5 atm

(D) 0.1 atm

(E) 0.03 atm

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64. Equal numbers of moles of He(g), Ar(g), and Ne(g) are placed in a glass vessel at room temperature.

If the vessel has a pinhole-sized leak, which of the following will be true regarding the relative

values of the partial pressures of the gases remaining in the vessel after some of the gas mixture has

effused?

(A) PHe < PNe < PAr

(B) PHe < PAr < PNe

(C) PNe < PAr < PHe

(D) PAr < PHe < PNe

(E) PHe = PAr = PNe

2002

Questions 8-10 refer to the following gases at 0°C and 1 atm.

(A) Ne

(B) Xe

(C) O2

(D) CO

(E) NO

8. Has an average atomic or molecular speed closest to that of N2 molecules at 0°C and 1 atm.

9. Has the greatest density.

10. Has the greatest rate of effusion through a pinhole.

20. A flask contains 0.25 mole of SO2(g), 0.50 mole of CH4(g), and 0.50 mole of O2(g). The total

pressure of the gases in the flask is 800 mm Hg. What is the partial pressure of the SO2(g) in the

flask?

(A) 800 mm Hg

(B) 600 mm Hg

(C) 250 mm Hg

(D) 200 mm Hg

(E) 160 mm Hg

CS2(l) + 3 O2(g) CO2(g) + 2 SO2(g)

31. What volume of O2(g) is required to react with excess CS2(l) to produce 4.0 L of CO2(g)? (Assume

all gases are measured at 0°C and 1 atm.

(A) 12 L

(B) 22.4 L

(C) 1/3 × 22.4 L

(D) 2 × 22.4 L

(E) 3 × 22.4 L

40. An excess of Mg(s) is added to 100. mL of 0.400 M HCl. At 0°C and 1 atm pressure, what volume

of H2 gas can be obtained?

(A) 22.4 mL

(B) 44.8 mL

(C) 224 mL

(D) 448 mL

(E) 896 mL

66. A 2 L container will hold about 4 g of which of the following gases at 0°C and 1 atm?

(A) SO2

(B) N2

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(C) CO2

(D) C4H8

(E) NH3

2008

9. Which of the following is the slowest to effuse through a small opening at 25°C and 1 atmosphere?

(A) NH3(g)

(B) BH3(g)

(C) H2(g)

(D) H2S(g)

(E) HBr(g)

Δ

2 KClO3(s) 2 KCl(s) + 3 O2(g) MnO

2

18. According to the equation above, how many moles of potassium chlorate, KClO3, must be

decomposed to generate 1.0 L of O2 gas at standard temperature and pressure?

(A)

4.22

1

3

1mol

(B)

4.22

1

2

1 mol

(C)

4.22

1

3

2 mol

(D)

4.22

1

2

3 mol

(E)

4.22

12 mol

25. The pressure, in atm, exerted by 1.85 mol of an ideal gas placed in a 3.00 L container at 35°C is

given by which of the following expressions?

(A)

00.3

3080821.085.1 atm

(B)

00.30821.0

0.3585.1 atm

(C) 3080821.085.1

00.3 atm

(D)

00.3

308314.885.1atm

(E)

0.350821.0

85.100.3 atm

42. Equal masses of He and Ne are placed in a sealed container. What is the partial pressure of He if the

total pressure in the container is 6 atm?

(A) 1 atm

(B) 2 atm

(C) 3 atm

(D) 4 atm

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(E) 5 atm

51. Under which of the following conditions of temperature and pressure would 1.0 mol of the real gas

CO2(g) behave most like an ideal gas?

Temperature Pressure

(K) (atm)

(A) 100 0.1

(B) 100 100

(C) 800 0.1

(D) 800 1

(E) 800 100

65. In a laboratory experiment, H2(g) is collected over water in a gas-collection tube as shown in the

diagram above. The temperature of the water is 21°C and the atmospheric pressure in the laboratory

is measured to be 772 torr. Before measuring the volume of gas collected in the tube, what step, if

any, must be taken to make it possible to determine the total gas pressure inside the tube?

(A) Tilt the tube to the side enough to let some air in to break the partial vacuum in the tube.

(B) Lift the tube upward until it is just barely immersed in the water.

(C) Move the tube downward until the water level is the same inside and outside the tube.

(D) Adjust the temperature of the water to 25°C

(E) No further steps need to be taken as long as the temperature of the water is known.

69. At standard temperature and pressure, a 0.50 mol sample of H2 gas and a separate 1.0 mol sample of

O2 gas have the same

(A) average molecular kinetic energy

(B) average molecular speed

(C) volume

(D) effusion rate

(E) density

Part II

1970

Gases

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A 2.000 gram sample containing graphite (carbon) and an inert substance was burned in oxygen and

produced a mixture of carbon dioxide and carbon monoxide in the mole ratio 2.00:1.00. The volume of

oxygen used was 747.0 milliliters at 1,092K and 12.00 atmospheres pressure. Calculate the percentage by

weight of graphite in the original mixture.

1971

2 HCOONa + H2SO4 2 CO + 2 H2O + Na2SO4

A 0.964 gram sample of a mixture of sodium formate and sodium chloride is analyzed by adding sulfuric

acid. The equation for the reaction for sodium formate with sulfuric acid is shown above. The carbon

monoxide formed measures 242 milliliters when collected over water at 752 torr and 22.0C. Calculate

the percentage of sodium formate in the original mixture.

1972

A 5.00 gram sample of a dry mixture of potassium hydroxide, potassium carbonate, and potassium

chloride is reacted with 0.100 liter of 2.0 molar HCl solution.

(a) A 249 milliliter sample of dry CO2 gas, measured at 22C and 740 torr, is obtained from the

reaction. What is the percentage of potassium carbonate in the mixture?

(b) The excess HCl is found by titration to be chemically equivalent to 86.6 milliliters of 1.50 molar

NaOH. Calculate the percentages of potassium hydroxide and of potassium chloride in the original

mixture.

1973

A 6.19 gram sample of PCl5 is placed in an evacuated 2.00 liter flask and is completely vaporized at

252C.

Calculate the pressure in the flask if no chemical reaction were to occur.

1976

When the molecular weight of a volatile liquid is calculated from the weight, volume, temperature, and

pressure of a sample of that liquid when vaporized, the assumption is usually made that the gas behaves

ideally. In fact at a temperature not far above the boiling point of the liquid, the gas is not ideal. Explain

how this would affect the results of the molecular weight determination.

1982

(a) From the standpoint of the kinetic-molecular theory, discuss briefly the properties of gas molecules

that cause deviations from ideal behavior.

(b) Real gases approach ideality at low pressure, high temperature, or both. Explain these observations.

1986 (1)

Three volatile compounds X, Y, and Z each contain element Q. The percent by weight of element Q in

each compound was determined. Some of the data obtained are given below.

Compound Percent by Weight

of Element Q

Molecular

Weight

X 64.8% ?

Y 73.0% 104.

Z 59.3% 64.0

(a) The vapor density of compound X at 27 degrees Celsius and 750. mm Hg was determined to be 3.53

grams per liter. Calculate the molecular weight of compound X.

(b) Determine the mass of element Q contained in 1.00 mole of each of the three compounds.

(c) Calculate the most probable value of the atomic weight of element Q.

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(d) Compound Z contains carbon, hydrogen, and element Q. When 1.00 gram of compound Z is

oxidized and all of the carbon and hydrogen are converted to oxides, 1.37 grams of CO2 and 0.281

gram of water are produced. Determine the most probable molecular formula.

1986 (2)

Give a scientific explanation for the following observations. Use equations or diagrams if they seem

relevant.

A hot-air balloon must be larger than a helium-filled balloon in order to lift the same weight.

1990

A mixture of H2(g), O2(g), and 2 milliliters of H2O(l) is present in a 0.500 liter rigid container at 25C.

The number of moles of H2 and the number of moles of O2 are equal. The total pressure is 1,146

millimeters mercury. (The equilibrium vapor pressure of pure water at 25C is 24 millimeters 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 90C. (The equilibrium vapor pressure of water at 90C is 526 millimeters mercury.)

(c) Calculate the number of moles of water present as vapor in the container at 90C.

1993

Observations about real gases can be explained at the molecular level according to the kinetic molecular

theory of gases and ideas about intermolecular forces. Explain how each of the following observations

can be interpreted according to these concepts, including how the observation supports the correctness of

these theories.

(a) When a gas-filled balloon is cooled, it shrinks in volume; this occurs no matter what gas is

originally placed in the balloon.

(b) When the balloon described in (a) is cooled further, the volume does not become zero; rather, the

gas becomes a liquid or solid.

(c) When NH3 gas is introduced at one end of a long tube while HCl gas is introduced simultaneously at

the other end, a ring of white ammonium chloride is observed to form in the tube after a few

minutes. This ring is closer to the HCl end of the tube than the NH3 end.

(d) A flag waves in the wind.

1994

A student collected a sample of hydrogen gas by the displacement of water as shown by the diagram

above. The relevant data are given in the following table.

GAS SAMPLE DATA

Volume of sample 90.0 mL

Temperature 25C

Atmospheric Pressure 745 mm Hg

Equilibrium Vapor Pressure of

H2O (25C)

23.8 mm Hg

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(a) Calculate the number of moles of hydrogen gas collected.

(b) Calculate the number of molecules of water vapor in the sample of gas.

(c) Calculate the ratio of the average speed of the hydrogen molecules to the average speed of the water

vapor molecules in the sample.

1995

Propane, C3H8, is a hydrocarbon that is commonly used as fuel for cooking.

(a) Write a balanced equation for the complete combustion of propane gas, which yields CO2(g) and

H2O(g).

(b) Calculate the volume of air at 30C and 1.00 atmosphere that is needed to burn completely 10.0

grams of propane. Assume that air is 21.0 percent O2 by volume.

1996

Represented above are five identical balloons, each filled to the same volume at 25C and 1.0 atmosphere

pressure with the pure gases indicated.

(a) Which balloon contains the greatest mass of gas? Explain.

(b) Compare the average kinetic energies of the gas molecules in the balloons. Explain.

(c) Which balloon contains the gas that would be expected to deviate most from the behavior of an ideal

gas? Explain.

(d) Twelve hours after being filled, all the balloons have decreased in size. Predict which balloon will

be the smallest. Explain your reasoning.

2003 (1)

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 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 2

2

( )

( )

N

O

g

g 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.

2003 (2)

For the following, use appropriate chemical principles to explain the observation. Include chemical

equations as appropriate.

Methane gas does not behave as an ideal gas at low temperatures and high pressures.

2004

Answer the following questions about carbon monoxide, CO(g), and carbon dioxide, CO2(g). Assume

that both gases exhibit ideal behavior.

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(a) A 1.0 mol sample of CO(g) is heated at constant pressure. On the graph on your answer sheet,

sketch the expected plot of volume verses temperature as the gas is heated.

(b) Samples of CO(g) and CO2(g) are placed in 1 L containers at the conditions in the diagram below.

(i) Indicate whether the average kinetic energy of the CO2 is greater than, equal to, or less than the

average kinetic energy of the CO(g) molecules. Justify your answer.

(ii) Indicate whether the root-mean-square speed of the CO2(g) molecules is greater than, equal to

or less than the root-mean-square speed of the CO(g) molecules. Justify your answer.

(iii) Indicate whether the number of CO2(g) molecules is greater than, equal, or less than the

number of CO(g) molecules. Justify your answer.

2009

A student was assigned the task of determining the molar mass of an unknown gas. The student

measured the mass of a sealed 843 mL rigid flask that contained dry air. The student then flushed the

flask with the unknown gas, released it, and measured the mass again. Both the air and the unknown gas

were at 23.0°C and 750 torr. The data for the experiment are shown in the table below.

Volume of sealed flask 843 mL

Mass of sealed flask and dry air 157.70 g

Mass of sealed flask and unknown gas 158.08 g

(a) Calculate the mass, in grams, of the dry air that was in the sealed flask. (The density of dry air is

1.18 g L–1

at 23.0°C and 750. torr.)

(b) Calculate the mass, in grams, of the sealed flask itself (i.e., if it had no air in it).

(c) Calculate the mass, in grams, of the unknown gas that was added to the sealed flask.

(d) Using the information above, calculate the value of the molar mass of the unknown gas.

After the experiment was completed, the instructor informed the student that the unknown gas was

carbon dioxide (44.0 g mol–1

).

(e) Calculate the percent error in the value of the molar mass calculated in part (d).

(f) For each of the following two possible occurrences, indicate whether it by itself could have been

responsible for the error in the student’s experimental result. You need not include any

calculations with your answer. For each of the possible occurrences, justify your answer.

Occurrence 1: The flask was incompletely flushed with CO2(g), resulting in some dry air

remaining in the flask.

Occurrence 2: The temperature of the air was 23.0°C, but the temperature of the CO2(g) was

lower than the reported 23.0°C.

(g) Describe the steps of a laboratory method that the student could use to verify that the volume of

the rigid flask is 843 mL at 23.0°C. You need not include any calculations with your answer.

2010

8 H+(aq) + 4 Cl

−(aq) + MnO4

−(aq) → 2 Cl2(g) + Mn

3+(aq) + 4 H2O(l)

Cl2(g) can be generated in the laboratory by reacting potassium permanganate with an acidified solution

of sodium chloride. The net-ionic equation for the reaction is given above.

A 25.00 mL sample of 0.250 M NaCl reacts completely with excess KMnO4(aq). The Cl2(g) produced is

dried and stored in a sealed container. At 22°C the pressure of the Cl2(g) in the container is 0.950 atm.

(a) Calculate the number of moles of Cl–(aq) present before any reaction occurs.

(b) Calculate the volume, in L, of the Cl2(g) in the sealed container.

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