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?
  2. It does not change.
  3. It is halved.
  4. It is doubled.
  5. It is squared.
  6. It cannot be predicted.
  7. A sealed, rigid container is filled with three ideal gases:A,B, andC. 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?
  8. the average distance traveled between molecular collisions
  9. the intermolecular forces
  10. the volume of the gas molecules
  11. the total pressure
  12. the molar masses of the gases
  13. 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 thefalsestatement from the following list.
  14. The density of the nitrogen sample is less than the density of the argon sample.
  15. The average speed of the nitrogen molecules is the same as the average speed of the argon molecules.
  16. The average kinetic energy of the nitrogen molecules is the same as the average kinetic energy of the argon molecules.
  17. The volume of the nitrogen container is the same as the volume of the argon container.
  18. The number of molecules in the nitrogen container is the same as the number of atoms in the argon container.
  19. 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?
  20. vapor pressure of water
  21. temperature of the displaced water
  22. barometric pressure in the room
  23. mass of the solid used
  24. volume of the displaced water
  25. 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:
  26. the attraction between the molecules
  27. the shape of the molecules
  28. the volume of the molecules
  29. the mass of the molecules
  30. the speed the molecules are moving
  31. 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?
  32. 18.0 g
  33. 35.0 g
  34. 27.0 g
  35. 4.50 g
  36. 9.00 g
  37. 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?
  38. 0.080 atm
  39. 0.34 atm
  40. 0.13 atm
  41. 0.24 atm
  42. 0.48 atm
  43. 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).
  44. 760 mm Hg
  45. 41 mm Hg
  46. 715 mm Hg
  47. 797 mm Hg
  48. 756 mm Hg
  49. 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.
  50. 727°C
  51. 318°C
  52. 1000°C
  53. 160°C
  54. 45°C
  55. The average kinetic energy of nitrogen molecules changes by what factor when the temperature is increased from 30°C to 60°C?
  1. 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?
  2. 0.290 atm
  3. 0.519 atm
  4. 0.206 atm
  5. 0.338 atm
  6. 0.310 atm
  7. 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.
  8. 322 g/mol
  9. 168 g/mol
  10. 0.00621 g/mol
  11. 80.5 g/mol
  12. 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?
  14. the average distance between the molecules
  15. the average speed of the molecules
  16. the average molecular mass of the gas
  17. III only
  18. I only
  19. II only
  20. II and III
  21. I and II
  22. 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?
  23. CO
  24. He
  25. O2
  26. F2
  27. SiH4
  28. 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?
  29. There is no change in the number of gas atoms.
  30. There is an increase in the volume of the gas.
  31. There is a decrease in the pressure exerted by the gas.
  32. The gas atoms travel with the same average speed.
  33. The gas atoms are separated by a greater average distance.
  34. 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–1K–1.)

  1. 0.6 atm
  2. 6 atm
  3. 0.05 atm
  4. 1.2 atm
  5. 3 atm
  1. 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?
  2. H2< NO2< O2
  3. NO2< H2< O2
  4. H2= NO2= O2
  5. O2< NO2< H2
  6. H2< O2< NO2
  7. Choose the gas that probably shows the greatest deviation from ideal gas behavior.
  8. He
  9. O2
  10. SF4
  11. SiH4
  12. Ar
  13. 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.
  14. SiH4
  15. Si2H6
  16. Si3H8
  17. Si4H10
  18. Si5H12
  19. Which of the following best explains why a hotair balloon rises?
  20. The heating of the air causes the pressure inside the balloon to increase.
  21. The cool outside air pushes the balloon higher.
  22. The temperature difference between the inside and outside air causes convection currents.
  23. Hot air has a lower density than cold air.
  24. 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 meansV2= 2V1. On substituting:T2= 2V1T1/V1; givingT2= 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, andTare 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—SinceTandPare 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 sameTandP, 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.

  1. 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.
  2. C—Using Dalton's law (PTotal=PA+PB+ …), the partial pressure may be found by: 756 mm Hg –41 mm Hg = 715 mm Hg.
  3. 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.
  4. 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.
  5. A—You can begin by removing the volume (constant) from the combined gas law to produce Gay-Lussac'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.
  6. 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.
  7. 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.
  8. 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:
  1. 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.
  2. 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 toP=nRT/V= ()(0.2 mol) (0.082 L atm mol–1K–1). (300. K)/(2 L) = 6 atm.
  3. 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.
  4. 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.
  5. 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.
  6. 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.