Gases and IMFs Unit Exam Review Guide
The following review guide does not necessarily contain all topics that will appear on the test. Make sure to review all notes and concepts from this unit. This review is intended to give you an idea of what AP questions look like. Some are very similar to the style of questioning you are used to and others will be very different. Your test will consist of AP style questions. Day 1 will be multiple choice questions. Day 2 will be free response questions (with multiple parts).
AP Style Multiple Choice Questions
1. 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
2. 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
3. Equal masses of three different ideal Gas, 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.
4. Two flexible containers for Gas 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.
5. 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
6. 24. A sample of 0.010 mole of oxygen gas is confined at 127 °C and 0.80 atmosphere. What would be the pressure of this sample at 27 °C and the same volume?
(A) 0.10 atm
(B) 0.20 atm
(C) 0.60 atm
(D) 0.80 atm
(E) 1.1 atm
7. The critical temperature of a substance is the
(A) temperature at which the vapor pressure of the liquid is equal to the external pressure
(B) temperature at which the vapor pressure of the liquid is equal to 760 mm Hg
(C) temperature at which the solid, liquid, and vapor phases are all in equilibrium
(D) Temperature at which liquid and vapor phases are in equilibrium at I atmosphere
(E) lowest temperature above which a substance cannot be liquified at any applied pressure
Use the following diagram for 8-10.
8. The normal boiling point of the substanced represented by the phase diagram above is
(A) -15 °C
(B) -10 °C
(C) 140 °C
(D) greater than 140 °C
(E) not determinable from the diagram
9. The phase diagram above provides sufficient information for determining the
(A) Entropy change on vaporization
(B) conditions necessary for sublimation
(C) deviations from ideal gas behavior of the gas phase
(D) latent heat of vaporization
(E) latent heat of fusion
10. For the substance represented in the diagram, which of the phases is most dense and which is least dense at - 15 °C.
Most Dense / Least Dense(A) / Solid / Gas
(B) / Solid / Liquid
(C) / Liquid / Solid
(D) / Liquid / Gas
(E) / The diagram gives no information about densities.
11. The phase diagram for a pure substance is shown above. Which point on the diagram corresponds to the equilibrium between the solid and liquid phases at the normal melting point?
(A) A
(B) B
(C) C
(D) D
(E) E
AP Style Free Response Questions
1. The normal boiling and freezing points of argon are 87.3 K and 84.0 K, respectively. The triple point is at 82.7 K and 0.68 atmosphere.
(a)Use the data above to draw a phase diagram for argon. Label the axes and label the regions in which the solid, liquid and gas phases are stable. On the phase diagram, show the position of the normal boiling point.
(b)Describe any changes that can be observed in a sample of solid argon when the temperature is increased from 40 K to 160 K at a constant pressure of 0.50 atmosphere.
The argon sublimes.
(c)Describe any changes that can be observed in a sample of liquid argon when the pressure is reduced from 10 atmospheres to 1 atmosphere at a constant temperature of 100 K, which is well below the critical temperature.
The argon vaporizes.
(d)Does the liquid phase of argon have a density greater than, equal to, or less than the density of the solid phase? Explain your answer, using information given in the introduction to this question.
The liquid phase is less dense than the solid phase. Since the freezing point of argon is higher than the triple point temperature, the solid-liquid equilibrium line slopes to the right with increasing pressure. Thus, if a sample of liquid argon is compressed (pressure increased) at constant temperature, the liquid becomes a solid. Because increasing pressure favors the denser phase, solid argon must be the denser phase.
2.
The phase diagram for a pure substance is shown above. Use this diagram and your knowledge about changes of phase to answer the following questions.
(a)What does point V represent? What characteristics are specific to the system only at point V?.
Triple point. All three states of the substance coexist.
(b)What does each point on the curve between V and W represent?
Curve VW represents the equilibrium between the liquid and its vapor. Along this line the liquid will be boiling.
(c)Describe the changes that the system undergoes as the temperature slowly increases from X to Y to Z at 1.0 atmosphere.
At point X the substance is a solid, as its temperature increases (at constant pressure), at point Y the solid is in equilibrium with its vapor and will sublime (there will be no temperature changed at point Y. After point Y to Z it exist only as a vapor.
(d)In a solid-liquid mixture of this substance, will the solid float or sink? Explain.
Sink. A positive slope of the solid-liquid line indicates that the solid is denser than it’s liquid and, therefore, will sink (solids are more dense than liquids)
3.
(a)From the standpoint of the kinetic-molecular theory, discuss briefly the properties of gas molecules that cause deviations from ideal behavior.
Real molecules exhibit finite volumes, thus excluding some volume from compression. Real molecules exhibit attractive forces, thus leading to fewer collisions with the walls and a lower pressure. Real molecules do have some extent of attraction for each other.
(b)At 25C and 1 atmosphere pressure, which of the following gases shows the greatest deviation from ideal behavior? Give two reasons for your choice.
CH4SO2O2H2
SO2 is the least ideal gas. By drawing out the molecules, you can see that it has the strongest IMF since it is polar and has DDF (the others are nonpolar and only have LDFs). Also, its LDFs are the strongest as it has the largest size of the 4.
(c)Real gases approach ideality at low pressure, high temperature, or both. Explain these observations.
High temperature result in high kinetic energies. This energy overcomes the attractive forces. Low pressure increases the distance between molecules. (So molecules comprise a small part of the volume or attractive forces are small.)
4. 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.
Reducing the temperature of a gas reduces the average kinetic energy (or velocity) of the gas molecules. This would reduce the number (or frequency) of collisions of gas molecules with the surface of the balloon; [OR decrease the momentum change that occurs when the gas molecules strike the balloon surface]. In order to maintain a constant pressure vs the external pressure, the volume must decrease.
(b)When the balloon described in (a) is cooled further, the volume does not become zero; rather, the gas becomes a liquid or solid.
The molecules of the gas do have volume, when they are cooled sufficiently, the forces of attraction that exist between them cause them to liquefy or solidify.
(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.
The molecules of gas are in constant motion so the HCl and NH3 diffuse along the tube. Where they meet, NH4Cl(s) is formed. Since HCl has a higher molar mass, its velocity (average) is lower, therefore, it doesn’t diffuse as fast as the NH3.
5.
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.
CO2; since they all have the same value, they all contain the same number of particles, therefore, the heaviest molecule, CO2 (molar mass = 44), will have the greatest mass.
(b)Compare the average kinetic energies of the gas molecules in the balloons. Explain.
All the same; at the same temperature all gases have the same kinetic energy.
(c)Which balloon contains the gas that would be expected to deviate most from the behavior of an ideal gas? Explain.
CO2; since they are all essentially non-polar, the largest intermolecular (London) force would be greatest in the molecule/atom with the largest number of electrons.
(d)Twelve hours after being filled, all the balloons have decreased in size. Predict which balloon will be the smallest. Explain your reasoning.
He; it has the smallest size and has the greatest particulate speed and, therefore, it’s the easiest to penetrate the wall and effuse.