Rucete ✏ AP Chemistry In a Nutshell
6. Gases — Practice Questions 2
This chapter introduces the laws and concepts governing the behavior of gases, including the ideal gas law, kinetic molecular theory, and properties of real gases.
(Multiple Choice — Click to Reveal Answer)
1. What happens to the pressure of a gas if the temperature is increased while volume and moles are constant?
(A) It decreases
(B) It remains unchanged
(C) It increases
(D) It becomes zero
Answer
(C) — Pressure increases because the kinetic energy and frequency of collisions increase with temperature.
2. Which of the following gas laws best explains why a balloon expands when heated?
(A) Boyle’s Law
(B) Gay-Lussac’s Law
(C) Avogadro’s Law
(D) Charles’s Law
Answer
(D) — Charles’s Law explains the direct relationship between volume and temperature.
3. Which of the following is the correct value of R when using mm Hg as the pressure unit?
(A) 8.31
(B) 0.0821
(C) 62.4
(D) 22.4
Answer
(C) — R = 62.4 L·mm Hg/mol·K when using mm Hg and liters.
4. The units of the gas constant R must cancel with all EXCEPT:
(A) L
(B) atm
(C) K
(D) mL
Answer
(D) — mL is not part of R's standard units; must be converted to liters.
5. The ideal gas law fails under conditions of:
(A) High temperature and low pressure
(B) Low temperature and high pressure
(C) STP
(D) Normal atmospheric conditions
Answer
(B) — Real gases deviate most from ideal behavior at low T and high P.
6. Which graph would best show Boyle’s Law?
(A) Linear increase of P vs. T
(B) Inverse curve of P vs. V
(C) Flat line of V vs. n
(D) Exponential increase of V vs. T
Answer
(B) — Boyle’s Law: pressure and volume are inversely related.
7. Graham’s Law shows that gas diffusion rate is:
(A) Directly proportional to molar mass
(B) Inversely proportional to molar mass
(C) Inversely proportional to the square root of molar mass
(D) Equal for all gases
Answer
(C) — Graham’s Law: rate ∝ 1/√(molar mass)
8. What must be true to use PV = nRT?
(A) The gas must be real
(B) The gas must be at STP
(C) The gas must behave ideally
(D) The gas must be diatomic
Answer
(C) — The equation is valid for ideal gases or real gases under ideal conditions.
9. Which quantity is NOT directly used in the ideal gas law?
(A) Temperature
(B) Volume
(C) Mass
(D) Pressure
Answer
(C) — Mass must first be converted to moles to use PV = nRT.
10. What is the main reason helium balloons rise in air?
(A) Helium is heavier than air
(B) Helium is more colorful
(C) Helium has lower density
(D) Helium reacts with oxygen
Answer
(C) — Helium’s low density causes it to rise in denser air.
11. What property of gas particles does temperature directly affect?
(A) Color
(B) Volume
(C) Average kinetic energy
(D) Surface area
Answer
(C) — Temperature increases the average kinetic energy of particles.
12. When the number of moles increases at constant pressure and temperature, what happens to volume?
(A) It decreases
(B) It stays the same
(C) It increases
(D) It becomes zero
Answer
(C) — From Avogadro’s Law: V ∝ n.
13. In which situation would a gas deviate most from ideal behavior?
(A) Helium at 500 K and 1 atm
(B) CO₂ at 273 K and 10 atm
(C) Neon at 298 K and 0.5 atm
(D) O₂ at 300 K and 1 atm
Answer
(B) — CO₂ at low T and high P deviates from ideal behavior.
14. Which factor most affects the average speed of gas molecules?
(A) Container shape
(B) Molecular weight
(C) Pressure
(D) Molar volume
Answer
(B) — Lighter molecules move faster, on average.
15. What is the main assumption of kinetic molecular theory?
(A) Gases have strong intermolecular forces
(B) Gases are made of slow, sticky molecules
(C) Gases consist of small particles in constant random motion
(D) Gases react with container walls
Answer
(C) — KMT assumes gas particles move randomly and collide elastically.
16. A gas exerts pressure because:
(A) The gas particles are heavy
(B) The gas particles rotate
(C) The gas particles collide with the container walls
(D) The gas particles emit radiation
Answer
(C) — Gas pressure results from collisions with container walls.
17. Which of the following is NOT part of the kinetic molecular theory?
(A) Collisions are elastic
(B) Gas particles occupy volume
(C) Gas motion is random
(D) No intermolecular forces exist
Answer
(B) — In KMT, gas particles are assumed to have negligible volume.
18. Which gas law explains why a bicycle tire bursts on a hot day?
(A) Boyle’s Law
(B) Dalton’s Law
(C) Gay-Lussac’s Law
(D) Avogadro’s Law
Answer
(C) — Pressure increases with temperature at constant volume.
19. Which would most increase the pressure in a sealed container of gas?
(A) Lowering the temperature
(B) Increasing volume
(C) Adding more gas
(D) Making the container larger
Answer
(C) — Adding moles increases pressure if volume and temperature are constant.
20. Why is Kelvin used in gas law calculations?
(A) It is a larger scale
(B) Kelvin starts at absolute zero
(C) Kelvin matches pressure
(D) It's easier to memorize
Answer
(B) — Kelvin is based on absolute zero, where molecular motion stops.
21. A gas has a volume of 2.0 L at 3.0 atm. What is its volume at 1.0 atm, assuming temperature is constant?
(A) 6.0 L
(B) 1.0 L
(C) 3.0 L
(D) 2.0 L
Answer
(A) — P₁V₁ = P₂V₂ → V₂ = 6.0 L.
22. What is the total pressure if a gas mixture contains 2.0 atm O₂ and 1.0 atm N₂?
(A) 3.0 atm
(B) 2.0 atm
(C) 1.5 atm
(D) 1.0 atm
Answer
(A) — Dalton’s Law: Total pressure = sum of partial pressures.
23. Which describes a real gas better than an ideal gas?
(A) No forces and no volume
(B) Strong forces and large volume
(C) Some attraction and finite volume
(D) Constant speed and high density
Answer
(C) — Real gases have some intermolecular forces and particle volume.
24. Which of the following is a pressure unit?
(A) L
(B) atm
(C) K
(D) mol
Answer
(B) — atm (atmospheres) is a unit of pressure.
25. Which variable is directly proportional to pressure in Gay-Lussac’s law?
(A) Volume
(B) Moles
(C) Temperature
(D) Density
Answer
(C) — P ∝ T when volume is constant.
26. What happens to real gases at very low temperatures?
(A) They expand
(B) They contract
(C) They behave ideally
(D) They liquefy or condense
Answer
(D) — At low temperatures, real gases deviate from ideal behavior and can condense into liquids.
27. If the temperature of a gas doubles (in K), what happens to the average kinetic energy?
(A) It is halved
(B) It remains the same
(C) It doubles
(D) It quadruples
Answer
(C) — Kinetic energy is directly proportional to absolute temperature.
28. Which of the following gases would diffuse slowest under the same conditions?
(A) He
(B) O₂
(C) N₂
(D) CH₄
Answer
(B) — Heavier gases like O₂ diffuse more slowly than lighter ones.
29. A gas at 2.00 atm in a 4.00 L container is transferred to an 8.00 L container. What is the new pressure (T constant)?
(A) 1.00 atm
(B) 4.00 atm
(C) 0.50 atm
(D) 2.00 atm
Answer
(A) — Boyle’s Law: doubling the volume halves the pressure.
30. Which law can be derived from combining Boyle’s, Charles’s, and Gay-Lussac’s Laws?
(A) Ideal Gas Law
(B) Avogadro’s Law
(C) Dalton’s Law
(D) Combined Gas Law
Answer
(D) — The Combined Gas Law incorporates P, V, and T: (P₁V₁)/T₁ = (P₂V₂)/T₂.
31. In which condition is gas behavior LEAST ideal?
(A) High temperature, low pressure
(B) Low temperature, high pressure
(C) Room temperature and 1 atm
(D) 0°C and 1 atm
Answer
(B) — Real gases deviate most under low T and high P.
32. A 10.0 L gas sample is cooled from 400 K to 200 K. What is the new volume (P constant)?
(A) 20.0 L
(B) 5.0 L
(C) 10.0 L
(D) 2.5 L
Answer
(B) — Charles’s Law: V ∝ T. Halving T halves V.
33. Which gas would exert the greatest pressure in a fixed volume at 300 K?
(A) 2.0 mol of He
(B) 2.0 mol of O₂
(C) 1.0 mol of CO₂
(D) 1.0 mol of CH₄
Answer
(A) or (B) — Since moles and T are the same, both He and O₂ would exert the same pressure assuming ideal behavior.
34. If 3 gases are mixed in a container, each with 2 atm pressure, what is the total pressure?
(A) 2 atm
(B) 4 atm
(C) 6 atm
(D) 1 atm
Answer
(C) — Dalton’s Law: Pₜₒₜ = 2 + 2 + 2 = 6 atm.
35. What is the main reason gases expand when heated?
(A) Increased collisions with each other
(B) Loss of intermolecular forces
(C) Increased kinetic energy and movement
(D) Decrease in density
Answer
(C) — Temperature increases kinetic energy, causing expansion.
36. What does "elastic collision" mean in kinetic molecular theory?
Answer
No net loss of kinetic energy — Energy is conserved during collisions.
37. How does increasing molar mass affect gas velocity?
Answer
It decreases velocity — Heavier gases move slower on average.
38. Why are gases compressible?
Answer
They have lots of empty space between particles.
39. What happens to pressure if volume is doubled at constant temperature?
Answer
It is halved — Inverse relationship per Boyle’s Law.
40. Convert 27°C to Kelvin.
Answer
300 K — Add 273 to Celsius.
41. How is pressure related to collision frequency?
Answer
Directly — More collisions = more pressure.
42. What is molar volume at STP?
Answer
22.4 L — 1 mol of ideal gas at STP.
43. Write the combined gas law formula.
Answer
(P₁V₁)/T₁ = (P₂V₂)/T₂ — Combines Boyle’s, Charles’s, and Gay-Lussac’s Laws.
44. Why does gas pressure increase in a shrinking container?
Answer
Particles collide more often with walls.
45. Which is denser at STP: O₂ or H₂?
Answer
O₂ — It has greater molar mass.
46. State one assumption of ideal gases that real gases violate.
Answer
Real gases have intermolecular forces.
47. A gas at 300 K and 1.0 atm occupies 2.0 L. Find the number of moles.
Answer
0.082 mol — Use PV = nRT.
48. How does temperature affect diffusion rate?
Answer
Higher temperature increases diffusion.
49. What is the partial pressure of CO₂ if it’s 25% of a 4.0 atm gas mixture?
Answer
1.0 atm — 25% of 4.0 atm.
50. What gas would you expect to deviate most from ideal behavior?
Answer
NH₃ — Strong hydrogen bonding causes deviation.