Rucete ✏ AP Chemistry In a Nutshell
6. Gases — Practice Questions
This chapter introduces the physical properties and behaviors of gases, including ideal gas laws, kinetic molecular theory, and gas-related calculations.
(Multiple Choice — Click to Reveal Answer)
1. Which gas law describes the inverse relationship between pressure and volume at constant temperature?
(A) Charles’s Law
(B) Boyle’s Law
(C) Avogadro’s Law
(D) Gay-Lussac’s Law
Answer
(B) — Boyle’s Law states that pressure and volume are inversely related when temperature and moles of gas are constant.
2. According to the kinetic molecular theory, what happens when the temperature of a gas increases?
(A) The volume of the gas decreases
(B) The average kinetic energy increases
(C) The pressure decreases
(D) The molar mass changes
Answer
(B) — Temperature is directly proportional to average kinetic energy.
3. Which unit is not commonly used to express pressure?
(A) atm
(B) mm Hg
(C) Kelvin
(D) torr
Answer
(C) — Kelvin is a unit for temperature, not pressure.
4. What is the value of the universal gas constant R when using units of atm, L, mol, and K?
(A) 0.0821
(B) 8.31
(C) 1.00
(D) 22.4
Answer
(A) — R = 0.0821 L·atm/mol·K when using atm, L, and K.
5. What property must remain constant for Gay-Lussac’s Law to apply?
(A) Temperature
(B) Volume
(C) Pressure
(D) Number of moles
Answer
(B) — Gay-Lussac’s Law describes the pressure-temperature relationship at constant volume.
6. Which gas would effuse the fastest through a small pinhole?
(A) Cl2
(B) O2
(C) He
(D) CO2
Answer
(C) — According to Graham’s Law, lighter gases effuse faster; helium is the lightest here.
7. What is the molar volume of an ideal gas at STP?
(A) 22.4 L
(B) 273 L
(C) 1.00 atm
(D) 0.0821 L
Answer
(A) — At STP, one mole of an ideal gas occupies 22.4 L.
8. What causes gas pressure in a container?
(A) Magnetic interactions
(B) Gravity
(C) Collisions of gas molecules with the container walls
(D) Rotation of the molecules
Answer
(C) — Pressure results from molecular collisions with container walls.
9. What is assumed to be negligible in an ideal gas?
(A) Mass
(B) Volume of the gas molecules
(C) Speed of molecules
(D) Temperature
Answer
(B) — Ideal gases are assumed to have no volume or intermolecular forces.
10. What is the definition of STP?
(A) 273 K and 1.00 atm
(B) 25°C and 1 atm
(C) 0°C and 760 mm Hg
(D) Both A and C
Answer
(D) — Both A and C describe STP: 273 K (0°C) and 1 atm = 760 mm Hg.
11. Which variable must be in Kelvin when using the ideal gas law?
(A) Volume
(B) Temperature
(C) Pressure
(D) Moles
Answer
(B) — Temperature must always be expressed in Kelvin in the ideal gas law.
12. What does Avogadro’s law state?
(A) Equal volumes of gases at the same temperature and pressure contain equal numbers of particles
(B) Gas pressure is inversely related to temperature
(C) Gases do not occupy volume
(D) The rate of effusion is inversely proportional to pressure
Answer
(A) — Avogadro’s law relates volume and number of moles directly.
13. Which law explains the increase in pressure when temperature is increased at constant volume?
(A) Boyle’s Law
(B) Charles’s Law
(C) Gay-Lussac’s Law
(D) Dalton’s Law
Answer
(C) — Gay-Lussac’s Law shows direct relationship between pressure and temperature.
14. If pressure is constant and temperature increases, what will happen to volume?
(A) Decrease
(B) Increase
(C) Stay the same
(D) Drop to zero
Answer
(B) — According to Charles’s Law, volume increases with temperature.
15. Which of the following would behave most like an ideal gas?
(A) CO2 at high pressure and low temperature
(B) NH3 at STP
(C) He at low pressure and high temperature
(D) H2O vapor near its boiling point
Answer
(C) — Gases behave ideally at high T and low P; helium is nonpolar and small.
16. What is the correct unit for the gas constant R if pressure is in atm and volume in liters?
(A) J/mol·K
(B) L·atm/mol·K
(C) kPa·L/mol
(D) mm Hg·L/mol·K
Answer
(B) — The correct unit is L·atm/mol·K when pressure is in atm and volume in L.
17. What is Dalton’s Law used to calculate?
(A) Molar mass of gas
(B) Density of gas
(C) Partial pressures in a gas mixture
(D) Rate of diffusion
Answer
(C) — Dalton’s Law determines partial and total pressures in gas mixtures.
18. What does Graham’s law compare?
(A) Pressure vs. volume
(B) Effusion rates of gases
(C) Molecular mass and volume
(D) Pressure and temperature
Answer
(B) — Graham’s law compares effusion or diffusion rates based on molar mass.
19. Which of the following causes real gases to deviate from ideal behavior?
(A) High temperature and low pressure
(B) Small molar mass
(C) Strong intermolecular forces
(D) Low density
Answer
(C) — Strong attractions or high molecular volume cause deviations from ideal gas law.
20. Which of the following statements is FALSE about kinetic molecular theory?
(A) Gas particles move in random, straight-line motion
(B) Collisions between gas molecules are inelastic
(C) Volume of gas particles is negligible
(D) Gas pressure is caused by collisions with container walls
Answer
(B) — Kinetic molecular theory assumes **elastic** collisions, not inelastic.
21. Which of the following variables is held constant in Boyle’s Law?
(A) Pressure
(B) Volume
(C) Temperature
(D) Moles and temperature
Answer
(D) — Boyle’s Law examines pressure-volume relationship at constant temperature and moles.
22. What is the shape of the curve for a P vs. V graph at constant T?
(A) Linear
(B) Exponential
(C) Inverse hyperbola
(D) Horizontal line
Answer
(C) — Boyle’s Law produces a hyperbolic curve when pressure and volume are graphed.
23. Which gas would have the highest density at STP?
(A) He
(B) N2
(C) Ar
(D) Xe
Answer
(D) — At equal conditions, the gas with greatest molar mass has highest density.
24. Which postulate of the kinetic molecular theory explains the relationship between temperature and velocity?
(A) Gas molecules occupy volume
(B) Collisions are elastic
(C) Average kinetic energy is proportional to temperature
(D) Gas molecules move slowly
Answer
(C) — Postulate 5: KE ∝ T (in Kelvin).
25. What is the density (in g/L) of a gas with a molar mass of 44.0 g/mol at STP?
(A) 1.96
(B) 0.44
(C) 44.0
(D) 22.4
Answer
(A) — Density = molar mass / molar volume = 44.0 g/mol ÷ 22.4 L = 1.96 g/L.
26. A 1.00 L gas sample at 300 K and 1.00 atm contains 0.040 mol. What is the molar mass if the sample has a mass of 1.60 g?
(A) 16.0 g/mol
(B) 20.0 g/mol
(C) 32.0 g/mol
(D) 40.0 g/mol
Answer
(D) — Molar mass = 1.60 g / 0.040 mol = 40.0 g/mol.
27. A 0.50 L container holds 0.25 mol of gas at 298 K. What is the pressure in atm?
(A) 6.1 atm
(B) 12.2 atm
(C) 1.2 atm
(D) 2.5 atm
Answer
(A) — Use PV = nRT ⇒ P = (0.25 mol)(0.0821)(298 K)/0.50 L ≈ 6.1 atm.
28. A sample of gas is transferred from a 5.0 L container to a 2.0 L container. If the initial pressure was 1.2 atm, what is the final pressure (T constant)?
(A) 0.48 atm
(B) 1.2 atm
(C) 3.0 atm
(D) 6.0 atm
Answer
(C) — P₁V₁ = P₂V₂ ⇒ (1.2)(5.0) = P₂(2.0) ⇒ P₂ = 3.0 atm.
29. What is the total pressure in a 2.0 L container that holds 0.20 mol of He and 0.30 mol of Ne at 300 K?
(A) 6.15 atm
(B) 1.23 atm
(C) 0.615 atm
(D) 2.45 atm
Answer
(A) — Total n = 0.5 mol ⇒ P = nRT/V = (0.5)(0.0821)(300)/2.0 ≈ 6.15 atm.
30. A gas has a density of 1.25 g/L at STP. What is its molar mass?
(A) 22.4 g/mol
(B) 28.0 g/mol
(C) 32.0 g/mol
(D) 44.8 g/mol
Answer
(B) — M = D × 22.4 = 1.25 × 22.4 = 28.0 g/mol.
31. Two gases, A and B, are in a 3:1 molar ratio. If total pressure is 800 torr, what is the partial pressure of gas A?
(A) 200 torr
(B) 400 torr
(C) 600 torr
(D) 700 torr
Answer
(C) — Gas A is ¾ of total moles, so ¾ × 800 = 600 torr.
32. What is the rate of effusion of hydrogen gas compared to oxygen gas?
(A) 4:1
(B) 2:1
(C) 1:1
(D) 1:4
Answer
(A) — √(M₂/M₁) = √(32/2) = 4 ⇒ H₂ effuses 4× faster than O₂.
33. A 1.0 g sample of a gas occupies 0.500 L at 1.0 atm and 273 K. What is the molar mass?
(A) 22 g/mol
(B) 44 g/mol
(C) 11 g/mol
(D) 88 g/mol
Answer
(B) — Use PV = nRT ⇒ n = 0.022 mol ⇒ M = 1.0 g / 0.022 ≈ 44 g/mol.
34. A gas has a molar mass of 16.0 g/mol. What is the density at 2.00 atm and 273 K?
(A) 1.31 g/L
(B) 2.62 g/L
(C) 3.22 g/L
(D) 0.524 g/L
Answer
(B) — Use D = PM/RT = (2.00 × 16)/(0.0821 × 273) ≈ 2.62 g/L.
35. At constant temperature, which gas will have the highest velocity?
(A) Cl₂
(B) CH₄
(C) O₂
(D) CO₂
Answer
(B) — Lighter gases move faster; CH₄ has lowest molar mass.
(Short Answer — Click to Reveal Answer)
36. State the formula of the ideal gas law.
Answer
PV = nRT — Pressure × Volume = Moles × Gas constant × Temperature.
37. What are standard temperature and pressure (STP) in chemistry?
Answer
0°C and 1 atm — Also expressed as 273 K and 760 mm Hg.
38. What is the volume of 2.00 mol of an ideal gas at STP?
Answer
44.8 L — One mol = 22.4 L, so 2.00 mol = 44.8 L.
39. What law relates the pressure and temperature of a gas?
Answer
Gay-Lussac’s Law — Pressure is directly proportional to temperature.
40. What variable must be converted to Kelvin in gas calculations?
Answer
Temperature — All gas laws require temperature in Kelvin.
41. Write the expression for Graham’s law.
Answer
Rate₁ / Rate₂ = √(M₂ / M₁) — Rate of effusion is inversely proportional to square root of molar mass.
42. If a gas occupies 500 mL at 1.0 atm, what volume will it occupy at 2.0 atm (T constant)?
Answer
250 mL — Inverse relationship (Boyle’s Law): P₁V₁ = P₂V₂.
43. Why do real gases deviate from ideal behavior?
Answer
They have intermolecular forces and volume — Especially under high pressure or low temperature.
44. What is the partial pressure of a gas that makes up 30% of a 1.0 atm mixture?
Answer
0.30 atm — Partial pressure = mole fraction × total pressure.
45. Which law explains that equal volumes of gases contain equal numbers of moles?
Answer
Avogadro’s Law — V ∝ n at constant T and P.
46. What is the pressure in atm of 2.0 mol of gas in a 10.0 L container at 300 K?
Answer
4.92 atm — Use PV = nRT ⇒ P = nRT/V = (2)(0.0821)(300)/10 = 4.92 atm.
47. A gas sample exerts 800 mm Hg. What is this in atm?
Answer
1.05 atm — 800 mm Hg ÷ 760 = 1.05 atm.
48. How do you calculate density of a gas?
Answer
D = PM/RT — Pressure × Molar Mass ÷ (R × Temperature).
49. What is the molar mass of a gas if 1.00 g occupies 0.50 L at STP?
Answer
44.8 g/mol — 0.50 L = 0.0223 mol ⇒ M = 1.00 / 0.0223 ≈ 44.8 g/mol.
50. Which factor has the greatest effect on rate of diffusion?
Answer
Molar mass — Lighter gases diffuse more quickly.