MDCAT Chemistry chapter 3: Chemistry · Gases MCQs with answers

By Team Megalecture · Updated 4 Nov 2026

30 exam-style questions on Chemistry · Gases, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.

30 questions · 2 topics · 10 per test

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What chapter 3 covers

Ten Chemistry · Gases questions with answers

Try each one, then open the answer. These ten stay the same; the test draws new ones every time.

1. The average kinetic energy of the molecules of a gas is E at −73 °C. What is it at 327 °C?

  1. A
    3E
  2. B
    4.5E
  3. C
    √3 E
  4. D
    9E
Show answer

Answer: A. Average kinetic energy is proportional to the absolute temperature: −73 °C = 200 K and 327 °C = 600 K, so it becomes 3E. Dividing the Celsius values (327 ÷ 73) gives about 4.5E, and √3 is the factor for the root mean square speed, not the energy.

2. A sample of nitrogen is studied under four sets of conditions. Under which conditions is its behaviour furthest from that of an ideal gas?

  1. A
    1 atm and 600 K
  2. B
    100 atm and 600 K
  3. C
    100 atm and 150 K
  4. D
    1 atm and 150 K
Show answer

Answer: C. Real gases deviate most at high pressure, where the volume of the molecules is no longer negligible, and at low temperature, where slow molecules feel each other's attractions. Nitrogen at 1 atm behaves almost ideally at either temperature.

3. The gas constant R is the energy per mole per kelvin. Which of these gives its value in SI units?

  1. A
    8.314 J K−1 mol−1
  2. B
    0.0821 dm3 atm K−1 mol−1
  3. C
    1.987 cal K−1 mol−1
  4. D
    62.4 dm3 torr K−1 mol−1
Show answer

Answer: A. All four are correct values of R, but only the one in joules uses SI units: R = 8.314 J K−1 mol−1, also written N m K−1 mol−1 because pressure × volume is energy. Atmospheres, torr and calories are not SI units.

4. At very high pressures the value of PV/nRT for every real gas becomes greater than 1. The main reason is that

  1. A
    the attractive forces between the molecules increase
  2. B
    the volume of the molecules is no longer negligible
  3. C
    the molecules move faster at high pressure
  4. D
    collisions between the molecules become inelastic
Show answer

Answer: B. When the gas is squeezed hard, the molecules themselves fill a significant part of the container, so the free space is less than V and the gas is harder to compress than an ideal gas (PV/nRT > 1). Stronger attractions would make PV/nRT smaller than 1.

5. Ammonia gas and hydrogen chloride gas are released at the same moment from opposite ends of a long glass tube. Where does the white ring of ammonium chloride first form?

  1. A
    nearer the NH3 end, because HCl diffuses faster
  2. B
    exactly in the middle of the tube
  3. C
    nearer the HCl end, because NH3 diffuses faster
  4. D
    nearer the NH3 end, because NH3 is lighter
Show answer

Answer: C. By Graham's law, rate ∝ 1/√M. NH3 (M = 17) diffuses faster than HCl (M = 36.5), so it travels further before the gases meet, and the ring forms nearer the HCl end.

6. Which property distinguishes a plasma from an ordinary gas?

  1. A
    it has a fixed volume but no fixed shape
  2. B
    it conducts electricity, having free electrons and ions
  3. C
    its particles are packed even more closely than in a liquid
  4. D
    it contains only neutral atoms moving at high speed
Show answer

Answer: B. A plasma is a gas so strongly heated or energised that many of its atoms are ionised; its free electrons and positive ions let it conduct electricity, although it is neutral overall. Like a gas, it has no fixed volume or shape.

7. A gas at 27 °C is heated at constant pressure until its volume has doubled. What is its final temperature?

  1. A
    54 °C
  2. B
    300 °C
  3. C
    600 °C
  4. D
    327 °C
Show answer

Answer: D. Charles's law uses absolute temperature: 27 °C = 300 K, doubling V needs 600 K = 327 °C. Doubling the Celsius value gives 54 °C, and 600 °C forgets to convert 600 K back.

8. A gas has a density of 1.25 g dm−3 at STP. Which gas could it be? (H = 1, C = 12, N = 14, O = 16)

  1. A
    NH3
  2. B
    O2
  3. C
    N2
  4. D
    CO2
Show answer

Answer: C. Molar mass = density × molar volume = 1.25 × 22.4 = 28 g mol−1, which fits N2. NH3 (17), O2 (32) and CO2 (44) would have densities of 0.76, 1.43 and 1.96 g dm−3.

9. Which pressure is equal to 2.0 atm? (1 atm = 760 torr = 101 325 Pa)

  1. A
    380 torr
  2. B
    1520 Pa
  3. C
    1520 torr
  4. D
    202.65 Pa
Show answer

Answer: C. 2.0 × 760 = 1520 torr, or 2.0 × 101 325 = 202 650 Pa (202.65 kPa). 380 torr divides by 2 instead of multiplying, 1520 Pa attaches the torr value to the wrong unit, and 202.65 Pa drops the kilo.

10. Boyle's law curves (P plotted against V) are drawn for the same sample of gas at 200 K and at 400 K. Compared with the 200 K curve, the 400 K curve

  1. A
    lies closer to both axes, because PV is halved
  2. B
    becomes a straight line through the origin
  3. C
    lies further from both axes, because PV is doubled
  4. D
    is exactly the same curve, because Boyle's law ignores temperature
Show answer

Answer: C. Each curve is a hyperbola with PV = nRT, so doubling the absolute temperature doubles the constant PV and the 400 K isotherm lies further from the axes. A straight line through the origin is obtained only when P is plotted against 1/V.

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Chemistry · Gases: questions students ask

How many Chemistry · Gases MCQs are there?

30, each with the answer and a short explanation, across 2 topics: Gas laws, Kinetic theory and real gases. Every chapter test draws ten of them, so you can take it many times.

Is the MDCAT Chemistry chapter 3 test free?

Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.

How is the chapter test marked?

The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.

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