FSc Part 1 Chemistry chapter 3: Gases MCQs with answers

By Team Megalecture · Updated 4 Nov 2026

30 exam-style questions on 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 · 3 topics · 10 per test

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What chapter 3 covers

Ten Gases questions with answers

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

1. 500 cm3 of a gas is at 1 atm and 27 °C. What is its volume at 2 atm and 327 °C?

  1. A
    250 cm3
  2. B
    500 cm3
  3. C
    1000 cm3
  4. D
    3028 cm3
Show answer

Answer: B. V2 = V1 × (P1/P2) × (T2/T1) = 500 × (1/2) × (600/300) = 500 cm3. Doubling pressure halves V, doubling kelvin temperature doubles it.

2. Deep-sea divers breathe a mixture of oxygen and helium rather than compressed air mainly because

  1. A
    helium increases the partial pressure of oxygen
  2. B
    helium supplies extra energy to the body
  3. C
    helium is much less soluble in blood than nitrogen at high pressure
  4. D
    nitrogen is poisonous at every pressure
Show answer

Answer: C. At high pressure nitrogen dissolves in blood and forms bubbles when the diver rises. Helium is far less soluble, so it is used to dilute the oxygen.

3. Samples of hydrogen and oxygen are at the same temperature. Which statement is correct?

  1. A
    they have the same average kinetic energy, but the hydrogen molecules move faster on average
  2. B
    the molecules of both gases have the same average speed
  3. C
    oxygen molecules have the greater average kinetic energy because they are heavier
  4. D
    hydrogen molecules have the greater average kinetic energy because they move faster
Show answer

Answer: A. Average KE depends only on temperature. Since KE = ½mv2, the lighter H2 molecules must move faster to have the same KE.

4. For a fixed mass of gas at constant temperature, what is the shape of a graph of pressure P against 1/V?

  1. A
    a hyperbola
  2. B
    a straight line parallel to the 1/V axis
  3. C
    a curve that falls to zero
  4. D
    a straight line passing through the origin
Show answer

Answer: D. Boyle's law gives P = k × (1/V), so P against 1/V is a straight line through the origin. P against V is the hyperbola.

5. Which of these gases is the easiest to liquefy?

  1. A
    H2
  2. B
    NH3
  3. C
    He
  4. D
    N2
Show answer

Answer: B. NH3 has hydrogen bonding, the strongest intermolecular forces here, and so the highest critical temperature (about 132 °C). He and H2 have very weak forces.

6. Absolute zero is the temperature at which

  1. A
    all gases become liquids
  2. B
    water freezes
  3. C
    the volume of an ideal gas would theoretically become zero
  4. D
    the pressure of any gas equals 1 atm
Show answer

Answer: C. Extrapolating Charles's law graphs gives zero volume at about −273 °C (0 K). Real gases liquefy before reaching it.

7. According to Charles's law, for each 1 °C rise in temperature at constant pressure, a gas expands by what fraction of its volume at 0 °C?

  1. A
    1/100
  2. B
    1/273
  3. C
    1/373
  4. D
    1/546
Show answer

Answer: B. The volume increases by 1/273 of its 0 °C volume per degree, which is why it would become zero at −273 °C.

8. If the absolute temperature of a gas is increased four times, its root mean square speed becomes

  1. A
    four times as great
  2. B
    twice as great
  3. C
    sixteen times as great
  4. D
    half as great
Show answer

Answer: B. crms = √(3RT/M), so crms ∝ √T; √4 = 2.

9. For an ideal gas, the compressibility factor Z = PV/nRT is

  1. A
    0
  2. B
    less than 1 at all pressures
  3. C
    greater than 1 at all pressures
  4. D
    equal to 1 at all pressures
Show answer

Answer: D. An ideal gas obeys PV = nRT exactly, so Z = 1. Real gases show Z below 1 (attractions dominate) or above 1 (molecular volume dominates).

10. In Linde's method for liquefying air, the compressed gas is cooled by

  1. A
    compressing it further at constant temperature
  2. B
    passing it over a catalyst
  3. C
    mixing it with an inert gas
  4. D
    allowing it to expand suddenly through a narrow jet into a region of low pressure
Show answer

Answer: D. This is the Joule-Thomson effect: on sudden expansion the molecules do work against their attractions and cool. Repeated cycles cool the gas until it liquefies.

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Ways to practise this chapter

Gases: questions students ask

How many Gases MCQs are there?

30, each with the answer and a short explanation, across 3 topics: Gas laws and the ideal gas equation, Dalton's law, Graham's law and kinetic molecular theory, Non-ideal behaviour, liquefaction and plasma. Every chapter test draws ten of them, so you can take it many times.

Is the FSc Part 1 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.

Can I read the explanations in Urdu?

Yes. In practice mode and on your results you can switch the explanations to Urdu.

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