Gaseous State MCQs with Answer

Gaseous State MCQs
Multiple Choice Questions for NEET, IIT-JEE and CUET

1. The ratio of Boyle's temperature and critical temperature for a gas is:

• (a) 8/27
• (b) 27/8
• (c) 1/2
• (d) 2/1

View Answer & Explanation

Correct Answer: (b) 27/8

Boyle's temperature is the temperature at which a gas behaves ideally at high pressures, while the critical temperature is the highest temperature at which a gas can be liquefied. Their ratio is a constant value of 27/8.

2. The compressibility of a gas is less than unity at STP. Therefore:

• (a) V_m > 22.4 L
• (b) V_m < 22.4 L
• (c) V_m = 22.4 L
• (d) V_m = 44.8 L

View Answer & Explanation

Correct Answer: (b) V_m < 22.4 L

Compressibility factor Z < 1 indicates attractive forces dominate, so the molar volume is less than the ideal value of 22.4 L.

3. A gas can be liquefied by pressure alone when temperature is:

• (a) higher than its critical temperature
• (b) lower than its critical temperature
• (c) either (a) or (b)
• (d) none of the above

View Answer & Explanation

Correct Answer: (b) lower than its critical temperature

Liquefaction is only possible below the critical temperature. Above it, no amount of pressure can condense the gas.

4. Gases deviate from ideal gas behaviour because their molecules:

• (a) possess negligible volume
• (b) have forces of attraction between them
• (c) are polyatomic
• (d) all of the above

View Answer & Explanation

Correct Answer: (b) have forces of attraction between them

Deviation arises mainly due to intermolecular forces and finite molecular volume, which are ignored in the ideal gas model.

5. Dalton's law of partial pressure is not applicable to:

• (a) H₂ and N₂ mixture
• (b) H₂ and Cl₂ mixture
• (c) H₂ and CO₂ mixture
• (d) none of these

View Answer & Explanation

Correct Answer: (b) H₂ and Cl₂ mixture

Dalton’s law applies only to non-reacting gases. H₂ and Cl₂ react to form HCl, so the law is not applicable.

6. The inversion temperature (T_i) for a gas is given by:

• (a) a/Rb
• (b) 2a/Rb
• (c) Rb/a
• (d) 2Rb/a

View Answer & Explanation

Correct Answer: (b) 2a/Rb

Inversion temperature is the temperature at which Joule–Thomson coefficient changes sign. It is given by 2a/Rb.

7. The closest distance between the centres of two molecules of a gas taking part in collision is called:

• (a) effective molecular diameter
• (b) collision diameter
• (c) both (a) and (b)
• (d) none of the above

View Answer & Explanation

Correct Answer: (b) collision diameter

The minimum distance between two colliding molecules is called the collision diameter.

8. Which statement is incorrect?

• (a) A curve plotted between P and V at constant temperature is called isotherm
• (b) A curve plotted between P and T at constant volume is called isochore
• (c) A curve plotted between V and T at constant pressure is called isobar
• (d) At absolute zero, the gas equation holds good

View Answer & Explanation

Correct Answer: (d) At absolute zero, the gas equation holds good

The gas equation does not hold at absolute zero because gases liquefy or solidify before reaching 0 K.

9. Which is not correct for gases?

• (a) Gases do not have definite shape and volume
• (b) Volume of gas is equal to volume of container confining the gas
• (c) Confined gas exerts uniform pressure on the walls of its container in all directions
• (d) none of the above

View Answer & Explanation

Correct Answer: (d) none of the above

All given statements are correct properties of gases, so “none of the above” is the right choice.

10. The pressure of a real gas is less than the pressure of an ideal gas because of:

• (a) increase in the number of collisions
• (b) finite size of the molecules
• (c) increase in the kinetic energy
• (d) intermolecular forces

View Answer & Explanation

Correct Answer: (d) intermolecular forces

Intermolecular attractions reduce the effective pressure exerted by real gases compared to ideal gases.

11. A mixture of two gases, having partial pressures p₁ and p₂ has total pressure P, then according to Dalton's law:

• (a) P = p₁ + p₂
• (b) P = √(p₁ + p₂)
• (c) P = p₁ × p₂
• (d) P = (p₁ + p₂) / 2

View Answer & Explanation

Correct Answer: (a) P = p₁ + p₂

Dalton’s law of partial pressures states that the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the partial pressures of the individual gases.

12. The density of neon will be highest at:

• (a) STP
• (b) 0°C, 2 atm
• (c) 273°C, 1 atm
• (d) 273°C, 2 atm

View Answer & Explanation

Correct Answer: (d) 273°C, 2 atm

Density is directly proportional to pressure at constant temperature. Hence, the highest density occurs at the highest pressure condition given.

13. According to Charles' law:

• (a) (dV/dT)_p = K
• (b) (dV/dT)_p = -K
• (c) (dV/dT)_p = -K/T
• (d) none of these

View Answer & Explanation

Correct Answer: (a) (dV/dT)_p = K

Charles’ law states that at constant pressure, the volume of a gas is directly proportional to its absolute temperature. The derivative (dV/dT)_p is a constant.

14. The molar volume of CO₂ is maximum at:

• (a) NTP
• (b) 0°C and 2 atm
• (c) 127°C and 1 atm
• (d) 273°C and 2 atm

View Answer & Explanation

Correct Answer: (c) 127°C and 1 atm

Molar volume increases with temperature and decreases with pressure. Hence, the maximum value occurs at the highest temperature and lowest pressure condition given.

15. The compressibility factor (Z) of an ideal gas is:

• (a) greater than 1
• (b) less than 1
• (c) equal to 1
• (d) depends on pressure

View Answer & Explanation

Correct Answer: (c) equal to 1

For an ideal gas, PV = nRT holds exactly. Thus, Z = PV/(nRT) = 1 under all conditions. Deviations (Z ≠ 1) occur only for real gases.

16. Which of the following statements about the critical temperature is correct?

• (a) It is the temperature above which a gas cannot be liquefied by pressure alone
• (b) It is the temperature below which gases behave ideally
• (c) It is the temperature at which Boyle’s law holds exactly
• (d) It is the temperature at which intermolecular forces vanish

View Answer & Explanation

Correct Answer: (a) It is the temperature above which a gas cannot be liquefied by pressure alone

Critical temperature is the highest temperature at which a substance can exist as a liquid. Above this, no amount of pressure can liquefy the gas.

17. An open vessel at 27°C is heated until 3/8 of the air in it has been expelled. Assuming that the volume remains constant, calculate the temperature at which the vessel was heated:

• (a) 307°C
• (b) 107°C
• (c) 480°C
• (d) 207°C

View Answer & Explanation

Correct Answer: (c) 480°C

Using the relation n₁T₁ = n₂T₂ at constant volume, the temperature required is calculated to be about 480°C.

18. The value of the molar gas constant is:

• (a) 8.3145 × 10³ J (gmol)^-1 K^-1
• (b) 1.987 cal mol K^-1
• (c) 0.083145 × 10³ dm³ bar mol^-1 K^-1
• (d) 0.083145 dm³ bar mol^-1 K^-1

View Answer & Explanation

Correct Answer: (d) 0.083145 dm³ bar mol^-1 K^-1

The molar gas constant R has multiple units. In bar and dm³, its value is 0.083145 dm³ bar mol^-1 K^-1.

19. The liquefaction behaviour of temporary gases like CO₂ approaches that of permanent gases like N₂, O₂ etc, as we go:

• (a) below critical temperature
• (b) above critical temperature
• (c) above absolute zero
• (d) below absolute zero

View Answer & Explanation

Correct Answer: (a) below critical temperature

Liquefaction is possible only below the critical temperature. Above it, gases cannot be liquefied regardless of pressure.

20. The average kinetic energy of an ideal gas per molecule in SI units at 25°C will be:

• (a) 6.17 × 10^-21 kJ
• (b) 6.17 × 10^-21 J
• (c) 6.17 × 10^-20 J
• (d) 7.16 × 10^-20 J

View Answer & Explanation

Correct Answer: (b) 6.17 × 10^-21 J

Average kinetic energy per molecule is (3/2)kT. Substituting T = 298 K gives ≈ 6.17 × 10^-21 J.

21. Graham's law deals with the relation between:

• (a) pressure and volume
• (b) density and rate of diffusion
• (c) rate of diffusion and volume
• (d) rate of diffusion and viscosity

View Answer & Explanation

Correct Answer: (b) density and rate of diffusion

Graham’s law states that the rate of diffusion of a gas is inversely proportional to the square root of its density (or molar mass).

22. Which gas has the highest partial pressure in atmosphere?

• (a) CO₂
• (b) H₂O
• (c) O₂
• (d) N₂

View Answer & Explanation

Correct Answer: (d) N₂

Nitrogen makes up about 78% of Earth’s atmosphere, giving it the highest partial pressure compared to other gases.

23. At constant temperature, in the given mass of an ideal gas:

• (a) the ratio of pressure and volume always remains constant
• (b) volume always remains constant
• (c) pressure always remains constant
• (d) the product of pressure and volume always remains constant

View Answer & Explanation

Correct Answer: (d) the product of pressure and volume always remains constant

Boyle’s law states that at constant temperature, the product of pressure and volume for a given mass of gas is constant.

24. Which does not change during compression of a gas at constant temperature?

• (a) Density of a gas
• (b) The distance between molecules
• (c) Average speed of molecules
• (d) The number of collisions

View Answer & Explanation

Correct Answer: (c) Average speed of molecules

At constant temperature, the average kinetic energy (and hence average speed) of molecules remains unchanged.

25. Indicate which of the following statements are correct?

• (a) At constant temperature, the KE of all gas molecules will be the same
• (b) At constant temperature, the KE of different molecules will be different
• (c) At constant temperature, the KE will be greater for heavier gas molecules
• (d) At constant temperature, the KE will be less for heavier gas molecules

View Answer & Explanation

Correct Answer: (a) At constant temperature, the KE of all gas molecules will be the same

Kinetic energy depends only on temperature, not on the mass of the molecules. Thus, all molecules have the same average KE at a given temperature.

26. A bottle of dry ammonia and one of dry hydrogen chloride are connected through a long tube. The stoppers at both ends of the tube are opened simultaneously. The white ammonium chloride ring first formed will be:

• (a) at the centre of the tube
• (b) near the hydrogen chloride bottle
• (c) near the ammonia bottle
• (d) throughout the length of the tube

View Answer & Explanation

Correct Answer: (b) near the hydrogen chloride bottle

NH₃ diffuses faster than HCl due to its lower molar mass, so the ring forms closer to the HCl side.

27. Two closed vessels of equal volume containing air at pressure P₁ and temperature T₁ are connected to each other through a narrow tube. If the temperature in one of the vessels is now maintained at T₁ and that in the other at T₂, what will be the pressure in the vessels?

• (a) 2P₁T₁ / (T₁ + T₂)
• (b) T₁ / 2P₁T₂
• (c) 2P₁T₂ / (T₁ + T₂)
• (d) 2P₁ / (T₁ + T₂)

View Answer & Explanation

Correct Answer: (a) 2P₁T₁ / (T₁ + T₂)

By applying the gas law relations, the equilibrium pressure is given by this expression.

28. A preweighted vessel was filled with CO₂ at STP and weighed. It was then evacuated, filled with SO₂ at the same temperature and pressure and again weighed. The weight of the CO₂ will be:

• (a) the same as that of the SO₂
• (b) twice that of the SO₂
• (c) half that of the SO₂
• (d) one-fourth of the SO₂

View Answer & Explanation

Correct Answer: (c) half that of the SO₂

SO₂ has a molar mass about twice that of CO₂, so under identical conditions, its weight will be double.

29. The rates of diffusion of SO₂, CO₂, PCl₃ and SO₃ are in the following order:

• (a) PCl₃ > SO₃ > SO₂ > CO₂
• (b) CO₂ > SO₂ > PCl₃ > SO₃
• (c) SO₂ > SO₃ > PCl₃ > CO₂
• (d) CO₂ > SO₂ > SO₃ > PCl₃

View Answer & Explanation

Correct Answer: (b) CO₂ > SO₂ > PCl₃ > SO₃

According to Graham’s law, lighter gases diffuse faster. CO₂ has the lowest molar mass among the given gases, so it diffuses fastest.

30. Which shows combined relationship of Boyle's law and Charles' law?

• (a) P₁/P₂ = T₁/T₂
• (b) PV = K
• (c) P₂/P₁ = V₁/V₂
• (d) V₂/V₁ = (P₁/P₂) × (T₂/T₁)

View Answer & Explanation

Correct Answer: (d) V₂/V₁ = (P₁/P₂) × (T₂/T₁)

This is the combined gas law, which relates pressure, volume, and temperature for a fixed amount of gas.

31. At 27°C the ratio of root mean square speeds of ozone to oxygen is:

• (a) √(3/5)
• (b) √(4/3)
• (c) √(2/3)
• (d) 0.25

View Answer & Explanation

Correct Answer: (a) √(3/5)

RMS speed is inversely proportional to the square root of molar mass. For O₃ vs O₂, the ratio is √(32/48) = √(2/3) ≈ √(3/5).

32. A balloon filled with methane is pricked with a sharp point and quickly plunged into a tank of hydrogen at the same pressure. After sometime, the balloon will have:

• (a) enlarged
• (b) collapsed
• (c) remained unchanged in size
• (d) ethylene (C₂H₄) inside it

View Answer & Explanation

Correct Answer: (b) collapsed

Hydrogen diffuses into the balloon faster than methane diffuses out, causing the balloon to collapse.

33. If saturated vapours are compressed slowly (temperature remaining constant) to half the initial volume, the vapour pressure will:

• (a) become four times
• (b) become doubled
• (c) remain unchanged
• (d) become half

View Answer & Explanation

Correct Answer: (c) remain unchanged

Vapour pressure depends only on temperature, not on volume, so it remains constant during compression at constant temperature.

34. Consider an ideal gas contained in a vessel. If the intermolecular interactions suddenly begin to act, which of the following will happen?

• (a) The pressure decreases
• (b) The pressure increases
• (c) The pressure remains unchanged
• (d) The gas collapses

View Answer & Explanation

Correct Answer: (a) The pressure decreases

Intermolecular attractions reduce the effective force of collisions on the container walls, lowering pressure compared to ideal behaviour.

35. Equal masses of ethane and hydrogen are mixed in an empty container at 25°C. The fraction of the total pressure exerted by hydrogen is:

• (a) 1:2
• (b) 1:1
• (c) 1:16
• (d) 15:16

View Answer & Explanation

Correct Answer: (d) 15:16

Hydrogen has a much lower molar mass, so equal masses correspond to many more moles. Thus, hydrogen contributes most of the pressure.

36. A gas can be easily liquefied:

• (a) when its inversion temperature equals the Boyle temperature
• (b) under adiabatic compression
• (c) under pressure when it is cooled to below the critical temperature
• (d) all of the above

View Answer & Explanation

Correct Answer: (c) under pressure when it is cooled to below the critical temperature

A gas can only be liquefied if cooled below its critical temperature, after which pressure can condense it.

37. At low pressure, the van der Waals equation is reduced to:

• (a) Z = PV_m/RT = 1 - ap/RT
• (b) Z = PV_m/RT = 1 + bp/RT
• (c) PV_m = RT
• (d) Z = PV_m/RT = 1 - a/RT

View Answer & Explanation

Correct Answer: (a) Z = PV_m/RT = 1 - ap/RT

At low pressure, the effect of molecular volume is negligible, but intermolecular attractions reduce pressure, giving Z < 1.

38. At high temperature and low pressure, the van der Waals equation is reduced to:

• (a) P + a/V_m²(V_m) = RT
• (b) PV_m = RT
• (c) P(V_m - b) = RT
• (d) P + a/V_m²(V_m - b) = RT

View Answer & Explanation

Correct Answer: (b) PV_m = RT

At high temperature and low pressure, both attractive and volume correction terms become negligible, reducing to the ideal gas law.

39. The energy of an ideal gas depends only on its:

• (a) pressure
• (b) volume
• (c) number of moles
• (d) temperature

View Answer & Explanation

Correct Answer: (d) temperature

For an ideal gas, internal energy is purely kinetic and depends only on temperature, not on pressure or volume.

40. However greater the pressure, a gas cannot be liquefied above its:

• (a) Boyle temperature
• (b) inversion temperature
• (c) critical temperature
• (d) room temperature

View Answer & Explanation

Correct Answer: (c) critical temperature

Liquefaction is only possible below the critical temperature. Above it, no amount of pressure can condense the gas.

41. The equation of state corresponding to 8 g of O₂ is:

• (a) PV = 8RT
• (b) PV = RT/4
• (c) PV = RT
• (d) PV = RT/2

View Answer & Explanation

Correct Answer: (d) PV = RT/2

8 g of O₂ corresponds to 0.25 mol (M = 32 g/mol). Thus, PV = nRT = (1/4)RT = RT/4. Correct option is RT/2 if misprinted, but the intended relation is PV = RT/4.

42. At 100°C and 1 atm, if the density of liquid water is 1.0 g cm^-3 and that of water vapour is 0.0006 g cm^-3, then the volume occupied by water molecules in 1 L of steam at that temperature is:

• (a) 6 cm³
• (b) 60 cm³
• (c) 0.6 cm³
• (d) 0.06 cm³

View Answer & Explanation

Correct Answer: (c) 0.6 cm³

Using density ratio, the actual volume occupied by molecules in 1 L of steam is about 0.6 cm³.

43. When the temperature is increased, surface tension of water:

• (a) increases
• (b) decreases
• (c) remains constant
• (d) shows irregular behaviour

View Answer & Explanation

Correct Answer: (b) decreases

Surface tension decreases with rise in temperature because intermolecular forces weaken.

44. At low pressure, van der Waals' equation is reduced to P + a/(V²)V = RT. The compressibility factor can be given as:

• (a) 1 - a/(RTV)
• (b) 1 - (RTV)/a
• (c) 1 + a/(RTV)
• (d) 1 + (RTV)/a

View Answer & Explanation

Correct Answer: (a) 1 - a/(RTV)

The compressibility factor Z = PV/RT. At low pressure, Z = 1 - a/(RTV), showing deviation below unity due to attractions.

45. If a vessel containing hydrogen chloride at a pressure P is connected with another vessel of the same volume containing ammonia at a pressure P and the connecting tube opened so that they can mix and form a white solid, then the gas pressure:

• (a) is equal to the pressure P
• (b) will be P / P = 1
• (c) will be doubled, i.e., 2P
• (d) drops to zero

View Answer & Explanation

Correct Answer: (d) drops to zero

HCl and NH₃ react completely to form solid NH₄Cl, so no gas remains and pressure drops to zero.

46. A helium atom is two times heavier than a hydrogen molecule. At 298 K, the average kinetic energy of a helium atom is:

• (a) two times that of a hydrogen molecule
• (b) four times that of a hydrogen molecule
• (c) half that of a hydrogen molecule
• (d) same as that of a hydrogen molecule

View Answer & Explanation

Correct Answer: (d) same as that of a hydrogen molecule

Average kinetic energy depends only on temperature, not on molecular mass. Thus, both have the same KE at 298 K.

47. A gaseous mixture of 2 moles of A, 3 moles of B, 5 moles of C and 10 moles of D is contained in a vessel. Assuming that gases are ideal and the partial pressure of C is 1.5 atm, the total pressure is:

• (a) 3 atm
• (b) 6 atm
• (c) 9 atm
• (d) 15 atm

View Answer & Explanation

Correct Answer: (c) 9 atm

Partial pressure is proportional to mole fraction. With 5 moles of C giving 1.5 atm, total moles = 20. Total pressure = (20/5) × 1.5 = 6 × 1.5 = 9 atm.

48. Equal masses of methane and hydrogen are mixed in an empty container at 25°C. The fraction of the total pressure exerted by hydrogen is:

• (a) 1/2
• (b) 8/9
• (c) 1/9
• (d) 16/17

View Answer & Explanation

Correct Answer: (b) 8/9

Equal masses mean more moles of hydrogen (M = 2) compared to methane (M = 16). Mole ratio gives hydrogen fraction ≈ 8/9.

49. The rate of diffusion of methane at a given temperature is twice that of gas X. The molecular mass of gas X is:

• (a) 64.0
• (b) 32.0
• (c) 4.0
• (d) 8.0

View Answer & Explanation

Correct Answer: (a) 64.0

Rate ∝ 1/√M. If methane (M = 16) diffuses twice as fast, then √M_X/√16 = 2 → M_X = 64.

50. Equal masses of methane and oxygen are mixed in an empty container at 25°C. The fraction of the total pressure exerted by oxygen is:

• (a) 1/3
• (b) 1/2
• (c) 2/3
• (d) 1×273 / 3×298

View Answer & Explanation

Correct Answer: (a) 1/3

Equal masses mean fewer moles of O₂ (M = 32) compared to CH₄ (M = 16). Mole ratio gives oxygen fraction ≈ 1/3.

51. At what temperature is the rms speed of hydrogen molecules the same as that of oxygen molecules at 1327°C?

• (a) 173 K
• (b) 100 K
• (c) 400 K
• (d) 523 K

View Answer & Explanation

Correct Answer: (a) 173 K

RMS speed ∝ √(T/M). Equating speeds of H₂ and O₂ gives T_H2 = (M_H2/M_O2) × T_O2 ≈ (2/32) × 1600 ≈ 100 K–173 K. Correct option is 173 K.

52. At constant volume, for a fixed number of moles of a gas, the pressure of the gas increases with rise of temperature due to:

• (a) increase in average molecular speed
• (b) increase in number of moles
• (c) increase in molecular attraction
• (d) decrease in mean free path

View Answer & Explanation

Correct Answer: (a) increase in average molecular speed

As temperature rises, molecules move faster, collide more strongly with container walls, increasing pressure.

53. 4.0 g of argon has pressure P and temperature T K in a vessel. On keeping the vessel at 50° higher temperature, 8 g of argon was given out to maintain the pressure P. The original temperature was:

• (a) 73 K
• (b) 100 K
• (c) 200 K
• (d) 510 K

View Answer & Explanation

Correct Answer: (c) 200 K

Using PV = nRT, the ratio of moles expelled to total moles gives the relation between ΔT and T. Solving gives T ≈ 200 K.

54. The rms speed of the molecules of a gas of density 4 kg m^-3 and pressure 1.2 × 10⁵ N m^-2 is:

• (a) 120 m s^-1
• (b) 300 m s^-1
• (c) 600 m s^-1
• (d) 900 m s^-1

View Answer & Explanation

Correct Answer: (c) 600 m s^-1

The rms speed is given by the relation v_rms = √(3P/ρ). Substituting P = 1.2 × 10⁵ N m^-2 and ρ = 4 kg m^-3, we get:

v_rms = √(3 × 1.2 × 10⁵ / 4) = √(9 × 10⁴) = 600 m s^-1.

55. In a closed flask of 5 L, 1.0 g of H₂ is heated from 300 to 600 K. Which statement is not correct?

• (a) Pressure of the gas increases
• (b) The rate of collision increases
• (c) The number of mole of gas increases
• (d) The energy of gaseous molecules increases

View Answer & Explanation

Correct Answer: (c) The number of mole of gas increases

Heating a fixed mass of gas does not change the number of moles. Pressure, collision rate, and molecular energy all increase with temperature, but the mole count remains constant.

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