Gases: MCQs and Answers with explanation

1. When compressed hydrogen is allowed to expand rapidly, it causes.

a) cooling
b) heating
c) liquefaction
d) solidification

... Answer is a)
Explanation: Option a is correct. According Joul-Thomson effect,when compressed gas is allowed to expand through "throttle" into a region of low pressure, the temperature of gas falls and it is cooled. Option c also looks possible answer but it can be rejected because liquifaction occures due to repetition of cooling process for several times.

2. 760 torr are equal to Pascal

a) 760 Pascal
b) 101325 Pascal
c) 1.01325 Pascal
d) one Pascal

... Answer is b)
Explanation: 1atm = 760 torr and 1atm = 101325 Nm^-2 or 1atm = 101325 Pa as 1Nm^-2 = 1Pa Hence, 760 torr = 101325 Pascal

3. A number of molecules of 4g H₂ are ------ number of molecules of 56 g of N₂

a) equal to
b) less than
c) greater than
d) none of these

... Answer is a)
Explanation: Number of moleucules of H₂ = (4/2)× N_A = 2 × N_A Number of moleucules of N₂ = (56/28)×N_A = 2 × N_A so both have same number of moleucles.

4. According to the Kinetic theory of gases, the molecular collisions are elastic, such collisions cause

a) No energy change
b) A small energy change
c) High energy change
d) None of these

... Answer is a)
Explanation:According to one of the postulates of Kinetic molecular theory, the collision between moleucules and wall of container are perfectly elastic.

5. What volume of gas would one mole of hydrogen occupy at S.T.P.?

a) 11.2 dm³
b) 22.4 dm³
c) 33.6 dm³
d) 44 dm³

... Answer is b)
Explanation: Molar volume or volume of one mole of any is dm₃ at S.T.P.

6. According to Grahms law of diffusion the rate of diffusion of H₂ and O₂ is

a) 1 : 4
b) 1 : √4
c) 4 : 1
d) 2 : 32

... Answer is c)
Explanation: According to Grahm law of diffusion,

7. Deep sea divers tank contains

a) 96% He + 4% O₂
b) 96% O₂ + 4% He
c) 50% O₂ + 50% He
d) none

... Answer is a)
Explanation: Deep sea divers use a mixture of oxygen gas along with an innert gas such as He. Purpose of innert gas is to adjust the partial pressure of oxygen suitable for breathing.

8. In SI units, the value of R is

a) 8.3413 NmK^-1mole^-1
b) 8.3143 NmK^-1mole^-1
c) 0.0821 dm³atm K^-1mole^-1
d) None of these

... Answer is b)
Explanation: In SI units, n = 1 mole, T = 273 K, V = 0.0224 m³, P = 101325 Nm ^-1

.

9. According to Kinetic Molecular Theory, K.E. of molecules increase when they

a) are mixed with other molecules at low temperature
b) are frozen into solid
c) are condensed into solid
d) are melted from solid to liquid state

... Answer is d)
Explanation: When solids are melted by increasing the temperature. According to Kinetic Molecular Theory, K.E. of molecules increase with increase in temperature.

10. Which gas is more ideal at S.T.P.?

a) SO₂
b) H₂S
c) NH₃
d) H₂

... Answer is d)
Explanation: Among these gases, H₂ is more ideal because of its small values of its "a" and "b" constants. Small values of these constants are due to non polar nature and small size of H₂ gas.

11. On heating direct conversion of a solid in to gas is called

a) evaporation
b) sublimation
c) diffusion
d) boiling

... Answer is b)
Explanation: The process of conversion of a solid directly into vapours and condensation of vapours dierctly into solid without passing through liquid state is known as sublimation.

12. In which of the following are the particles the most disorderd.

a) water
b) steam at 100˚C
c) impure water at 100 ˚C
d) water at 0˚C

... Answer is b)
Explanation: As steam is a gaseous phase of water and hence, gas molecules are more random and disordered than liquid phase of water.

13. When steam condenses, the particles

a) shrink to a smaller size
b) loss energy to the surrounding
c) move further apart
d) vibrate about fixed positions

... Answer is b)
Explanation: The steam condense only when its molecules lose their thermal and kinetic energy to its surrounding.

14. The particles of a gas can be described as

a) only moving outward in direction
b) vibrate about defined position
c) rising upwards
d) moving randomly in all directions

... Answer is d)
Explanation: According to one of the postulates of KMT, particles of a gas are in constant random motion. They collide with one an other and with walls of container and change their direction of motion.

15. Which of these changes would speed up the rate of diffusion the most.

a)
b)
c)
d)

... Answer is b)
Explanation: According to Grahm law of diffusion and effusion, rate of of diffusion is inversly proportional to square root of the molar mass of the gas. Thus rate of diffusion will incrase with deacrease in molar mass of the gas. By considering this law, options c and d can be excluded. According KMT, the K.E (rate of diffusion) of gas molecules increase with increase in temperature. By considering, KMT, option a can also be excluded. So the correct option is b).

16. Which of the following gases has the lowest denstiy under room conditions

a) CO
b) N₂
c) Ne
d) NH₃
d) O₂

... Answer is d)
Explanation: As derived from general gas equation

At constant temperature and pressure,the density of a gas is directly proportional to its molar mass. Since, among these gases molar mass of NH₃ is least, so its density will also be the lowest.

17. Which statment provides the best evidence that matter may exist as tiny particles moving at "random motion".

a) many elements conduct electricity.
b) air can be readily compressed.
c) a small mass of water produces a much larger volume of steam.
d) if a bottle of the ether is opened, the smell is quickly detected in all parts of the room.

... Answer is d)
Explanation: If a bottle of the ether is opened, the smell is quickly detected in all parts of the room. This is the best evidence that a matter may exist as tiny particles moving randomly in all direction.

18. Which of the following is an example of diffusion?

a) bubble rising in a beaker of boiling water.
b) steam condensing on a cold window.
c) water spreading out on the surface of a table.
d) hot air rising above a burning candle.
e) The spreading of the smell of flowers in a garden .

... Answer is e)
Explanation: The spreading of the fragrance of flower in the garden is due to spontaneous intermixing of vapours of flowers into air. So, this is the example of diffusion.

19. Which statement explains why the compounds propane and CO₂ diffuse at the same rate?

a) They are both gases.
b) Their molecules contain carbon.
c) They have the same relative molecular masses.
d) Both are denser than air.
e) Both molecules contain covalent bonds.

... Answer is c)
Explanation: By Grahm law of diffusion, the rate of diffusion or effusion of a gas is inversly proportional to square root of molar mass of the gas. As both gases propance (C₃H₈ and CO₂ have equal molecular masses i.e. 44gmole_-1, so they have equal rate of diffusion.