# Engineering Physics Questions and Answers – Ideal Gases – 2

This set of Engineering Physics Quiz focuses on “Ideal Gases – 2”.

1. In the upper part of the atmosphere, the kinetic temperature of the air is of the order 1000K. Therefore one feels extremely hot.
a) True
b) False

Explanation: As we go up the atmosphere, the number of air molecules per unit volume decreases. The quantity oh heat per unit volume or the heat density is low. But the translational kinetic energy per molecule is quite large. As the kinetic temperature is the measure of translational kinetic energy, so the kinetic temperature is quite high in the upper atmosphere but one feels severe cold there due to low density.

2. What type of motion is associated with the molecules of gas?
a) Newton’s law of motion
b) Conservative motion
c) Linear motion
d) Brownian motion

Explanation: Brownian motion is associated with the molecules of a gas. In this motion, any particular molecule will follow a zigzag path due to the collisions with the other molecules or with the walls of the container.

3. At room temperature, the rms speed of the molecules of a certain diatomic gas is found to be 1920ms-1. Which is the gas?
a) H2
b) F2
c) O2
d) Cl2

Explanation: vrms=√(3RT/M)
M=3RT/(vrms)2 = (3×8.3×300)/(1920)2 = 2
Hence the gas is H2.

4. A vessel contains a mixture of 1 mole of oxygen and 2 moles of nitrogen at 300 K. The ratio of the average rotational kinetic energy per O2 molecule to that per N2 molecule is?
a) 1:1
b) 1: 2
c) 2:1
d) Depends on the moment of inertia of the two molecules

Explanation: Both gases are diatomic. Both have two rotational degrees of freedom at 300K. Average rotational kinetic energy per molecule of each gas will be same=2×1/2 kT=1kT
Hence the required ratio is 1:1.

5. Statement: For an ideal gas at constant temperature, the product of the pressure and volume is a constant.
Reason: The mean square velocity of the molecules is inversely proportional to mass.
a) Both statement and reason are true and explanation is the correct explanation of the statement
b) Both statement and reason are true but the explanation is not a correct explanation of the statement
c) Statement it true but the reason is false
d) Both statement and reason are false

Explanation: Both the statement and reason are true but the explanation is not a correct explanation of the statement. According to Boyle’s law, PV=constant, at a given temperature.
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6. An ant is walking on the horizontal surface. The number of degrees of freedom of ant will be ___________
a) 1
b) 2
c) 3
d) 6

Explanation: As the ant can move on a plane, it has 2 degrees of freedom.

7. The number of degrees of freedom for a diatomic gas molecule is ___________
a) 2
b) 3
c) 5
d) 6

Explanation: A diatomic molecule has 3 degrees of freedom due to translator motion and 2 degrees of motion due to rotator motion.

8. A gas at one atmosphere and having volume 100ml is mixed with another gas of equal moles at 0.5atm and having volume 50ml in flask of one litre, what is the final pressure?
a) 0.5atm
b) 1atm
c) 0.75atm
d) 0.125atm

Explanation: Total number of moles is conserved.
(P1 V1)/RT+(P2 V2)/RT=PV/RT
(1×100)/RT+(0.5×50)/RT=(P×1000)/RT
P=0.125atm.

9. Temperature of oxygen kept in a vessel is raised by 1°C at constant volume. Heat supplied to the gas may be taken partly as translational and partly rotational kinetic energies. Their respective shares are ___________
a) 60%, 40%
b) 50%, 50%
c) 100%, zero
d) 40%, 60%

Explanation: A diatomic oxygen molecule has 3 degrees of freedom due to translator motion and 2 degrees of freedom due to rotator motion. Their associated kinetic energies will be in the ratio 3:2 or 60% and 40%.

10. An ideal gas is heated from 27°C to 627°C at constant pressure. If initial volume was 4m3, then the final volume of the gas will be?
a) 2m3
b) 4m3
c) 6m3
d) 12m3

Explanation: Using Charle’s law,
V1/T1 = V2/T2
V2=T2/T1 ×V1=((273+627))/((273+27))×4=900/300×4=12m3.

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