# Engineering Physics Questions and Answers – Gases

This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Gases”.

1. Assertion: In the pressure-temperature phase diagram of water, the slope of the melting curve is found to be negative.
Reason: Ice contracts on melting to water.
a) Both assertion and reason are true and the reason is the correct explanation of the assertion
b) Both assertion and reason are true but the reason is not a correct explanation of the assertion
c) Assertion it true but the reason is false
d) Both assertion and reason are false

Explanation: Both the assertion and reason are true but the reason is not the correct explanation of the assertion. The negative slope of the melting curve indicates that the melting point of ice decreases with the increase in pressure.

2. At constant volume temperature is increases, then ___________
a) Collisions on walls will be less
b) Number of collisions per unit time will increase
c) Collisions will be in straight line
d) Collisions will not change

Explanation: As the temperature increases, the average molecular velocity increases. This increases collisions frequency.

3. Gases at low temperature and high pressure show a large deviation from an ideal behaviour.
a) True
b) False

Explanation: At low temperature and high pressure, the intermolecular attractions become appreciable. Moreover, the volume occupied by the gas molecules cannot be neglected in comparison to the volume of the gas. Hence the real gases show large deviations from an ideal gas.

4. When an automobile travels for a long distance, the pressure in the gas?
a) Increases slightly
b) Decreases slightly
c) Remains the same
d) Decreases and then increases

Explanation: Due to the friction between the tyres and the road, the tyres get heated. The temperature of the air inside the tyres increases. Consequently, the air pressure in the tyres increases slight.

5. A vessel contains one mole of O2 gas at a temperature T. The pressure of the gas is P. An identical vessel containing one hole of He gas at a temperature 2T has a pressure of?
a) P/8
b) P
c) 2P
d) 8P

Explanation: PV = nRT or P=nRT/V
For same n, R and V, P is proportional to T
P2/P1 =T2/T1
Or
P2/P=2T/T or P2=2P.

6. The average translational kinetic energy of O2 molecules (relative molar mass 32) at a particular temperature is 0.048eV. The translational kinetic energy of N2 molecules (relative molar mass) in eV at the same temperature is?
a) 0.0015
b) 0.003
c) 0.048
d) 0.768

Explanation: Average translational kinetic energy of a molecule=3/2kT
It depends on temperature and not on molecular mass. Hence average translational kinetic energy for N2 will also be 0.048 eV.

7. A gas mixture consists of 2 moles of oxygen and 4 moles of argon at temperature T. Neglecting all vibrational modes, the total internal energy of the system is?
a) 4RT
b) 9RT
c) 11RT
d) 15RT

Explanation: For two moles of diatomic nitrogen with no vibrational mode,
U1=2×5/2 RT=5RT
For four moles of monoatomic argon,
U2=4×3/2 RT=6RT
Therefore, U=U1+U2=11RT.

8. Surface the lake is at 2°C and depth of the lake is 20m. Find the temperature of the bottom of the lake?
a) 2°C
b) 3°C
c) 4°C
d) 552°C

Explanation: P1/T1 =P2/T2
(1×105)/(273+2)=(hρg+1×105)/T2
(1×105)/275=(20×103×10+1×105)/T2
T2=275×3=825K=552℃.

9. The degree of freedom in case of a monoatomic gas is ___________
a) 1
b) 3
c) 5
d) 2

Explanation: A monoatomic gas has 3 degrees of freedom due to a translator motion.

10. The mean kinetic energy of one mole of gas per degree of freedom (on the basis of a kinetic theory of gases) is ___________
a) (1/2) kT
b) (3/2) kT
c) (3/2) RT
d) (1/2) RT

Explanation: Mean kinetic energy per mole pf a gas per degree of freedom = (1/2) RT.

Sanfoundry Global Education & Learning Series – Engineering Physics.

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