# Engineering Physics Questions and Answers – Ideal Gases – 1

This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Ideal Gases – 1”.

1. The equation of state for g of oxygen at a pressure P, and temperature T, when occupying a volume V, will be?
a) PV = (5/32)RT
b) PV = 5RT
c) PV = (5/2)RT
d) PV = (5/16)RT

Explanation: Number of moles in 5g of oxygen=5/32
As PV = nRT
Therefore,
PV = (5/32) RT.

2. Two gases are at absolute temperatures 300K and 350K respectively. What is the ratio of average kinetic energy of their molecules?
a) 7:6
b) 6:7
c) 36:49
d) 49:36

Explanation: E1/E2 =T1/T2
300/350=6/7
6:7.

3. The gas having average speed four times as that of SO2 (molecular mass 64) is ___________
a) He(molecular mass 64)
b) O2(molecular mass 4)
c) H2(molecular mass 32)
d) CH4(molecular mass 16)

Explanation: vrms=√(3RT/M)
v1/v2 =√(M2/M1)
v/4v=√(M2/64)
M2=4
Hence the gas is He which has molecule mass 4.

4. Assertion: Air pressure in a car tyre increases during driving.
Reason: Absolute zero temperature is not zero energy temperature.
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 a correct explanation of the assertion. Air pressure in a car tyre increases due to rise in temperature during driving.

5. R/Cv =0.67 for a gas. Then this gas is made up of which molecules?
a) Diatomic
b) Mixture of diatomic and polyatomic
c) Monoatomic
d) Polyatomic

Explanation: Cv=f/2×R=1/0.67×R
Therefore,
f=2/0.67=3
Hence the gas is monoatomic.
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6. If a given mass of gas occupies a volume of 10cc at 1 atmospheric pressure and temperature 100°C, what will be its volume at 4 atmospheric pressure, the temperature being the same?
a) 100cc
b) 400cc
c) 104cc
d) 2.5cc

Explanation: From Boyle’s law,
P1 V1=P2 V2
1×10=4×V2
V2=10/4=2.5cc.

7. If an ideal gas has a volume V at 27°C and it is heated at a constant pressure, so that its volume becomes 1.5V, then the value of final temperature is?
a) 327K
b) 177°C
c) 873K
d) 600°C

Explanation: At constant pressure,
V2/V1 =T2/T1
1.5V/V=T2/(273+27)
T2=300×1.5=450K=177℃.

8. Average velocity of a gas becomes 4 times. What will be the effect on rms velocity at the same temperature?
a) 1.4 times
b) 4 times
c) 3 times
d) 2 times

Explanation: v=0.92vrms
vrms=1/0.92×v
When v becomes 4 times,
(vrms)=4/0.92×v=4vrms.

9. The temperature of H2 at which the rms velocity of its molecules is seven times the rms velocity of the molecules of nitrogen at 300K is?
a) 2100K
b) 1700K
c) 1350K
d) 1050K

Explanation: vrms=√(3RT/M)
v1/v2 =√((T1 M2)/(T2 M1 ))
V(H2)/V(N2) =√((T(H2)×28)/(300×2))=7
T(H2)=(49×600)/28=1050K.

10. We have a jar filled with gas characterised by parameters P, V and T and another jar B filled with a gas with parameters 2P, V/4,2T; where the symbols have their usual meanings. The ratio of the number of molecules of jar A to those of jar B is?
a) 1:1
b) 1:2
c) 2:1
d) 4:1

Explanation: n1 r=PV/T
n2 r=(2P)(V/2)/2T=PV/4T=1/4 n1 r
n1/n2 = 4/1=4:1.

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