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Junior Inter Chemistry
t
e
States of Matter
n
.
a
4 Marks Questions
Q: Write the postulates of Kinetic molecular theory of gases.
h
b
i
A: Particles of a gas are in constant & random motion. Collisions of gas molecules
are perfectly elastic.
t
a
r
★ Average kinetic energy of gas molecules ∝ T.
★ There is no force of attraction between the molecules of the gas at ordinary
temperature and pressure.
p
u
d
Q: Deduce a) Boyle's law b) Charle's law from Kinetic gas equation.
A: a) Boyle's law:
a
n
e
e
.
w
w
2
1
pV =  mnurms
3
t
e
n
2
2
1
pV =  ×  mnurms
3
2
w
.
a
h
2
1
 mnurms = K.E.
2
b
i
t
But K.E. ∝ T or K.E. = KT
2
∴ pV =  KT
3
At constant T,
a
r
p
pV = Constant
u
d
a
b) Charle's law:
2
1
pV =  mnurms
3
n
e
e
.
ww
2
2
1
pV =  ×  mnurms
3
2
as KE = KT
w
2
pV =  KT
3
V
2 K
 =  
T
3 p
R-23-11-14
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V
At Constant p,  = Constant
T
Q. State Graham's law of diffusion.
t
e
360 cm3 of CH4 gas diffused through a porus membrane in 15 minutes.
Under similar conditions, 120 cm3 of another gas diffused in 10 minutes.
Find the molar mass of the gas.
n
.
a
h
b
i
A: At constant P & T, the rate of diffusion (or effusion) of a gas is inversely
proportional to the square root of its density.
t
a
r
1
r ∝ 

√d

rCH
Munknown
4

= 
r unknown
MCH
4
√
p
u
d
a
n

Munknown
4
 ×  =

tCH
MCH
Vunknown
4
4

Munknown
360
10
 ×  = 2 = 
15
120
16
√
e
e
.
w
√
w
w
tunknown
VCH
t
e
n
.
a
h
∴ Munknown = 2 × 2 × 16 = 64
Q: What is R.M.S. speed? Find R.M.S., most probable and average speeds of
SO2 at 27°°C.
b
i
t
A: Square root of mean of square of speeds of all the molecules of a gas is called
R.M.S. speed.

3RT
R.M.S. speed = Urms = 
M

3 × 8.314 × 300
 = 3.42 × 102 m sec−1
Urms =
64 / 1000

2RT
Most probable speed = Ump = 
M
a
r
p
√
u
d
a
√
n
e
e
.
ww
√
(or) 0.8166 Urms
w
= 0.8166 × 3.42 × 102 = 2.79 × 102 msec−1

8RT
Average speed = Uav = 
ΠM
√
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(or) 0.9213 Urms
= 0.9213 × 3.42 × 102 = 3.15 × 102 m sec−1
t
e
2 Marks Questions
n
.
a
Q: Calculate the Kinetic energy of 5 moles of Di Nitrogen at 27°°C.
3
A: K.E. =  nTR
2
n=5
h
b
i
T = 27 + 273 = 300 K
t
a
r
R = 8.314 J mol−1 K−1
3
=  × 5 × 300 × 8.314
2
p
u
d
= 18706.5 J
a
n
Q: Calculate the ratio of Kinetic energies of 3 g of H2 and 4 g of O2 at a given
temperature.
e
e
.
w
w
3
3
A: K.E.H : K.E.O =  nH TR :  nO TR
2
2
2
2
2
2
= nH : nO
2
2
WH WO
3
4
1
2
= 2 : 
=  :  = 1.5 :  = 12 : 1
2 32
8
MH MO
2
2
Q: Which of the gases diffuses faster among N2, O2, CH4? Why?
w
.
a
h
b
i
t
A: As per Graham's law
1
Diffusion rate ∝ 

√M
t
e
n
a
r
p
u
d
a
Molar masses of N2 = 28, O2 = 32, CH4 = 16
∴ CH4 diffuses faster among them due to lowest molar mass.
n
e
e
.
w√ w √ √
Q: Give the ratio of R.M.S., Average, Most probable speeds of gas molecules.
A: UM.P. : UAV : UR.M.S



2RT
8RT
3RT
=
 :  : 
M
ΠM
M
  
= √ 2 : √ 8/Π : √ 3
w
= 1.000 : 1.128 : 1.224
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Q: What is critical temperature? Give its value for CO2.
A: The temperature above which any gas cannot be liquified even by applying high
pressure.
t
e
TC of CO2 = 30.98°C
n
.
a
Q: What is Boltzman's constant? Give its value.
A: The gas constant per molecule (K).
R
8.314
K =  = 
N
6.023 × 1023
= 1.38 × 10−23 J K−1 molecule−1
h
b
i
t
a
r
Q: Why pressure cooker used for cooking food on hills?
p
u
d
A: Atmospheric pressure is low on hills. So liquids boil at low temperature. In
pressure cooker, boiling point of water is increased by increasing pressure above
the atmospheric pressure. Hence pressure cooker is used for cooking food.
a
n
e
e
.
w
w
Q: State Dalton's law of partial pressures.
A: At constant volume & temperature, the total pressure exerted by the non reacting
gaseous mixture is equal to the sum of partial pressures of individual gases
present in it.
Ptotal = P1 + P2 + P3 + ...
t
e
n
w
Where P1 is partial pressure of 1st gas.
.
a
h
b
i
t
Partial pressure = mole fraction × total pressure
Q: What is surface tension of liquids? Give units.
a
r
p
A: The force acting per unit length perperpendicular the line drawn.
Units: N m−1 (in SI System)
du
Writer: A.N.S. Sankara Rao
a
n
e
e
.
ww
w
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