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Published on: 04/12/2019
Kinetic Theory
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1.
Explain, why it is not possible to increase the temperature of a gas while keeping its volume and pressure constant?
2.
A vessel contains two nonreactive gases : neon (monatomic) and oxygen (diatomic). The ratio of their partial pressures is 3:2. Estimate the ratio of (i) number of molecules and (ii) mass density of neon and oxygen in the vessel. Atomic mass of Ne = 20.2 u, molecular mass of O2 = 32.0 u.
3.
At what temperature would the root-mean square speed of a gas molecule have twice its value at 100°C?
4.
A gas is filled in a cylinder at 300k. Calculate the temperature upto which it should be heated so that its volume becomes \(\frac { 4 }{ 3 } \) of its initial volume.
5.
If the ratio of molecular weights of two gases is 4.What will be ratio of the Vrms values for the molecules of those two gases?
6.
If the pressure of a gas filled in closed container is increased by 0.2 %. When temperature is increased by 1K,calculate the initial temperature of the gas.
7.
The density of water is 1000 kgm-3. The density of water vapour at 1000C and 1 atm pressure is 0.6 kgm -3 .The volume of a molecule multiplied by the total number gives ,what is called, molecular volume. Estimate the ratio (or fraction) of the molecular volume to the total volume occupied by the water vapour under the above conditions of temperature and pressure.
8.
A gas in equilibrium has uniform density and pressure throughout its volume. This is strictly true only if there are no external influences. A gas column under gravity,e.g. does not have uniform density(and pressure). As you might except, its density decreases with height. The precise dependence is given by the so called law of atmosphere.
n2 = n1 exp [- mg(h2 - h1)/ kBT]
Where, n2 and n1 refer to number density at heights h2 and h1, respectively. Use this relation to derive the equation for sedimentation equilibrium of a suspension in a liquid column.
\({ n }_{ 2 }={ n }_{ 1 }\quad exp\left[ -mg\quad { N }_{ A }(\rho -{ \rho }^{ ' })({ h }_{ 2 }-{ h }_{ 1 })/(\rho RT) \right] \)
Where, \(\rho \) is the density of the suspended particle and \({ \rho }^{ ' }\), that of surrounding medium.
[\(\because \) NA is Avogadro's number and R is the universal gas constant.]
9.
Chlorine and carbon dioxide gases are maintained at 27°C. Which gas will have higher average molar kinetic energy of translation and why?
10.
Mention two conditions when real gases obey the ideal gas equation PV = RT?
11.
What would be the effect on the rms velocity of gas molecules if the temperature of the gas is increased by a factor of 4?
12.
What is basic law followed by equipartition of energy?
13.
Three vessels of equal capacity have gases at the same temperature and pressure. The first vessel contains neon (monatomic), the second contains chlorine (diatomic), and the third contains uranium hexafluoride (polyatomic). Do the vessels contain equal number of respective molecules ? Is the root mean square speed of molecules the same in the three cases? If not, in which case is vrms the largest ?
14.
Oxygen and hydrogen gases are at the same temperature T. The kinetic energy of an oxygen molecule will be equal to
16 times the kinetic energy of a hydrogen molecule
5 times the kinetic energy of a hydrogen molecule
the kinetic energy of a hydrogen molecule
one-fourth the kinetic energy of a hydrogen molecule
15.
The energy density \(\frac { u }{ v } \)of an ideal gas is related to its pressure P as
\(\frac { u }{ v } \)=3p
\(\frac { u }{ v } =\frac { 3 }{ 2 } p\)
\(\frac { u }{ v } =\frac { 1 }{ 3 } p\)
\(\frac { u }{ v } =\frac { 2 }{ 3 } p\)
16.
Two vessels having equal volume contain molecular hydrogen at one atmosphere and helium at two atmosphere pressure respectively.If both samples are at the same temperature the mean velocity of hydrogen molecule is
equal to that of helium
twice that of helium
half that of helium
\(\sqrt { 2 } \) times that of helium
17.
During an adiabatic process, the pressure of a gas is proportional to the cube of its absolute temperature. The value of Cp/ C v for that gas is
3/5
4/3
5/3
3/2
18.
A sealed container with negligible thermal coefficient of expansion contains helium (a monoatomic gas). When it is heated from 300 to 600 K, the average kinetic energy of the helium atom is
halved
left unchanged
doubled
becomes \(\sqrt { 2 } \)times
19.
The speed of sound in a gas is v. The rms speed of molecules of this gas is C. If \(\Upsilon =\frac { { C }_{ P } }{ C_{ V } } \) then the ratio of v and C is
\(\frac { 3 }{ \Upsilon } \)
0.33 \(\Upsilon \)
\(\sqrt { \frac { 3 }{ \Upsilon } } \)
\(\sqrt { \frac { \Upsilon }{ 3 } } \)
20.
According to kinetic theory of gases the r.m.s. velocity of the gas molecules is directly proportional to
\(\sqrt { T } \)
T4
T
T2
1.
According to kinetic theory of gases,
\(p=\frac { 1 }{ 3 } { P }^{ { C }^{ 2 } }=\frac { 1 }{ 3 } \frac { M }{ V } { C }^{ 2 }\)
\(=\frac { 1 }{ 3 } \frac { M }{ V } KT\)
\( T\propto PV\) \((\because { C }^{ 2 }=KT.\)When k is constant)
Now as T is directly proportional to the product of P and V. If P and V are constant, then T is also constant.
2.
Partial pressure of a gas in a mixture is the pressure it would have for the same volume and temperature if it alone occupied the vessel. (The total pressure of a mixture of non-reactive gases is the sum of partial pressures due to its constituent gases.) Each gas (assumed ideal) obeys the gas law. Since V and T are common to the two gases, we have P1V = µ1 RT and P2V = µ2 RT, i.e. (P1 /P2 ) = (µ1 / µ2 ). Here 1 and 2 refer to neon and oxygen respectively. Since (P1 /P2 ) = (3/2) (given), (µ1 / µ2 ) = 3/2.
(i) By definition µ1 = (N1 /NA ) and µ2 = (N2 /NA ) where N1 and N2 are the number of molecules of 1 and 2, and NA is the Avogadro’s number. Therefore, (N1 /N2 ) = (µ1 / µ2 ) = 3/2.
(ii) We can also write µ1 = (m1 /M1 ) and µ2 = (m2 /M2 ) where m1 and m2 are the masses of 1 and 2; and M1 and M2 are their molecular masses. (Both m1 and M1 ; as well as m2 and M2 should be expressed in the same units). If ρ1 and ρ2 are the mass densities of 1 and 2 respectively, we have \( \frac{\rho_1}{\rho_2}=\frac{m_1 / V}{m_2 / V}=\frac{m_1}{m_2}=\frac{\mu_1}{\mu_2} \times\left(\frac{M_1}{M_2}\right) \)
\(=\frac{3}{2} \times \frac{20.2}{32.0}=0.947
\)
3.
We know that
\({ C }^{ 2 }=3\frac { RT }{ Nm } =3\frac { kT }{ m } \)
Thus \({ C }_{ 1 }^{ 2 }=\frac { { 3kT }_{ 1 } }{ m } \)
and \({ C }_{ 2 }^{ 2 }=\frac { { 3kT }_{ 2 } }{ m } \)
\(\frac { { C }_{ 1 }^{ 2 } }{ { C }_{ 2 }^{ 2 } } =\frac { { T }_{ 1 } }{ { T }_{ 2 } } \)
Here C2 = 2C1,T - 273 + 100 = 373 K
T2 = T1 x \(\frac { { C }_{ 2 }^{ 2 } }{ { C }_{ 1 }^{ 2 } } \) = 373 x 4
= 1492 K = 12190C
4.
Given, \({ T }_{ 1 }=300k,{ T }_{ 2 }=?\)
\(\\ { V }_{ 1 }=V,{ V }_{ 2 }=\frac { 4 }{ 3 } V\)
According to Charles law, we get
\(\frac { { V }_{ 2 } }{ { V }_{ 1 } } =\frac { { T }_{ 2 } }{ { T }_{ 1 } } \)
\(\\ \Rightarrow { T }_{ 2 }={ T }_{ 1 }\frac { { V }_{ 2 } }{ { V }_{ 1 } } =300\times \frac { 4 }{ 3 } =400\ k\)
5.
0.5
6.
500 K
7.
For a given mass of water molecules, the density is less if volume is large. So the volume of the vapour is 1000/0.6 = 1/(6 × 10 -4 ) times larger. If densities of bulk water and water molecules are same, then the fraction of molecular volume to the total volume in liquid state is 1. As volume in vapour state has increased, the fractional volume is less by the same amount, i.e. 6×10-4 .
8.
According to the law of atmospheres.
\({ n }_{ 2 }={ n }_{ 1 }\quad exp.\left[ -\frac { mg }{ { K }_{ B }T } \left( h_{ 2 }-{ h }_{ 1 } \right) \right] --\quad (i)\)
where, n2 and n1refer to number density of particles at heights h2 and h1, respectively.
If we consider the sedimentation equilibrium of suspended particles in a liquid, then in place of mg, we will have to take effective weight of the suspended particles.
Let, V = average volume of a suspended particle,
\(\rho \) = density of suspended particle, \({ \rho }^{ ' }\)= density of liquid, m = mass of one suspended particle, \({ m }^{ ' }\)= mass of equal volume of liquid displaced.
According to Archimedes' priciple, effective weight of one suspended particle
= Actual weight-weight of liquid displaced = mg-m'g
\(=mg-V{ \rho }^{ ' }g=mg-\left( \frac { m }{ \rho } \right) { \rho }^{ ' }g=mg\left( 1-\frac { { \rho }^{ ' } }{ \rho } \right) \)
\(\\ Also,\ Boltzmann\ constant,{ K }_{ B }=\frac { R }{ { N }_{ A } } \)
where, R is gas constant and NA is Avogardro's number.
\(putting,\ mg\left( 1-\frac { { \rho }^{ ' } }{ \rho } \right) in\ place\ of\ mg\ and\ value\ of\ { K }_{ B }\quad in\)
\( Eq.(i)\ we\ get\)
\( { n }_{ 2 }={ n }_{ 1 }exp\left[ -\frac { mg{ N }_{ A } }{ RT } \left( 1-\frac { { \rho }^{ ' } }{ \rho } \right) \left( { h }_{ 2 }-{ h }_{ 1 } \right) \right] ,\ which\ id\ required\ relation.\)
9.
Both gases have same value of average translational kinetic energy per mole because their temperatures are equal and \(\bar { { E } } =\frac { 3 }{ 2 } RT\)
10.
(i) Low pressure
(ii) High temperature
11.
Since \(C\propto \sqrt { T } \)
Clearly, C will be doubled
12.
The law of equipartition of energy for any dynamical system in thermal equilibrium, the total energy is distributed q = equally amongst all the degrees of freedom.
The energy associated with each molecule per degree of freedom is \(\frac { 1 }{ 2 } { k }_{ B }T\), where KB is Boltzmann's constant and T is temperature of the system.
13.
As three vessels are identical i.e., they have same volume now at constant pressure, temperature and volume the three vessels will contain equal number of molecules (by Avogadro’s law) and is equal to Avogadro's number, NA = 6.023 x 1023
\(\because { V }_{ rms }=\sqrt { \frac { 3{ k }_{ B }T }{ m } } \Rightarrow { V }_{ rms }\propto \frac { 1 }{ \sqrt { m } }\)
where, m is mass of single gas molecule as neon has the smallest mass, so rms speed will be greatest in case of neon.
14.
(c)
the kinetic energy of a hydrogen molecule
15.
(b)
\(\frac { u }{ v } =\frac { 3 }{ 2 } p\)
16.
(d)
\(\sqrt { 2 } \) times that of helium
17.
(d)
3/2
18.
(c)
doubled
19.
(d)
\(\sqrt { \frac { \Upsilon }{ 3 } } \)
20.
(c)
T
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