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Physics NEET MCQ
Quiz 1
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Q.1
In the following questions, a statement of assertion is followed by a statement of reason. You are required to choose the correct one out of the given four responses and mark it as (a) If both assertion and reason are true and reason is the correct explanation of the assertion. (b) If both assertion and reason are true but reason is not correct explanation of the assertion. (c) If assertion is true, but reason is false. (d) If both assertion and reason are false. (e) If reason is true but assertion is false. Assertion: The ratio Cp/Cv is more for helium gas than for hydrogen gas. Reason: ATomic mass of helium is more than that of Hydrogen
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(a)
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(b)
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(c)
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(d)
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(e)
Explanation
Helium (monatomic, γ ≈ 1.67) does have a higher Cp/Cv than hydrogen (diatomic, γ = 1.4), so the Assertion is true. The Reason (helium's atomic mass is greater) is also a true fact on its own, but molecular MASS has nothing to do with the value of γ — γ depends entirely on the number of degrees of freedom (i.e. whether the gas is monatomic, diatomic, etc.), not on how heavy the molecules are. So the Reason doesn't correctly explain the Assertion.
Q.2
The rate of diffusion, is
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equal in solids, liquids, and gases
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faster in liquids than in solids and gases
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faster in solids than in liquids and gases
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faster in gases than liquids and solids
Explanation
Molecules in a gas are far apart, move freely, and have weak intermolecular forces, so they mix (diffuse) fastest. Liquids diffuse more slowly (molecules are closer together, more frequent interactions), and diffusion in solids is by far the slowest (particles are essentially fixed in place, only slow lattice-level migration occurs). So diffusion is fastest in gases, then liquids, then solids.
Q.3
Two balloons are filled, on with pure He gas and other by air, respectively. If the pressure and temperature of these balloons are same then the number of molecules per unit volume is
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more in the He filled balloon
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same in both balloons
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more in air-filled balloon
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in the ratio of 1: 4
Explanation
By the ideal gas law PV = nRT, at the same pressure, volume and temperature, the number of moles n is the same no matter which gas is used — and since the number of molecules is just n × Avogadro's number, the number of molecules per unit volume is also the same in both balloons (Avogadro's law: equal volumes of any ideal gas at the same T and P contain equal numbers of molecules).
Q.4
If one mole of monoatomic gas( $\gamma =5/3$) is mixed with one mole of a diatomic gas ($\gamma = 7/5$), the value of adiabatic exponent $\gamma$ for mixture is
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1.35
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1.40
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1.5
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1.75
Explanation
For a mixture, Cv and Cp are mole-weighted averages. Monatomic (n₁=1 mol): Cv₁ = (3/2)R, Cp₁ = (5/2)R. Diatomic (n₂=1 mol): Cv₂ = (5/2)R, Cp₂ = (7/2)R. Cv(mix) = [(3/2)R + (5/2)R]/2 = 2R Cp(mix) = [(5/2)R + (7/2)R]/2 = 3R γ(mix) = Cp/Cv = 3R/2R = 1.5.
Q.5
A thermally insulated vessel contains an ideal gas of Molecular mass M and a ratio of specific heats $\gamma$. It is moving with speed b and is suddenly brought to rest.Assuming no heat lost to the surroundings, its temperature increases by
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$ \frac { (\gamma -1}{2(\gamma + 2)R} Mv^2 \; K$
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$ \frac { (\gamma -1}{2\gamma R} Mv^2 \; K$
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$ \frac { \gamma Mv^2} {2R} \; K$
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$ \frac { (\gamma -1}{2 R} Mv^2 \; K$
Explanation
When the gas suddenly stops, all its bulk kinetic energy (per mole, ½Mv²) converts into extra internal (thermal) energy, since the vessel is insulated (no heat escapes): ½Mv² = Cv·ΔT = [R/(γ−1)]·ΔT ΔT = (γ−1)Mv² / (2R).
Q.6
A gas behaves as an ideal gas at
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high pressure and low temperature
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low pressure and high temperature
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high pressure and high temperature
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low pressure and low temperature
Explanation
A real gas behaves most like an ideal gas when its molecules are far apart and moving fast — that means low pressure (plenty of space between molecules, so their own volume barely matters) and high temperature (enough kinetic energy that weak intermolecular attractions become negligible).
Q.7
If the rms velocity of the molecules of a gas in a container be doubled then pressure of the gas will
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becomes 4 times of its previous value
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becomes 2 times of its previous value
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remains same
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becomes 1/4 of its previous value
Explanation
Pressure of a gas is proportional to the mean square speed of its molecules: P ∝ v_rms² (at fixed density/volume). If v_rms doubles, v_rms² becomes 4 times as large, so pressure becomes 4 times its previous value.
Q.8
In a vessel , the gas is at a pressure P. If the mass of all the molecules is halved and their speed is doubled, then the resultant pressure will be
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4P
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2P
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P
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P/2
Explanation
Pressure P ∝ (mass per molecule) × (mean square speed), i.e. P ∝ m·v². If mass per molecule is halved (m → m/2) and speed is doubled (v → 2v): New P ∝ (m/2)(2v)² = (m/2)(4v²) = 2mv² = 2 × (original m·v²) So the new pressure is 2P.
Q.9
On colliding in a closed container the gas molecules
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transfer momentum to the walls
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momentum becomes zero
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momentum becomes zero
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perform brownian motion
Explanation
Gas pressure arises precisely because molecules colliding with the container walls transfer momentum to them — that repeated, rapid transfer of momentum, averaged over huge numbers of collisions, is what we measure as pressure.
Q.10
The degree of Freedom of a triatomic gas is
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1
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2
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6
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8
Explanation
A nonlinear triatomic molecule (like water vapour) has 3 translational and 3 rotational degrees of freedom, giving 3 + 3 = 6 total (vibrational modes are typically excluded in this basic count).
Q.11
The root mean square and most probable speed of the molecules in a gas are
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same
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different
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cannot say
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depends on nature of the gas
Explanation
The Maxwell-Boltzmann speed distribution is not symmetric — it's skewed with a longer tail toward higher speeds — so its characteristic speeds (most probable speed, average speed, and rms speed) are all genuinely different from one another, with v_rms > v_average > v_most-probable.
Q.12
In the following questions, a statement of assertion is followed by a statement of reason. You are required to choose the correct one out of the given four responses and mark it as (a) If both assertion and reason are true and reason is the correct explanation of the assertion. (b) If both assertion and reason are true but reason is not correct explanation of the assertion. (c) If assertion is true, but reason is false. (d) If both assertion and reason are false. (e) If reason is true but assertion is false. Assertion: The ratio Cp/Cv for a diatomic gas is more than that for a monatomic gas. Reason: The molecules of a monatomic gas have more degrees of freedom than those of a diatomic gas.
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(a)
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(b)
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(c)
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(d)
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(e)
Explanation
Actual values: monatomic γ = 5/3 ≈ 1.67, diatomic γ = 7/5 = 1.4 — so diatomic gas actually has a LOWER Cp/Cv than monatomic gas, making the Assertion false (it has the comparison backwards). The Reason is also false the same way round: diatomic molecules have MORE degrees of freedom (5) than monatomic ones (3), not fewer — and more degrees of freedom is exactly why diatomic gases have the lower γ (since γ = 1 + 2/f). Both statements are false.
Q.13
In the following questions, a statement of assertion is followed by a statement of reason. You are required to choose the correct one out of the given four responses and mark it as (a) If both assertion and reason are true and reason is the correct explanation of the assertion. (b) If both assertion and reason are true but reason is not correct explanation of the assertion. (c) If assertion is true, but reason is false. (d) If both assertion and reason are false. (e) If reason is true but assertion is false. Assertion: The root mean square and most probable speeds of the molecules in a gas are the same. Reason: The Maxwell distribution for the speed of molecules in a gas is symmetrical.
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(a)
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(b)
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(c)
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(d)
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(e)
Explanation
As established, the rms speed and most probable speed of gas molecules are genuinely different (the Assertion is false), and the Maxwell speed distribution is asymmetric/skewed, not symmetrical (the Reason is false too) — both statements are incorrect.
Q.14
In the following questions, a statement of assertion is followed by a statement of reason. You are required to choose the correct one out of the given four responses and mark it as (a) If both assertion and reason are true and reason is the correct explanation of the assertion. (b) If both assertion and reason are true but reason is not correct explanation of the assertion. (c) If assertion is true, but reason is false. (d) If both assertion and reason are false. (e) If reason is true but assertion is false. Assertion: Root mean square velocity of the gas does not change in an Isothermal process Reason: Isothermal process may be achieved by immersing the system in a large reservoir and performing the process slowly
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(a)
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(b)
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(c)
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(d)
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(e)
Explanation
The rms speed formula v_rms = √(3RT/M) depends only on temperature (and molar mass) — so in an isothermal process, where T stays fixed, v_rms genuinely doesn't change, making the Assertion true. The Reason (using a large thermal reservoir and a slow process to maintain constant temperature) correctly describes a standard practical METHOD for achieving isothermal conditions, and is true — but it doesn't explain WHY v_rms specifically depends only on temperature, which is the actual reason behind the Assertion. So both are true, but the Reason isn't the correct explanation.
Q.15
A cubical box of side 1 meter contains helium gas (atomic weight 4) at a pressure of 100 N/mDuring an observation time of 1 second, an atom travelling with the root-mean-square speed parallel to one of the edges of the cube, was found to make 500 hits with a particular wall, without any collision with other atoms. Take R=25/3 J/mol-K and $k=1.38 \times 10^{−23}$ J/K. Evaluate the temperature of the gas
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120 K
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200 K
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160 K
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150 K
Explanation
An atom bouncing between two opposite walls of the 1 m cube (with no collisions with other atoms) makes exactly one hit on a given wall per round trip, covering a distance of 2 × 1 m = 2 m each round trip. 500 hits in 1 second means 500 round trips per second, so time per round trip = 1/500 s. Speed = distance/time = 2 / (1/500) = 1000 m/s — this is the rms speed. Using v_rms² = 3RT/M (M = 4 g/mol = 0.004 kg/mol for helium): (1000)² = 3 × (25/3) × T / 0.004 1,000,000 = 25T/0.004 = 6250T T = 1,000,000/6250 = 160 K.
Q.16
Evaluate the average KE per atom
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$3.312 \times 10^{-21} $J
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$2.312 \times 10^{-21} $J
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$3.312 \times 10^{21} $J
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$3.312 \times 10^{-23} $J
Explanation
Average kinetic energy per atom = (3/2)kT = 1.5 × (1.38×10⁻²³) × 160 ≈ 3.312 × 10⁻²¹ J.
Q.17
Evaluate the total mass of the helium gas
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$ 3 \times 10^{-4}$ kg
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$ 2 \times 10^{-4}$ kg
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$ 4 \times 10^{-4}$ kg
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$ 5 \times 10^{-4}$ kg
Explanation
Using PV = nRT with the temperature found above (T = 160 K): n = PV/(RT) = (100 × 1) / ((25/3) × 160) = 100 / (4000/3) = 300/4000 = 0.075 mol Mass = n × M = 0.075 mol × 4 g/mol = 0.3 g = 3 × 10⁻⁴ kg.
Q.18
For Boyle’s law to hold good, the gas should be
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perfect and at constant temperature but variable mass
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perfect and of constant mass and temperature
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real and at constant temperature but variable mass
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real and of constant mass and temperature.
Explanation
Boyle's law (PV = constant at fixed T) strictly applies to an ideal ("perfect") gas, and requires both the amount of gas (mass/moles) and the temperature to be held constant while pressure and volume vary.
Q.19
A vessel of volume V contains a mixture of 1 mole of Hydrogen and 1 mole of Oxygen (both considered as ideal). Let f1(v)dv, denote the fraction of molecules with speed between v and (v + dv) with f2 (v)dv, similarly for oxygen. Then
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f1(v) + f2 (v) =f(v) obeys the Maxwell’s distribution law.
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f1(v), f2(v) will obey the Maxwell’s distribution law separately.
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Neither f1 (v), nor f2 (v) will obey Maxwell’s distribution law.
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f2(v) and f1 (v) will be the same
Explanation
Each gas species in an ideal-gas mixture independently follows its own Maxwell-Boltzmann speed distribution, based on its own molecular mass — the presence of the other gas doesn't change this (assuming only elastic collisions, no chemical interaction). So f₁(v) for hydrogen and f₂(v) for oxygen each separately obey Maxwell's distribution law, using their own respective molar masses.
Q.20
When an ideal gas is compressed adiabatically, its temperature rises: the molecules on average have more kinetic energy than before. The kinetic energy increases
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because of collisions with moving parts of the wall only.
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because of collisions with the entire wall.
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because the molecules gets accelerated in their motion inside the volume
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because of the redistribution of energy amongst the molecules.
Explanation
A gas molecule bouncing off a STATIONARY wall just reverses its velocity component with no change in speed or energy. But a molecule that collides with a MOVING wall (the piston advancing inward during compression) rebounds with EXTRA speed, gaining kinetic energy from the piston's motion — much like a ball gains speed bouncing off an approaching bat. This is exactly the mechanism by which adiabatic compression raises a gas's average molecular kinetic energy (and hence its temperature).
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