Thermal Physics - NEET Physics Questions
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Thermal Physics

Question 391: easy

At constant volume temperature is increased then: (1989)

1. Collision on walls will be less
2. Number of collisions per unit time will increase
3. Collisions will be in straight lines
4. Collisions will not change
View Answer

An increase in temperature raises the thermal agitation (root-mean-square speed) of gas molecules. This causes them to travel faster, increasing the frequency of their collisions with the container walls.

Question 392: easy

The volume occupied by the molecules contained in $4.5 \text{ kg}$ water at STP, if the intermolecular forces vanish away is: (2022)

1. $5.6 \text{ m}^3$
2. $5.6 \times 10^6 \text{ m}^3$
3. $5.6 \times 10^3 \text{ m}^3$
4. $5.6 \times 10^{-3} \text{ m}^3$
View Answer

At STP, 1 mole of an ideal gas occupies $22.4 \text{ L}$. The number of moles in $4.5 \text{ kg}$ of water is $n = \frac{4500 \text{ g}}{18 \text{ g/mol}} = 250 \text{ moles}$. The volume is $V = 250 \times 22.4 \text{ L} = 5600 \text{ L} = 5.6 \text{ m}^3$.

Question 393: easy

A cylinder contains hydrogen gas at pressure of $249 \text{ kPa}$ and temperature $27^\circ\text{C}$. Its density is : ($R = 8.3 \text{ J mol}^{-1} \text{ K}^{-1}$) (2020)

1. $0.2 \text{ kg/m}^3$
2. $0.1 \text{ kg/m}^3$
3. $0.02 \text{ kg/m}^3$
4. $0.5 \text{ kg/m}^3$
View Answer

Using the ideal gas law in terms of density, $P = \frac{\rho RT}{M}$, we get $\rho = \frac{PM}{RT}$. For hydrogen, $M = 2 \times 10^{-3} \text{ kg/mol}$. Thus, $\rho = \frac{249 \times 10^3 \times 2 \times 10^{-3}}{8.3 \times 300} = 0.2 \text{ kg/m}^3$.

Question 394: easy

An ideal gas equation can be written as $P = \frac{\rho RT}{M_0}$ where $\rho$ and $M_0$ are respectively, (2020-Covid)

1. Number density, molar mass
2. Mass density, molar mass
3. Number density, mass of the gas
4. Mass density, mass of the gas
View Answer

In the equation $P = \frac{\rho RT}{M_0}$, $\rho$ represents the mass density (mass per unit volume) of the gas, and $M_0$ is the molar mass of the gas.

Question 395: easy

Increase in temperature of a gas filled in a container would lead to: (2019)

1. Increase in its mass
2. Increase in its kinetic energy
3. Decrease in its pressure
4. Decrease in intermolecular distance
View Answer

According to the kinetic theory of gases, the average kinetic energy of gas molecules is directly proportional to its absolute temperature ($E_k \propto T$). Therefore, an increase in temperature increases its kinetic energy.

Question 396: easy

A given sample of an ideal gas occupies a volume $V$ at a pressure $P$ and absolute temperature $T$. The mass of each molecule of the gas is $m$. Which of the following gives the density of the gas? (2016 – II)

1. $P/(kTV)$
2. $mkT$
3. $P/(kT)$
4. $Pm/(kT)$
View Answer

From the ideal gas equation $PV = NkT$, the number density is $n = \frac{N}{V} = \frac{P}{kT}$. The mass density $\rho$ is mass per unit volume, so $\rho = m \times n = \frac{Pm}{kT}$.

Question 397: easy

Two vessels separately contain two ideal gases $A$ and $B$ at the same temperature, the pressure of $A$ being twice that of $B$. Under such conditions, the density of $A$ is found to be $1.5$ times the density of $B$. The ratio of molecular weight of $A$ and $B$ is: (2015 Re)

1. $1/2$
2. $2/3$
3. $3/4$
4. $2$
View Answer

We know $M = \frac{\rho RT}{P}$. Given $T_A = T_B$, $P_A = 2P_B$, and $\rho_A = 1.5\rho_B$. The ratio of molecular weights is $\frac{M_A}{M_B} = (\frac{\rho_A}{\rho_B}) \times (\frac{P_B}{P_A}) = 1.5 \times \frac{1}{2} = 0.75 = \frac{3}{4}$.

Question 398: easy

At $10^\circ\text{C}$ the value of the density of a fixed mass of an ideal gas divided by its pressure is $x$. At $110^\circ\text{C}$ this ratio is (2008)

1. $\frac{283}{383}x$
2. $x$
3. $\frac{383}{283}x$
4. $\frac{10}{100}x$
View Answer

Since $\frac{\rho}{P} = \frac{M}{RT}$, the ratio is inversely proportional to the absolute temperature $T$. Thus, $x_2 = x_1 (\frac{T_1}{T_2}) = x \times \frac{10 + 273}{110 + 273} = x \times \frac{283}{383}$.

Question 399: easy

The equation of state for $5 \text{ g}$ of oxygen at a pressure $P$ and temperature $T$, when occupying a volume $V$, will be: (2004)

1. $PV = 5 RT$
2. $PV = (5/2) RT$
3. $PV = (5/16) RT$
4. $PV = (5/32)RT$
View Answer

The number of moles $n = \frac{\text{given mass}}{\text{molar mass}}$. For $5 \text{ g}$ of $O_2$ gas, $n = \frac{5}{32}$. Substituting this into the ideal gas equation $PV = nRT$, we get $PV = (\frac{5}{32})RT$.

Question 400: moderate

At $0\text{ K}$ which of the following properties of a gas will be zero? (1996)

1. Vibrational energy
2. Density
3. Kinetic energy
4. Potential energy
View Answer

According to kinetic theory of gases, at absolute zero ($0\text{ K}$), all molecular motion stops. Thus, kinetic energy becomes zero.