Class NEET 2026 ALL Q #1960
COMPETENCY BASED
APPLY
4 Marks 2026 NTA-RE-NEET-2026 MCQ SINGLE
Two moles of an ideal gas undergo free expansion from 10 L to 100 L at 300 K. The values of $\Delta S_{system}$ and $\Delta S_{surroundings}$ are (R is universal gas constant)
(A) $\Delta S_{system}=4.606~R$; $\Delta S_{surroundings}=0$
(B) $\Delta S_{system}=0$; $\Delta S_{surroundings}=0$
(C) $\Delta S_{system}=4.606~R$; $\Delta S_{surroundings}=-4.606~R$
(D) $\Delta S_{system}=0$; $\Delta S_{surroundings}=4.606~R$
Correct Answer: A

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Detailed Solution

Step 1: Analyze the process

The process described is free expansion of an ideal gas. Free expansion occurs against zero external pressure ($P_{ext} = 0$). Since the gas expands into a vacuum, no work is done ($W = 0$).

Step 2: Calculate Entropy change of the system

For an ideal gas, the change in entropy ($\Delta S_{system}$) for an isothermal process is given by the formula: $$ \Delta S_{system} = nR \ln\left(\frac{V_f}{V_i}\right) $$ Given $n = 2$ moles, $V_i = 10$ L, and $V_f = 100$ L: $$ \Delta S_{system} = 2 \times R \times \ln\left(\frac{100}{10}\right) $$ $$ \Delta S_{system} = 2 \times R \times \ln(10) $$ Using $\ln(10) \approx 2.303$: $$ \Delta S_{system} = 2 \times R \times 2.303 = 4.606 R $$

Step 3: Calculate Entropy change of the surroundings

In free expansion, the gas expands into a vacuum. Since $P_{ext} = 0$, the work done by the system on the surroundings is zero. Because the process is isothermal and no work is exchanged, there is no heat exchange with the surroundings ($q_{surr} = 0$). Therefore: $$ \Delta S_{surroundings} = \frac{-q_{sys}}{T} = 0 $$

Final Answer: Option (A)

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student must identify the specific thermodynamic conditions of free expansion and apply the entropy formula for isothermal expansion.
Knowledge Dimension: PROCEDURAL
Justification: The student must follow a sequence of steps: recognizing the nature of the process, selecting the correct entropy formula, and performing the logarithmic calculation.
Syllabus Audit: In the context of NEET, this is classified as COMPETENCY. It tests the conceptual understanding of the Second Law of Thermodynamics and the distinction between system and surroundings in irreversible processes.

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