NEET 2016 Phase 2 · ChemistryPrevious Year Question

For a sample of perfect gas when its pressure is changed isothermally from $p_{i}$ to $p_{f}$ , the entropy change is given by

Answer: (B) $Δ S = n R ln (\frac{p _{i}}{p _{f}})$ . \textbf{Answer:} (B) $Δ S = n R ln (\frac{p _{i}}{p _{f}})$ \textbf{Solution:} For an isothermal change of an ideal gas, $Δ S = n R ln (\frac{V _{f}}{V _{i}})$ .

A. $Δ S = n R ln (\frac{p _{f}}{p _{i}})$
B. $Δ S = n R ln (\frac{p _{i}}{p _{f}})$ ✓
C. $Δ S = n R T ln (\frac{p _{f}}{p _{i}})$
D. $Δ S = R T ln (\frac{p _{i}}{p _{f}})$

Correct Answer

(B) $Δ S = n R ln (\frac{p _{i}}{p _{f}})$

Solution & Explanation

\textbf{Answer:} (B) $Δ S = n R ln (\frac{p _{i}}{p _{f}})$ \textbf{Solution:} For an isothermal change of an ideal gas, $Δ S = n R ln (\frac{V _{f}}{V _{i}})$ . At constant temperature, $p V =$ constant, so $V \propto 1/ p$ , giving $\frac{V _{f}}{V _{i}} = \frac{p _{i}}{p _{f}}$ . Therefore $Δ S = n R ln (\frac{p _{i}}{p _{f}})$ . (The entropy increases on expansion, when $p_{f} < p_{i}$ .)

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For a sample of perfect gas when its pressure is changed isothermally from pi​ to pf​, the entropy change is given by

Solution & Explanation

More NEET PYQ solutions

For a sample of perfect gas when its pressure is changed isothermally from $p_{i}$ to $p_{f}$ , the entropy change is given by