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GATE 2019 ME (ME1) – Question 34

Thermodynamics · Entropy changes, closed and open systems · 1 mark · Numerical answer

One kg air initially at 300 K and 10 bar expands isothermally to 1 bar. Treat it as ideal with R = 0.287 kJ/(kg·K). Find entropy change (kJ/(kg·K)), to two decimal places.

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Correct answer: .64 to .68

Explanation

For isothermal ideal-gas expansion, $\Delta S=mR\ln(p_1/p_2)=1(0.287)\ln10=0.660842$ kJ/K. To two decimals, **0.66 kJ/K**.