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Thermodynamics question

2016 · Q107
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Thermodynamics question

2016 · Q107

NEETChemistryThermodynamicsMCQ+4 / −1
For a sample of perfect gas when its pressure is changed isothermally from pi to pf, the entropy change is given by
  1. A
    ΔS=nR ln⁡(pfpi)\Delta S = nR\,\ln \left( {{{{p_f}} \over {{p_i}}}} \right)ΔS=nRln(pi​pf​​)
  2. B
    ΔS=nR ln⁡(pipf)\Delta S = nR\,\ln \left( {{{{p_i}} \over {{p_f}}}} \right)ΔS=nRln(pf​pi​​)
  3. C
    ΔS=nRT ln⁡(pfpi)\Delta S = nRT\,\ln \left( {{{{p_f}} \over {{p_i}}}} \right)ΔS=nRTln(pi​pf​​)
  4. D
    ΔS=RT ln⁡(pipf)\Delta S = RT\,\ln \left( {{{{p_i}} \over {{p_f}}}} \right)ΔS=RTln(pf​pi​​)
View written solutionFree

Correct answer: B

For an ideal gas undergoing reversible expansion, when temperature changes from Ti to Tf and pressure changes from Pi to Pf.

Δ\Delta ΔS = nCp ln TfTi{{{T_f}} \over {{T_i}}}Ti​Tf​​ + nR ln PiPf{{{P_i}} \over {{P_f}}}Pf​Pi​​

For an isothermal process, Ti = Tf so, ln 1 = 0

∴\therefore∴ Δ\Delta ΔS = nR ln PiPf{{{P_i}} \over {{P_f}}}Pf​Pi​​

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