Entropy change for isothermal expansion of a perfect gas

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SUMMARY

The discussion focuses on calculating the entropy change for the isothermal expansion of an ideal gas, specifically 0.85 mol of gas transitioning from 350 Torr to 125 Torr. The relevant equations used include the ideal gas law (PV=nRT) and the entropy change formula (ΔS = nRln(Vf/Vi)). The solution involves recognizing that the ratio of volumes can be derived from the pressure change without needing to find the actual volumes, leading to an entropy change of 7.276 J/K.

PREREQUISITES
  • Understanding of the ideal gas law (PV=nRT)
  • Knowledge of entropy change calculation (ΔS = nRln(Vf/Vi))
  • Familiarity with isothermal processes in thermodynamics
  • Basic algebra for manipulating equations and ratios
NEXT STEPS
  • Study the derivation of the ideal gas law and its applications
  • Learn about isothermal processes and their characteristics in thermodynamics
  • Explore advanced entropy calculations for different thermodynamic processes
  • Investigate the implications of entropy change in real-world gas expansion scenarios
USEFUL FOR

This discussion is beneficial for students in thermodynamics, particularly those studying ideal gas behavior, as well as educators and professionals involved in teaching or applying thermodynamic principles.

Pete_01
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Homework Statement



At a constant temperature, 0.85 mol of an ideal gas changes its pressure from 350 Torr to 125 Torr. Calculate the entropy change for this expansion process.


Homework Equations



Ideal gas: PV=nRT
ΔS = nRln(Vf/Vi)

The Attempt at a Solution



I'm stuck on how to find the volume given the change in pressure, and it being an isothermal process. What am I missing?
 
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Since T is constant, so is the righthand side of the ideal gas law. Hence you know that P1V1=P2V2. Note you don't have to find the actual volumes; you just need their ratio.
 
vela said:
Since T is constant, so is the righthand side of the ideal gas law. Hence you know that P1V1=P2V2. Note you don't have to find the actual volumes; you just need their ratio.

Ok, perfect thank you. I was wondering if that was how to solve it. So then it would become:

(0.85 mol)(8.314 J/Kmol)ln(2.8) = 7.276 J/K

Correct?
 
Looks good. (I'm assuming you have the right equation for the entropy. I don't know them off the top of my head.)
 
vela said:
Looks good. (I'm assuming you have the right equation for the entropy. I don't know them off the top of my head.)

Yes, I do. Thanks again!
 

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