Entropy change for isothermal expansion of a perfect gas

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Discussion Overview

The discussion revolves around calculating the entropy change for the isothermal expansion of an ideal gas, specifically focusing on a scenario where 0.85 mol of gas changes its pressure from 350 Torr to 125 Torr. Participants explore the relationship between pressure and volume in the context of the ideal gas law and the relevant equations for entropy change.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant expresses uncertainty about how to find the volume given the change in pressure during the isothermal process.
  • Another participant clarifies that since the temperature is constant, the relationship P1V1 = P2V2 can be used, indicating that the actual volumes are not necessary, only their ratio.
  • A participant attempts to calculate the entropy change using the formula ΔS = nRln(Vf/Vi) and presents a numerical result based on their understanding of the gas behavior.
  • Several participants confirm the approach and calculations, expressing confidence in the use of the entropy equation, although one notes uncertainty about the equation itself.

Areas of Agreement / Disagreement

Participants generally agree on the approach to solving the problem using the ideal gas law and the entropy equation, but there is no explicit consensus on the final numerical result or the specific values used in the calculations.

Contextual Notes

The discussion does not resolve the specific values for volume or the final entropy change, as these depend on the ratio of pressures and the assumptions made regarding the ideal gas behavior.

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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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