Entropy change for isothermal expansion of a perfect gas

AI Thread Summary
The discussion focuses on calculating the entropy change during the isothermal expansion of an ideal gas, where 0.85 mol of gas expands from 350 Torr to 125 Torr. The key formula used is ΔS = nRln(Vf/Vi), with the relationship between pressure and volume (P1V1 = P2V2) being crucial for solving the problem. Participants clarify that the actual volumes are not needed, only their ratio, since the temperature remains constant. The final calculation yields an entropy change of approximately 7.276 J/K. This confirms the correct application of the ideal gas law and entropy equations for the scenario presented.
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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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