Entropy change for isothermal expansion of a perfect gas

[Pete_01]

1. The problem statement, all variables and given/known data

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.


2. Relevant equations

Ideal gas: PV=nRT
ΔS = nRln(Vf/Vi)

3. 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?

[vela]

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.

[Pete_01]

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?

[vela]

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

[Pete_01]

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!