Homework Help: Isobaric compression from isotherm to adiabat

1. Sep 1, 2012

nlingraham

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

The figure(Figure 1) shows a thermodynamic process followed by 150 mg of helium. Determine the pressure (in atm) at points 1, 2, and 3. Determine the temperature (in °C) at points 1, 2, and 3.

http://imgur.com/B7lwQ

n=.0375 mol
P1=3 atm
P2=1 atm
T1=971.85 k
T2=971.85 k
V1=1000 cm^3
V2=V3=3000 cm^3

2. Relevant equations

PV=nRT

3. The attempt at a solution

I'm only on the pressure part right now. From the given chart P1=3 atm. I used PV=nRT, and got that T1 and T2 are 971.85 K. With that, I used ideal gas law again to get that P2=1 atm. However, I don't know how to get the pressure at point 3 since I don't know the temperature at that point. It looks like it goes through an isobaric compression from the isotherm to the adiabat to me. Any help is appreciated.

(303000)(.001)=(.0375)(8.314)T, T1=T2=971.85 K

P(.003)=(.0375)(8.314)(971.85), P2=1 atm

Last edited: Sep 1, 2012
2. Sep 1, 2012

rock.freak667

From 1 to 3, the He goes through an adiabatic process such that

PVγ= constant

So you will have to use this to get P3

3. Sep 1, 2012

nlingraham

Ok, I get the PVγ, where γ=5/3 since its a monatomic gas, but what exactly is the constant? Is that supposed to be nR?

4. Sep 1, 2012

nlingraham

Ok, I think I got it. So it would be P2V2γ=P1V1γ and then just solve for P2

5. Sep 1, 2012