# Homework Help: Entropy of gas at constant pressure and volume

1. Nov 25, 2009

### ricof

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

1m^3 of air is heated reversibly at constant pressure from 288K to 573K. Then it is cooled reversibly at constant volume back to the initial T. Initial P is 103kPa Calculate overall change in entropy.
Cp=1.02
Cv=0.702

2. Relevant equations

dS=Cp x ln(T2/T1)-R x ln(P2/P1)

3. The attempt at a solution

I have found P2 to be 911kPa but when I put all the data into the above equation I end up with the wrong answer (0.076kJ/K). Please help!

I have found

2. Nov 25, 2009

### vishal007win

tell me how u got 911kPa..
its wrong from thr only...

3. Nov 25, 2009

### ricof

I used T2/T1 = (P2/P2)^(n-1/n)

and n = Cp/Cv

4. Nov 25, 2009

### vishal007win

for 1st proces which is isobaric....
V1/T1=V3/T3... V1= 1m^3 so one can find V3.....3 is the intermediate stage
and then for isochoric process....
P3/T3=P2/T2.....
n bcoz T2= T1=288 and P3=P1.....
n T3=573
so jus find value n den see...
u r goin in wrong direction...dats actually for adiabatic processes