# Homework Help: Ideal gas entropy

1. Apr 8, 2010

### mahdert

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

what is the entropy of 1.0 litre of gasoues n2 at Ta=350 K and Pa=2.0 bar given that So = 191.61 J/(K gmol) at To = 298.15 K and Po = 1 bar?

2. Relevant equations
I think : S(at 350) - So = NC_p ln (Ta/To) - NR ln(Pa/po)

3. The attempt at a solution
My problem is, C_p is not given and can we use this formula if p is not constant.

2. Apr 8, 2010

### Mapes

1) Are you sure you don't know $c_P$ for an ideal gas?
2) To see if you can use this equation even if P is changing, how about starting from $dU=T\,dS-P\,dV$, isolating dS, and integrating?

3. Apr 8, 2010

### mahdert

C_p = 5/2 R;

I did the integration and I got the right answer. But the question remains, the specific heat capacity at constant pressure; C_p should be used at constant pressure. Here, the pressure is changing? A qualitative expalnation would suffice. Thanks

4. Apr 8, 2010

### Mapes

The pressure isn't changing; there's no process occurring. $c_P$ is just a variable.