 #1
miekier
 2
 0
 Homework Statement:
 500 kg of O2 gas with a molar mass of 32 g/mol is compressed by a polytropic compressor with constant k = 1.3. Initial pressure is 1 bar, pressure after compression is 3 bar. The initial temperature is 15°C = 288.15 K. This process takes 50 minutes. Calculate the work done and the power.
 Relevant Equations:

P = W/t
p*V^(k) = constant
W = (p1*V1p2*V2)/(k1)
p*V = n*R*T
First i calculate n:
500000 g / 32 g/mol = 15625 mol
Then I calculate V1:
(100000 Pa * V1)/288.15 K = 15625 mol * 8.31 J/mol.k
so V1 = 374.145 m^3
I calculate V2:
100000¨ Pa * (374.145 m^3)^1.3 = 300000 Pa * (V2)^1.3
So V2 = 160.703 m^3
W = ((100000 Pa * 374.145 m^3)(300000 Pa * 160.703 m^3))/(1.31) = 35988000 J?
I know my work is supposed to be positive, and the only value that can be wrong is V2 I think? Although that calculation seems fine to me, so I don't understand why the result for W is negative.
500000 g / 32 g/mol = 15625 mol
Then I calculate V1:
(100000 Pa * V1)/288.15 K = 15625 mol * 8.31 J/mol.k
so V1 = 374.145 m^3
I calculate V2:
100000¨ Pa * (374.145 m^3)^1.3 = 300000 Pa * (V2)^1.3
So V2 = 160.703 m^3
W = ((100000 Pa * 374.145 m^3)(300000 Pa * 160.703 m^3))/(1.31) = 35988000 J?
I know my work is supposed to be positive, and the only value that can be wrong is V2 I think? Although that calculation seems fine to me, so I don't understand why the result for W is negative.