0.75 lbm of oxygen is initially at 30 psi and 50oF. The oxygen expands in an isobaric process until its volume is 6.2 ft3. Assuming the oxygen behaves as an ideal gas with constant specific heats, determine the change in total internal energy and total enthalpy of the oxygen during the process.
Given: m = 0.75 lbm; P1 = P2 = 30 psi (isobaric process); T1 = 50oF = 510 R; V2 = 6.2 ft3
What will be an ideal response?
First find the temperature at state 2, from the ideal gas law:
T2 = P2V2/mR
For O2, R = 0.3353 psi-ft3/lbm-R
So, T2 = (30 psi)(6.2 ft3)/(0.75 lbm)(0.3353 psi-ft3/lbm-R) = 739.6 R = 280°F
Also, for O2, cp = 0.219 Btu/lbm-R, cv = 0.157 Btu/lbm-R at 540 R
For an ideal gas with constant specific heats:
?H = m cp (T2 – T1) = 37.7 Btu
?U = m cv (T2 – T1) = 27.0 Btu
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