The heat released to change the state of 2 kg of air from 400°C and 400 kPa to 100 kPa in a rigid container is nearest:

A) 800 kJ
B) 725 kJ
C) 690 kJ
D) 520 kJ


B) 725 kJ

The temperature at the final state must be known. It is

Q = mC ?T = 2× 0.717× (673-168) = 724 kJ

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