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Work an engine does per cycle

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Here's the actual problem:

A heat engine using 3.30 grams of helium gas is initially at STP. The gas goes through the following closed cycle:
-Isothermal compression until the volume is halved.
-Isobaric expansion until the volume is restored to its initial value.
-Isochoric cooling until the pressure is restored to its initial value.

A) How much work does this engine due per cycle? Express your answer in joules.

B) What is its thermal efficiency? Express your answer in percent.

Solution Preview

A) Molecular weight of Helium = 4 grams/mol
Number of moles of Helium = 3.30/4 = 0.825 mol
Initial Volume = V1 = 22.4L = 0.0224 m^3
V2 = V1/2 = ...