One mole of an ideal gas undergoes reversible isothermal expansion at 27 deg C until its volume becomes three times the original volume. The entropy change of the universe (delta_S_universe) is:

0
R*ln(3)
-R*ln(3)
R/3

Correct answer: 0

Solution

For any reversible process, by definition delta_S_universe = delta_S_system + delta_S_surroundings = 0. The gas gains entropy R*ln(3) and the surroundings lose the same amount since heat transferred reversibly Q_rev = -Q_surroundings.

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