Final answer:
To calculate the entropy change of the universe for a not-completely-irreversible isothermic expansion, consider the entropy change of the system and surroundings; the entropy change of an ideal gas in a free expansion is the entropy change of the system, which is positive, since the surroundings' entropy does not change.
Step-by-step explanation:
To find the entropy change of the universe for a not-completely-irreversible isothermic expansion, one must consider both the entropy change of the system and its surroundings. Entropy is a measure of disorder, and its change is given by the formula ΔS = q/T for a reversible process, where q is the heat absorbed by the system and T is the absolute temperature at which the process occurs.
An isothermic expansion means the process occurs at a constant temperature. If the process is not completely reversible, there will be a net increase in entropy. For an ideal gas expanding into a vacuum, the entropy change of the surroundings is zero because no heat is exchanged with the surroundings. Thus, the entropy change of the universe would be the sum of the entropy change of the system, which is positive, and the entropy change of the surroundings, which is zero.
The total entropy change of the universe ΔSuniverse is given as ΔSuniverse = ΔSsystem + ΔSsurroundings. In cases where it is not a free expansion (i.e., the gas does work on the surroundings or work is done on the gas), you would have to calculate the entropy change for the surroundings based on the heat exchange and add it to the system's entropy change.