Final answer:
The change in internal energy of an ideal gas during an isothermal expansion at constant temperature is zero, because all the heat absorbed is used to do work on the surroundings.
Step-by-step explanation:
The question pertains to the change in internal energy (ΔU) of an ideal gas during an isothermal expansion. According to the first law of thermodynamics, for any system the change in internal energy is equal to the heat added to the system minus the work done by the system: ΔU = Q - W. In an isothermal process for an ideal gas, the temperature remains constant, and because internal energy is a function of temperature for an ideal gas, the change in internal energy is zero.
In the scenario described (expanding an ideal gas isothermally from 1L to 10L at 300K), any heat absorbed by the gas would be entirely used to do work on the surroundings during the expansion and therefore, the internal energy remains unchanged. So, the correct answer to the student's question is b. zero.