Change in internal energy for isothermal process A gas expands isothermally against a constant external pressure of $1\ \mathrm{atm}$ from a volume of $10\ \mathrm{dm^3}$ to a volume of $20\ \mathrm{dm^3}$. It absorbs $800\ \mathrm J$ of thermal energy from its surrounding.
The change in internal energy is:

I came across this question and my doubt is, for isothermal process, $\Delta U=0$ ( change in internal energy) for ideal gas is zero right? Then how can we calculate $\Delta U=0$ here, assuming the gas to be ideal?
Is the error with the question or am I missing something?
It must not be an ideal gas.  This is irrespective or whether the process is reversible or irreversible.   That is the only possible conclusion.  Otherwise the amount of heat received would adjust  until it was equal to the work done.