0.80 mol of an ideal gas expands at constant temperature from 4.0 L into an evacuated vessel, until…

Work against a constant outside pressure: $w = -p_\text{ex}\Delta V$, with $1$ L bar $= 100$ J and $1$ L atm $= 101.3$ J. Free expansion: $w = 0$. Reversible isothermal ideal gas: $w = -2.303\,nRT\log\dfrac{V_f}{V_i}$, and $\Delta U = 0$, so $q = -w$.