• # question_answer 73)   Expansion of a gas in vacuum is called free expansion. Calculate work done and the change in internal energy when 1 litre of ideal gas expands isothermally into vacuum until its total volume is 5 litre.

In free isothermal expansion, external pressure is zero hence work will also be zero. $w=P\Delta V=0$ $\Delta U=0$for all isothermal processes.