In case of expansion, work is done by system.

Now, as we know that,

$\mathrm{W}=-{\mathrm{P}}_{\mathrm{ext}.} ∆\mathrm{V}$

As the gas is expanding into vacuums which has no pressure, i.e.,

${\mathrm{P}}_{\mathrm{ext}.}=0$

$\therefore \mathrm{W}=0$

Hence, no work is done.

From first law of thermodynamics,

$\mathrm{\Delta U}=\mathrm{q}+\mathrm{W}$

As the system is working at constant temperature, i.e., isothermally.

$\therefore \mathrm{\Delta U}=0$

Hence $\mathrm{q}=-\mathrm{W}=0$