Expansion of a gas into a vacuum is called free expansion. In a vacuum, the external pressure ($p_{ex}$) is zero. The work done during the expansion of a gas is given by the equation $w = -p_{ex}\Delta V$. Since $p_{ex} = 0$, the work done $w$ is also zero, regardless of the number of moles of the gas or whether the process is reversible or irreversible.