We have 2 (insulated) partitions, one with the gas, another vacuum, separated by a stop cock. We remove the cock, letting the gas expand into vacuum . I understand that since there is no opposing force on the expanding gas ,the work done by the gas is zero . But work done by gas is given by PgasΔV . Now Pgas≠0 and ΔV≠0 , so PgasΔV≠0 , so mathematically how is work done by gas zero ? Please help me clear this doubt ?