Question about today's lecture 1/24/18

[Michelle Chernyak 1J]

In today's lecture, Dr. Lavelle stated that in the isothermal reversible reaction, the gas expanding against the vacuum is exerting work. This confused me because I thought that with a vacuum, there is no opposing force, therefore work cannot be done because work= opposing force X distance. Can someone please explain to me why this expanding gas is exerting work?

[Esin Gumustekin 2J]

In that example, the gas was doing work by pulling a weight a vertical distance up. Thus work was done by pulling the mass (force) over a specific distance (d).