Final answer:
No work is done by the gas when it expands against a vacuum as there is no external pressure opposing the expansion.
Step-by-step explanation:
The question is asking to calculate the work done by a gas that expands against a vacuum. When a gas expands against a vacuum, there is no external pressure opposing the expansion, and thus, no work is done. The formula for work done by the gas at constant temperature is W = PΔV, where P is the external pressure and ΔV is the change in volume. Since the external pressure is zero in a vacuum, no matter how much the volume changes, the work done will be zero joules.