CHEMISTRY COMMUNITY

Could someone explain why we can replace PΔV = ΔnRT for P(ex)Δv in the equation for work at a constant pressure?

For example, say you had a balloon with an ideal gas expanding to twice its volume at an unstated, constant external pressure. The pressure inside the balloon decreases as volume increases. Then, if you calculate PV = nRT, wouldn't this be different for internal vs external throughout the volume change? Why can we replace it for the external pressure in the work equation?

Work is the ability to move/push against an external force/pressure, which is why P external is used. In chemistry we view work from the point of view of the system.

To add onto this using gasses as an example, P external must be used because work is achieved through expansion or compression against a constant external pressure or force, not against the internal pressure of the system.