The volume of the container decreases to one-half its previous volume.
Work Step by Step
In this reaction, we are converting two moles of gas to one mole. Because we are able to use a moveable piston in order to keep the pressure constant, the container would now have a smaller volume as we have fewer moles of gas. Because we are halving the number of moles of gas, the volume of the container is also halved to maintain constant pressure.