A gas occupies 500 cm3 at 2 atm. At constant temperature, what volume will it occupy at 1 atm?

Explanation

Using P1V1 equals P2V2, the new volume is 2 multiplied by 500 divided by 1, which is 1000 cm3. Halving the pressure doubles the volume, so an answer smaller than the original must be wrong at a glance. Checking the direction of the change before calculating catches most errors.