A gas occupies 500 cm3 at 2 atm. At constant temperature, what volume will it occupy at 5 atm?

Explanation

Applying P1V1 equal to P2V2 gives V2 equal to 2 times 500 divided by 5, which is 200 cm3. The pressure rose by a factor of 2.5, so the volume must fall by the same factor, and 500 divided by 2.5 is 200. Getting 1250 means the ratio was applied the wrong way round.