Pressure P and Volume V are inversely proportional when Temperature T and mass n are held constant:

P ∝ 1/V

where V changes by a factor of z

P_{final} = 1/z x V_{initial}

For example, calculate the final volume of a gas if the pressure of a 4.0 L sample is changed from 2.5 atm to 5.0 atm.

You calculate z = P_{final}/P_{initial}

z = 5.0 / 2.5

z = 2

P_{final} = 1/z x V_{initial}

P_{final} = 1/2 x 4.0 L

P_{final} = 2.0 L