combined law

The combined gas law is a combination of Boyle's Law and Charles's Law; hence its name the combined gas law. In the combined gas law, the volume of gas is directly proportional to the absolute temperature and inversely proportional to the pressure.

This can be written as PV / T = constant. Since for a given amount of gas there is a constant then we can write P1V1 / T1 = P2V2 / T2.
  • P1 is the initial pressure
  • V1 is the initial volume
  • T1 is the initial temperature (in Kelvin)
  • P2 is the final pressure
  • V2 is the final volume
  • T2 is the final temperature (in Kelvin)

explanation of combined gas law variables


This equation is useful if you have the current volume, temperature, and pressure of a gas, and if you have two of the three final values of the gas.

For example if you have 4.0 liters of gas at STP, and you want to know the volume of the gas at 2.0 atm of pressure and 30o C, the equation can be setup as follows:
(1.0)(4.0) / 273 = (2.0)(V2) / 303
(V2)(2)(273) = (1)(4)(303)
V2 = 2.2
Therefore the new volume is 2.2 liters.