Gases: The Combined Gas Law   
The combined gas law is not a new law but a combination of Boyle's and Charles' laws, hence the name the combined gas law.   In short, this combined gas law is used when it is difficult to keep either the temperature or pressure constant.  In many experiments with gases, keeping either the pressure or temperature constant is not even attempted.  The combined gas law equation is:
P1V1 T2 =  P2V2T1
The temperature must be in kelvins, but the pressure and volume can be in any units as long as they are used consistently; that is, both pressure values must be in the same pressure units and both volume values must be in the same volume units.
This law also contains a third law that is implied but not explicitly stated.  The pressure-temperature law was discovered by Jacques Charles and Joseph Gay-Lussac (1778-1850), a French scientist.  It was not given a name because it is easily obtained by using Boyle's and Charles' Laws properly, however, it is given below for your enjoyment.  If the volume of a gas is fixed during an experiment then the pressure and temperature vary in a specific way.
This law, known as Gay-Lussac's Law is:         P1T2 = P2T1

           Go to the Combined Gas Law Worksheet