The Combined Gas Law is not one law, but a combination of Boyle’s and
Charles’ laws. Boyle’s law describes the relationship between a
pressure exerted on a gas and it’s volume. Charles’ law describes
the relationship between the temperature of a gas and it’s volume. The
Combined Gas Law also involves another gas law made by Joseph Gay-Luusac, Lussac’s
Law. Lussac’s Law describes the relationship between the pressure and
temperature of a gas. The Combined Gas Law says that the pressure of a gas
times the volume of a gas, then that number divided by the temperature of a
gas, will equal the same as the pressure of another gas multiplied by the volume,
then divided by the temperature. In this law all temperature units must be
in degrees Kelvin, and pressure units can be anything as long as they are constant.
The law is written as (P1 V1)/T1=(P2 V20/T2

For example,

If one gas has a pressure of 10 psi, volume of 30 ml, and temperature of 200
degrees; and the same gas has a pressure of 20 psi, volume of 30 ml, and temperature
of unknown degrees. Then

10 psi x 30 ml 20 psi x 20 ml

------------------- = --------------------

200 Degrees K unknown Degrees K

300 400

----- = --------

200 X

300X=80,000

X= 266.33 Degrees K

or

One gas has a pressure of 20 psi, volume of 50 ml, and temperature of 100 degrees;
when that gas has a pressure of 40 psi, unknown volume, and temperature of
400 degrees, what is the volume?

20 x 50 40X

----------- = ---------

100 400

1000 40X

-------- = ----------

100 400

10=.1X

X=100 ml