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The combined gas law is the compilation of Boyle's, Charles', and Gay-Lussac's laws.

+    +    =  

The combined gas law equation is the relationship of changing pressure, temperature, and volume of a gas.  Two identical quantities of the same gas are measured at two different conditions where pressure, temperature, and volume all change at once due to the effects they have on each other.  These three variables, therefore, must be dealt with at the same time.  To do this, set the two sets of conditions equal to each other.  If five of the six variables are known, you will be able to solve for the unknown variable.  

If one variable is held constant in both sets of conditions, you can cancel that variable on both sides of the equation, thereby reducing the combined gas law into  Boyle's, Charles', or Gay-Lussac's law.


Example:

Condition One:  
Volume (V):  205 mL
Temperature (T):  295 K
Pressure (P):  30.8 kPa

Condition Two:
Volume (V):  Unknown
Temperature (T):  273 K
Pressure (P):  101.3 kPa

The equation:



Now solve for the unknown volume (V2).