# Question #d7edd

##### 1 Answer
Apr 10, 2015

The gas relationship $\frac{{P}_{1} {V}_{1}}{T} _ 1 = \frac{{P}_{2} {V}_{2}}{T} _ 2$

is only applicable to fixed quantities (number of moles or mass) of gas.

This relationship is closely related to the ideal gas law $P V = n R T$

which makes assumptions (like the volume of the gas molecule itself is negligible) and may break down under low temperature or high pressure conditions. This equation considers the quantity of gas (n = number of moles) and might be the 4th variable you are looking for.