# What variable stays constant when using the combined gas law?

Jun 5, 2017

There is no single variable that never changes in the combined gas law:

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

This rule applies only to an unchanged sample of gas (none added or removed), though, so the number of moles of gas remains constant.

#### Explanation:

I do wonder whether the questioner had the Ideal Gas Law in mind:

$P V = n R T$

In that case, $R$, the ideal gas constant, remains constant, but may be expressed in different units.