Question

What variable stays constant when using the combined gas law?

Answer 1

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

`(P_1V_1)/T_1 = (P_2V_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:

`PV = nRT`

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