We can do even better than this. Given the pressure and temperature, we can calculate the volume that a mole of gas should occupy, and can find out how the volume changes as a gas is expanded or compressed, and heated or cooled. The relationship between pressure (P), volume (V), temperature (T), and number of moles (n), is given by the ideal gas law,

PV = nRT

in which R is a constant. But to understand what this gas law means and how to use it, we must back up a step or two.