For gases, the density is obtained by using an equation of state.

An **equation of state** relates the molar density (or
specific molar volume) of a fluid (so they sometimes work for
liquids, too!) to the temperature and pressure of the fluid.

simplest -> ideal gas law (PV=nRT)!

The ideal gas law may also be written as $P \dotV =\dot n RT$ for a flowing system or $P \hat V = RT$, where $\hat V$ is the specific molar volume.

The ideal gas law is an approximation (!) that has only limited applicability. It is usually used for diatomic gas when (RT/P)>5 L/mol and for other gases when (RT/P) > 20 L/mol (i.e., at high specific volumes!).

A technique for dealing with ideal gases that is of dubious utility is to utilize the reference point of standard temperature and pressure.

**STP** is an arbitrary reference point chosen to be T = 273K
(0C) and P = 1 atm.

Use the ideal gas law to determine P, V, or T of a single component