Gas density ($\rho =\frac{p\overline{M}}{RT}$)

Gas density is the mass density of a gas. For an ideal gas sample of amount of substance $n$, mean molar mass $\overline{M}$ and volume $V$, combine $m=n\overline{M}$ with $pV=nRT$ to obtainHere $R$ is the molar gas constant. For a homogeneous mixture of gases, $\overline{M}={\sum }_i{x}_i{M}_i$, where $x_i$ is the mole fraction of component $i$. Equal pressure and temperature make a gas density ratio equal to a mean-molar mass ratio within this model. Argon detection in a binary gas mixture gives a worked example and a temperature confounder.

In an idealized rigid-globe weighing, subtract the evacuated assembly's mass from the filled assembly's mass, then divide by the internal volume. Establish the filled sample's pressure and temperature before comparing gases. The subtraction removes the container's large contribution, making a small gas contribution accessible. A real measurement also needs corrections or bounds for residual gas, adsorption, leaks, changing vessel volume and external buoyancy. The Rayleigh-Ramsay argon density experiment shows why preparation-route comparisons add information beyond repeated weighings of one sample.