= Gas density
{title2=$\rho=\frac{p\overline M}{RT}$}
<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 (thermodynamics)> $V$, combine $m=n\overline M$ with $pV=nRT$ to obtain
$$
\rho=\frac{m}{V}=\frac{p\overline M}{RT}.
$$
Here $R$ is the <molar gas constant>. For a homogeneous <mixture> of <gases>, $\overline M=\sum_i x_iM_i$, 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.
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