= Mean particle mass
{title2=$\bar m=\rho/n$}
The <mean particle mass> is the <mass density> divided by the total <number density> of freely moving particles. It enters $P=\rho k_BT/\bar m$. <ionization> changes the particle count without significantly changing the <mass>, so it changes $\bar m$. The <dimensionless> <mean molecular weight> is $\bar m$ divided by a stated reference <mass>, commonly the atomic <mass> unit; it must not be substituted as a <dimensionless> number into a formula requiring a <mass>.
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