Mean particle mass ($\bar{m}=\rho /n$)

The mean particle mass is the mass density divided by the total number density of freely moving particles. It enters $P=\rho k_{B}T/\bar{m}$. 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.