## College Physics (4th Edition)

The SI units of mass density are: $~kg~m^{-3}$ The SI units of number density are: $~m^{-3}$ If the number density is equal while the masses of the two different kinds of gas particles are different, then the gases will have different mass densities.
The SI units of mass density are: $~kg~m^{-3}$ The SI units of number density are: $~m^{-3}$ If two different gases have the same number density, it means that the number of particles per unit volume is equal for each gas. However, if the masses of the two different kinds of gas particles are different, then the gases will have different mass densities.