why is the molecular formula of oide of arsenic in its gaseous state is not neccessarialy the same as its empirical formula

Question

what does this mean and how do i do it

Molecules are known to dimerize or trimerize. We actually don't know what the molecular formula is until we measure it. The empirical formula is just the simplest formula possible based on numbers; however, a molecule may be forced to dimerize in order to satisfy bonding requirements.

Which dimerizes, No2, No2-, or No2+