Is the proper formula for phosphorus oxide(V) P4O10 or P2O5?
The simplest types of chemical formulas are called empirical formulas, which use only letters and numbers indicating atomic proportional ratios (the numerical proportions of atoms of one type to those of other types). Molecular formulas indicate the simple numbers of each type of atom in a molecule of a molecular substance, and are thus sometimes the same as empirical formulas (for molecules that only have one atom of a particular type), and at other times require larger numbers than do empirical formulas. (Wikipedia)


For phosphorus pentoxide the molecule seems to be an adamantane-like compound with 4 phosphorus atoms.

So the correct molecular formula of the compound would be $\ce{P4O10}$.