According to the Encyclopaedia Britannica, N2O5 is the chemical formula for dinitrogen pentoxide. It is one of the oxides of nitrogen, a group of compounds that are made up exclusively of nitrogen and oxygen.
Dinitrogen pentoxide exists as a colorless solid that is formed when phosphorus oxide reacts with nitric acid. It is unstable at temperatures higher than room temperature and breaks down into N204, dinitrogen tetroxide, and O2, an oxygen molecule.
According to Reference.com, dinitrogen pentoxide is a potentially dangerous oxidizer that was previously used as a reagent for nitration. It has since been almost entirely replaced in this capacity by NO2BF4, nitronium tetrafluoroborate.