Phosphorus Pentoxide (P2O5)

What is Phosphorus Pentoxide?

Phosphorus Pentoxide is a chemical compound whose empirical formula is P2O5 and whose molecular formula is P4O10. At ambient temperatures, this compound exists as a white solid that is crystalline in nature. Phosphorus pentoxide is an acid anhydride that is obtained from phosphoric acid. It is highly hygroscopic and is, therefore, used as a dehydrating agent and as a desiccant.


P2O5 (or P4O10) can be prepared by burning elemental P4 (tetra-phosphorus) in an environment containing a sufficient amount of oxygen. The chemical reaction for the preparation of P2O5 is provided below.

P4 + 5O2 → P4O10

This compound is a source of pure phosphoric acid.

Structure of Phosphorus Pentoxide

The structure of a phosphorus pentoxide molecule is illustrated below.

Phosphorus Pentoxide Structure

Structure of Phosphorus Pentoxide

It can be noted that the P=O bond length in this compound is 143 picometers and the P-O bond length is 160 picometers.

Properties of Phosphorus Pentoxide

Physical Properties

  • Phosphorus pentoxide is a white solid which does not have any distinct odour.
  • The chemical formula of this compound is P4O10. However, it is named after its empirical formula, which is P2O5.
  • The molar mass of phosphorus pentoxide corresponds to 283.9 g/mol.
  • The density of this solid is 2.39 g/cm3.
  • The melting point of this P2O5 is 613K. Its boiling point corresponds to 633K (however, it undergoes sublimation at this temperature).

Chemical Properties

  • Phosphorus pentoxide undergoes violent hydrolysis when exposed to water. This reaction is highly exothermic.
  • This compound crystallizes in four different polymorphs.
  • It is highly corrosive and must, therefore, be kept away from metals.

Uses of P2O5

  • This compound is a very potent dehydrating agent (since it undergoes hydrolysis in an exothermic manner).
  • Phosphorus pentoxide is also used in some organic synthesis reactions. For example, it is used in the preparation of nitriles from primary amides.
  • It is a vital component of the Onodera reagent, which is widely used in the oxidation of alcohols.
  • This compound can also be used to convert certain mineral acids into their anhydrides. For example, N2O5 can be prepared from HNO3 with the help of P2O5.

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