Hypophosphorous Acid Formula

Hypophosphorous Acid

Hypophosphorous acid (HPA) also known as phosphonic acid is a phosphorous oxoacid and also acts as a good reducing agent. It is basically a colourless aqueous solution that is soluble in water, dioxin as well as in alcohol. The hypophosphorous acid formula which is the same for phosphinic acid formula is H3PO2 is a neutral molecule with no spare charge on it. Thus the oxidation number of the central atom phosphorus is +1 which can be described as follows.

As the whole charge on the H3PO2 molecule is zero. Charge on a single hydrogen atom is one and there are three atoms present in the molecules that make it 3. Similarly, the valency of a single oxygen atom is (-2) and there are 2 oxygen atoms present in the molecule. Thus the formula would be considering the oxidation state of the phosphorus atom as unknown as therefore represented as X.

[1x3] + X + [(-2) x 2] = 3 - 4 + X = -1 + X or, X = 1.

Hypophosphorous acid formula or phosphinic acid formula that has a more precise and descriptive presentation as HOP(O)H2 which is the actual hypophosphorous acid chemical formula that actually represents the monoprotic characteristics by the later molecular formula.

HOP(O)H2 is in equilibrium with the minor structural formula of hypophosphorous acid that is interconvertible like HP(OH)2. The minor tautomers are referred to as hypophosphorous acid and the major monomers are called phosphinic acid. 

The Structural Formula For Hypophosphorous Acid

The structural phosphinic acid formula that is also referred to as the structural representation of the chemical formula of hypophosphoric acid is pseudo tetrahedral in nature as illustrated below with 2D and 3D models.

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According to the chemical formula of Hypophosphorous acid, it is an oxoacid that has pentavalent phosphorus that is covalently bonded with the two hydrogen atoms, one hydroxyl atom, and with an oxygen atom via a double bond that is present in the H3PO2. Phosphinic acids are the parent of the hypophosphorous acid and are conjugate acid of phosphinate.

Properties and Uses 



Molar mass

66.0 g / mol


Colourless hygroscopic crystal or oily liquid

Molecular shape 

Pseudo - tetrahedral

Melting point

25.6oC or 79.7oC or 299.6K

Boiling point

130oC or 266oC or 403K


Highly soluble in alcohol and ether 

Solubility in water

Miscible in water



Conjugate base


Uses of Hypophosphorous Acid

  1. It is used as a salt (sodium hypophosphite) in industries for electroless nickel plating

  2. H3PO2  is used to reduce arene diazonium salt in organic solvents from ArN2+ to Ar-H. when diazotization is carried out with concentrated H3PO2 amine substitutes are removed from arenes.

  3. It is used for the synthesis of phosphinic derivatives

  4. Because of its mild reducing agent property, it is used in Fischer’s esterification reactions.

Hypophosphoric Acid Formula 

Hypophosphoric acid is a mineral acid where the central atom phosphorus shows a +4 oxidation state. The chemical formula of hypophosphoric acid is H4O2P6. In a solid state, the hypophosphoric acid is present in the form of anhydride and the molecular representation hypophosphoric acid formula for the solid-state is H4O2P6・H2O. The acid is synthesized by reacting red phosphorus with sodium chlorite at room temperature. In the hypophosphoric acid formula both the phosphorus atoms are alike and they make direct bonds with each other (P-P). but isohyphosphoric acid that is an isomeric form of hypophosphoric acid do not have alike phosphorus atom where one phosphorus is directly attached to a hydrogen atom and exhibit a +3 oxidation state while the other phosphorus atom is attached with an oxygen atom that forms a bridge between the phosphorus atoms like (P-O-P) and has a +5 oxidation state.   

The Structural Formula For Hypophosphoric Acid

The structural molecular formula of hypophosphoric acid is given below in 2D and 3D structures.

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Hypophosphoric acid possesses oxonium that can be represented as [H3O]+[ H4O2P6]2- that has an isostructural representation with its diammonium salt that consists of [HOPO2PO2OH] which is an anion with P-P bond length equals 219 pm. It has a staggering structure that is very similar to the molecular structure of methane. Each phosphorus atom attached to the oxygen shares a bond length of P-O equals 151 pm and the P-OH bond equals 159 pm.

Properties and Uses



Molecular formula 


Molar mass

161.98 g / mol


White dihydrate solid

Melting point

54oC or 129oC or 327K

Conjugate base 


 Uses of Hypophosphoric Acid are:

  1. It is used as a bleaching agent as well as a very good reducing agent

  2. In industries, it is mostly used as a wetting agent

  3. In most pharmaceutical agents it is applied as stimulants.

FAQs (Frequently Asked Questions)

1. How is Hypophosphoric Acid Synthesized?

Ans. The pure form of hypophosphoric acid is synthesized by the extraction of its aqueous solution with the help of diethyl ether that is chemically represented as (C2H5)2O. The pure form of hypophosphoric acid is a white crystal that melts at 26.5oC.

2. Is Hypophosphorous Acid Monobasic in Nature?

Ans. Yes, hypophosphorous acid is monobasic in nature with the oxidation state of phosphorus being the central atom is +1 and it has a conjugate base phosphinate.