Oxoacids of Phosphorus: Oxoacids are the acids that contain the element oxygen in their compound. Like Sulphur and chlorine, phosphorus also forms a number of oxoacids. In general, any hydrogen atom bonded to an oxygen atom is acidic. This is because the hydroxyl group or the \( – {\rm{OH}}\) group can lose a proton \({{\rm{H}}^{\rm{ + }}}\) leaving a negatively charged \( – {{\rm{O}}^ – }\) group and thus turning the acid into a phosphorus oxoanion. However, the hydrogen atoms bonded directly to phosphorus atoms are generally not acidic. Read on to find more about these compounds.

What are Oxoacids?

As the name suggests, oxoacids are the acids that contain the element oxygen. In oxoacids of phosphorus, the central atom is phosphorus, and all other atoms present in the compound surround it tetrahedrally. In all these acids, there is at least one \({\rm{P}} = {\rm{O}}\) bond and one \({\rm{P}} – {\rm{OH}}\) bond.

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Classification of Oxoacids of Phosphorus

The phosphorus oxoacids are classified based on the oxidation state(s) of the phosphorus atom(s), which may vary from \(+1\) to \(+5\). The oxidation state of oxygen atoms in these compounds is usually \(-2\); however, it may be in a \( – 1\) oxidation state if the molecule includes peroxide groups.

Oxidation State +1

Hypophosphorous or phosphinic acid, \({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{2}}}\) a monoprotic acid (meaning that only one of the hydrogen atoms is acidic).

Oxidation State +3

OrthoPhosphorous or phosphonic acid, \({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}}\), a diprotic acid (with only two acidic hydrogens). 

Oxidation State +4

Hypophosphoric acid, \({{\rm{H}}_{\rm{4}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{6}}}\). All four hydrogens are acidic.

Oxidation State +5

1. Phosphoric acid, also called as orthophosphoric or monophosphoric acid, \({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}\) is a triprotic acid (with three acidic hydrogens).
2. Pyrophosphoric acid, \({{\rm{H}}_{\rm{4}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{7}}}\), is a tetraprotic acid with four acid hydrogens.
3. Trimetaphosphoric, or cyclo-triphosphoric acid, \({{\rm{H}}_{\rm{3}}}{{\rm{P}}_{\rm{3}}}{{\rm{O}}_{\rm{9}}}\) or \({\left( {{\rm{HP}}{{\rm{O}}_{\rm{3}}}} \right)_{\rm{3}}}\), is a cyclic molecule with three acidic hydrogens.

The oxoacids in which phosphorus has an oxidation state less than \(+5\) contain, in addition to \({\rm{P}} = {\rm{O}}\) and \({\rm{P}} – {\rm{OH}}\) bonds, either \({\rm{P}} – {\rm{P}}\) or \({\rm{P}} – {\rm{H}}\) bonds can be present but not both. These acids tend to be disproportionate to higher and lower oxidation states. These \({\rm{P}} – {\rm{H}}\) bonds are not ionisable and do not play any role in acidity. Only those H atoms attached with oxygen in \({\rm{P}} – {\rm{OH}}\) form are ionisable and cause acidity.

Moreover, the acids which contain the \({\rm{P}} – {\rm{H}}\) bond have strong reducing properties.

Hypophosphorous Acid (H₃PO₂) or Phosphinic Acid

1. It is a powerful reducing agent with a molecular formula \({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{2}}}\).
2. The structure of hypophosphorous acid is shown below. 

3. The Oxidation state of phosphorus in \({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{2}}}\) is \(+1\).
4. It has one \({\rm{P}} – {\rm{OH}}\) bond, two \({\rm{P}} – {\rm{H}}\) bonds and one \({\rm{P}} = {\rm{O}}\) bond.

Preparation

1. By the oxidation of phosphine
Hypophosphorous Acid is prepared by the oxidation of phosphine in the presence of iodine with a calculated amount of water.
\({\rm{P}}{{\rm{H}}_{\rm{3}}} + {\rm{2}}{{\rm{I}}_{\rm{2}}} + {\rm{2}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{2}}} + {\rm{4HI}}\)

2. From white phosphorus
When phosphorus is heated with hydroxides of alkali or alkaline earth metals such as Barium hydroxide, barium hypophosphite is formed. When the precipitate is heated with a calculated amount of sulphuric acid, phosphinic acid or hypophosphorous acid is obtained.

\({\rm{3Ba}}{\left( {{\rm{OH}}} \right)_{\rm{2}}} + {\rm{2}}{{\rm{P}}_{\rm{4}}} + {\rm{6}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{2P}}{{\rm{H}}_{\rm{3}}} + {\rm{3Ba}}{\left( {{{\rm{H}}_{\rm{2}}}{\rm{P}}{{\rm{O}}_{\rm{2}}}} \right)_{\rm{2}}}\)
\({\rm{Ba}}{\left( {{{\rm{H}}_{\rm{2}}}{\rm{P}}{{\rm{O}}_{\rm{2}}}} \right)_{\rm{2}}} + {{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}} \to {\rm{BaS}}{{\rm{O}}_{\rm{4}}} + {\rm{2}}{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{2}}}\)

Properties

1. As it has one \({\rm{P}} – {\rm{OH}}\) bond, there is only one ionisable hydrogen atom. Therefore it behaves as monobasic acid, ionising as-

Due to the presence of two \({\rm{P}} – {\rm{H}}\) bonds, the acids as well as their salts (hypophosphites), behave as strong reducing agents.

2. On heating at \(313\,{\rm{K}}\), it decomposes to phosphine.

\({\rm{3}}{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{2}}} \to {\rm{P}}{{\rm{H}}_{\rm{3}}} + {\rm{2}}{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}}\)

3. However, on heating above \(333\,{\rm{K}}\), it decomposes to phosphoric acid and phosphine.

\({\rm{2}}{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{2}}} \to {\rm{P}}{{\rm{H}}_{\rm{3}}} + {{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}\)

Phosphorus Acid or Ortho-Phosphorus (H₃PO₃) or Phosphonic Acid

1. The molecular formula of phosphorous acid is \({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}}\).
2. The structure of phosphorus acid is shown below

3. The oxidation state of phosphorus in \({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}}\) is \(+3\).
4. It has two \({\rm{P}} – {\rm{OH}}\) bonds, one \({\rm{P}} – {\rm{H}}\) bond and one \({\rm{P}} = {\rm{O}}\) bond.

Preparation

1. The acid is prepared by hydrolysis of phosphorus trichloride or trioxide with water or steam.

\({\rm{PC}}{{\rm{l}}_{\rm{3}}} + {\rm{3}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}} + {\rm{3HCl}}\)
\({{\rm{P}}_{\rm{4}}}{{\rm{O}}_{\rm{6}}} + {\rm{6}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{4}}{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}}\)

2. Phosphorous acid can also be prepared from Potassium phosphite.

\({{\rm{K}}_{\rm{2}}}{\rm{HP}}{{\rm{O}}_{\rm{3}}} + {\rm{2HCl}} \to {{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}} + {\rm{2KCl}}\)

Properties

1. As it has two \({\rm{P}} – {\rm{OH}}\) bonds, there are two ionisable hydrogen atoms. Therefore it behaves as dibasic acid, ionising as-

Hence, it forms two types of salts.

\({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}} + {\rm{NaOH}} \to {\rm{Na}}{{\rm{H}}_{\rm{2}}}{\rm{P}}{{\rm{O}}_{\rm{3}}} + {{\rm{H}}_{\rm{2}}}{\rm{O}}\)

\({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}} + 2{\rm{NaOH}} \to {\rm{N}}{{\rm{a}}_2}{\rm{HP}}{{\rm{O}}_{\rm{3}}} + 2{{\rm{H}}_{\rm{2}}}{\rm{O}}\)

2. On heating, it undergoes auto-oxidation and reduction to form phosphoric acid and phosphine.

\({\rm{4}}{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}} \to {\rm{3}}{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}} + {\rm{P}}{{\rm{H}}_{\rm{3}}}\)

3. It is a powerful reducing agent due to the \({\rm{P}} – {\rm{H}}\) bond. Hence, it reduces silver nitrate solution to silver.

\({\rm{2AgN}}{{\rm{O}}_{\rm{3}}} + {{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}} + {{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{2Ag}} + {{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}} + {\rm{2HN}}{{\rm{O}}_{\rm{3}}}\)

3. It is readily oxidised by air to phosphoric acid in the presence of iodine.

\({\rm{2}}{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}} + {{\rm{O}}_{\rm{2}}} \to {\rm{2}}{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}\)

Orthophosphoric Acid (H₃PO₄)

1. Phosphoric acid, also known as orthophosphoric acid or phosphoric(V) acid, is a weak acid with a molecular formula \({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}\).
2. The pure compound is a colourless solid with a structure as shown below.

3. The oxidation state of phosphorus in \({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}\) is \(+5\).
4. There are three \({\rm{P}} – {\rm{OH}}\) bonds and one \({\rm{P}} = {\rm{O}}\) bond.

Preparation

1. It is prepared by boiling the solution of phosphorus pentoxide and water.

\({{\rm{P}}_{\rm{4}}}{{\rm{O}}_{{\rm{10}}}} + {\rm{6}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{4}}{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}\)

2. It can also be prepared by heating phosphorus with conc. \({\rm{HN}}{{\rm{O}}_{\rm{3}}}\) or by dissolving \({\rm{PC}}{{\rm{l}}_{\rm{5}}}\) in water.

\({\rm{P}} + {\rm{5HN}}{{\rm{O}}_{\rm{3}}} \to {{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}} + {\rm{5N}}{{\rm{O}}_{\rm{2}}} + {{\rm{H}}_{\rm{2}}}{\rm{O}}\)

\({\rm{PC}}{{\rm{l}}_{\rm{5}}} + {\rm{4}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}} + {\rm{5HCl}}\)

Properties

1. It is a soluble, deliquescent crystalline solid.

2. As it has three \({\rm{P}} – {\rm{OH}}\) bonds, there are three ionisable hydrogen atoms. Therefore it behaves as tribasic acid, ionising as-

Hence, it combines with alkalies like \({\rm{NaOH}}\) to form three series of salts.

\({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}} + {\rm{NaOH}} \to {\rm{Na}}{{\rm{H}}_{\rm{2}}}{\rm{P}}{{\rm{O}}_{\rm{4}}} + {{\rm{H}}_{\rm{2}}}{\rm{O}}\)

\({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}} + 2{\rm{NaOH}} \to {\rm{N}}{{\rm{a}}_2}{\rm{HP}}{{\rm{O}}_{\rm{4}}} + 2{{\rm{H}}_{\rm{2}}}{\rm{O}}\)

\({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}} + 3{\rm{NaOH}} \to {\rm{N}}{{\rm{a}}_3}{\rm{P}}{{\rm{O}}_{\rm{4}}} + 3{{\rm{H}}_{\rm{2}}}{\rm{O}}\)

3. On heating at \(523\,{\rm{K}}\), it gives pyrophosphoric acid and forms metaphosphoric acid at \(589\,{\rm{K}}\).

4. It reacts with silver nitrate to form a yellow precipitate of silver phosphate.

\({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}} + {\rm{3AgN}}{{\rm{O}}_{\rm{3}}} \to {\rm{A}}{{\rm{g}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}} + {\rm{3HN}}{{\rm{O}}_{\rm{3}}}\)

Pyrophosphorus Acid (H₄P₂O₅)

1. The molecular formula of pyrophosphorus acid is \({{\rm{H}}_{\rm{4}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}}\).

2. The structural formula of pyrophosphorus acid is shown below:

3. The oxidation state of phosphorus in \({{\rm{H}}_{\rm{4}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}}\) is \(+3\)

4. There are two \({\rm{P}} – {\rm{OH}}\) bonds, two \({\rm{P}} – {\rm{H}}\) bonds, two \({\rm{P}} = {\rm{H}}\) bonds and one \({\rm{P}} – {\rm{O}} – {\rm{P}}\) bond.

Preparation

1. Phosphorous acid reacts with phosphorus(V) chloride and water to produce pyrophosphorous acid and hydrogen chloride.

\({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}} + {\rm{PC}}{{\rm{l}}_{\rm{3}}} + {\rm{2}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {{\rm{H}}_{\rm{4}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}} + {\rm{3HCl}}\)

2. Pyrophosphorous acid is also prepared by dehydrating two moles of phosphorous acid.

Properties

1. Pyrophosphorus acid is a glassy solid and a powerful reducing agent because it has a \({\rm{P}} – {\rm{H}}\) bond.

Hypophosphoric Acid (H₄P₂O₆)

1. Hypo-phosphoric acid is a mineral acid with the molecular formula \({{\rm{H}}_{\rm{4}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{6}}}\). In the solid-state, it is present as the dihydrate, \({{\rm{H}}_{\rm{4}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{6}}}.{\rm{2}}{{\rm{H}}_{\rm{2}}}{\rm{O}}\).

2. The structural formula of hypophosphoric acid is shown below:

3. The oxidation state of phosphorus in hypophosphoric acid is \(+4\).

4. There are four \({\rm{P}} – {\rm{OH}}\) bonds, two \({\rm{P}} = {\rm{O}}\) bonds and the phosphorus atoms are joined directly with a \({\rm{P}} – {\rm{P}}\) bond.

Preparation

Hypophosphoric acid is prepared by the controlled oxidation of red phosphorus with sodium chlorite.

\({\rm{2P}} + {\rm{2NaCl}}{{\rm{O}}_{\rm{2}}} + {\rm{2}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{N}}{{\rm{a}}_{\rm{2}}}{{\rm{H}}_{\rm{2}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{6}}} + {\rm{2HCl}}\)

\({\rm{N}}{{\rm{a}}_{\rm{2}}}{{\rm{H}}_{\rm{2}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{6}}} + {\rm{2H}} – {\rm{resin}} \to {{\rm{H}}_{\rm{4}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{6}}} + {\rm{2Na}} – {\rm{resin}}\)

Properties

Hypophosphoric acid has four \({\rm{P}} – {\rm{OH}}\) bonds; hence, there are four ionisable hydrogen atoms. Therefore, it behaves as tetrabasic acid, ionising as-

Pyrophosphoric Acid (H₄P₂O₇)

1. Pyrophosphoric acid is also known as diphosphoric acid.

2. It is an inorganic compound with the molecular formula \({{\rm{H}}_{\rm{4}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{7}}}\).

3. The structural formula of pyrophosphoric acid is shown below-

4. The oxidation state of phosphorus in pyrophosphoric acid is \(+5\).

5. It has four \({\rm{P}} – {\rm{OH}}\) bonds, two \({\rm{P}} = {\rm{O}}\) bonds and one \({\rm{P}} – {\rm{O}} – {\rm{P}}\) bond.

Preparation

Pyrophosphoric acid is prepared by heating orthophosphoric acid at about \(250\,^\circ {\rm{C}}\).

\({\rm{2}}{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}} \to {{\rm{H}}_{\rm{4}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{7}}} + {{\rm{H}}_{\rm{2}}}{\rm{O}}\)

Properties

1. It is colourless, odourless and soluble in water, diethyl ether, and ethyl alcohol.

2. The acid is formed by the loss of a molecule of water from two molecules of phosphoric acid.

3. As it contains four \({\rm{P}} – {\rm{OH}}\) bonds, it is tetrabasic.

4. It dissolves in water to form orthophosphoric acid.

\({{\rm{H}}_{\rm{4}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{7}}} + {{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{2}}{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}\)

5. On heating, it decomposes into metaphosphoric acid.

\({{\rm{H}}_{\rm{4}}}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{7}}} \to {\rm{2HP}}{{\rm{O}}_{\rm{3}}} + {{\rm{H}}_{\rm{2}}}{\rm{O}}\)

Meta Phosphoric Acid (HPO₃)_n

1. Metaphosphoric acid can exist as a cyclic trimer or as a polymer.

2. The cyclic trimer form of metaphosphoric acid is known as Cyclotrimetaphosphoric acid.

3. The polymeric form of metaphosphoric acid is known as poly metaphosphoric acid

4. Metaphosphoric acid is formed by warming orthophosphoric acid to around \(850\,{\rm{K}}\).

\({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}} \to {\rm{HP}}{{\rm{O}}_{\rm{3}}} + {{\rm{H}}_{\rm{2}}}{\rm{O}}\)

Cyclotrimetaphosphoric Acid

The molecular formula of cyclotrimetaphosphoric acid is \({\left( {{\rm{HP}}{{\rm{O}}_{\rm{3}}}} \right)_{\rm{3}}}\).

The structural formula of \({\left( {{\rm{HP}}{{\rm{O}}_{\rm{3}}}} \right)_{\rm{3}}}\) is shown below-

1. It has three \({\rm{P}} – {\rm{OH}}\) bonds, three \({\rm{P}} = {\rm{O}}\) bonds and three \({\rm{P}} – {\rm{O}} – {\rm{P}}\) bonds.

2. It is tribasic, as there are three hydrogen atoms present in the \({\rm{P}} – {\rm{OH}}\) bond that are ionisable.

Poly Metaphosphoric Acid

The structure of poly metaphosphoric acid is shown as follows-

1. The oxidation state of phosphorus in metaphosphoric acid is \(+5\)

Uses of Oxoacids of Phosphorus

1.   It is used in the preparation of HBr and HI as a substitute for sulphuric acid.
2.   It is used as a souring agent in the preparation of soft drinks.
3.   It is used in the preparation of phosphate salts of sodium, potassium and ammonium. 4.   It is used in the manufacture of phosphatic fertilisers.

Summary

In oxoacids of phosphorus, the central atom is phosphorus which is tetrahedrally surrounded by other atoms. The \({\rm{P}} – {\rm{H}}\) bonds impart reducing property to the oxoacids, whereas the presence of \({\rm{P}} – {\rm{OH}}\) bonds imparts basicity to these acidic compounds. The oxidation state of phosphorus widely varies from \(+1\) to \(+5\). In this article, we learnt the various oxoacids of phosphorus such as phosphoric acid, orthophosphoric acid, phosphinic acid etc. We also learnt how these compounds exhibit basicity and reducing properties.

Frequently Asked Questions

Q.1. In which oxidation state does phosphorus form Oxyacids?
Ans: The oxidation state(s) of the phosphorus atom(s) in its oxoacids varies from \(+1\) to \(+5\). The oxidation state of oxygen atoms in these compounds is usually \(-2\); however, it may be in a \(-1\) oxidation state if the molecule includes peroxide groups.

Q.2. Which oxoacids of phosphorus are called glacial phosphoric acid?
Ans: \({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}}\) is known as glacial phosphoric acid. It is also known as metaphosphoric acid. It is a monobasic acid.

Q.3. What is the structure of \({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}}\)?
Ans: 1. The oxidation state of phosphorus in \({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}}\) is \(+3\).
2. It has two \({\rm{P}} – {\rm{OH}}\) bonds, one \({\rm{P}} – {\rm{H}}\) bond and one \({\rm{P}} = {\rm{O}}\) bond.

Q.4. Which oxoacid of phosphorus is most acidic?
Ans: Orthophosphoric acid \(\left( {{{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{3}}}} \right)\) is the most acidic. All hydrogen atoms in orthophosphoric acid are attached to an oxygen atom as an \( – {\rm{OH}}\) group. Hence, it can be easily released as protons. Therefore, the acidity of orthophosphoric acid is the highest among all oxyacids of phosphorus.

Q.5. Which oxoacid of phosphorus does not act as a reducing agent?
Ans: Metaphosphoric acid does not act as a reducing agent because there is no \({\rm{P}} – {\rm{H}}\) bond in the molecule.

Now you are provided with all the necessary information on the oxoacids of phosphorous and we hope this detailed article is helpful to you. If you have any queries regarding this article, please ping us through the comment section below and we will get back to you as soon as possible.