The main cause of basicity of oxoacids of phosphorus is:

\(P-OH\) bonds

The correct answer is option 2. \(P-OH\) bonds.

Oxoacids of phosphorus are compounds where phosphorus is bonded to oxygen atoms and other groups like hydroxyl groups \((OH)\) and sometimes hydrogen atoms directly attached to phosphorus. The most common oxoacids of phosphorus include:

Phosphoric acid \((H_3PO_4)\)

Phosphorous acid \((H_3PO_3)\)

Hypophosphorous acid \((H_3PO_2)\)

These acids differ in the number of oxygen atoms and the number of \(P-OH\) groups attached to the phosphorus atom, which directly affects their acidity and basicity.

Basicity of an Acid

Basicity in the context of acids refers to the number of protons \((H^+)\) that an acid can donate in an aqueous solution. This is often referred to as the number of ionizable hydrogen atoms in the molecule.

Structure of Oxoacids and Their Basicity

Phosphoric Acid \((H_3PO_4)\):

\(H_3PO_4\) has the structure:

In this structure, phosphorus is bonded to one oxygen atom with a double bond (\(P=O\)) and three hydroxyl groups (\(P-OH\)). Each \(P-OH\) group has a hydrogen atom that can dissociate as \(H^+\) in a solution. Therefore, phosphoric acid has a basicity of 3, meaning it can donate 3 protons.

Phosphorous Acid \((H_3PO_3)\):

\(H_3PO_3\) has the structure:

Here, phosphorus is bonded to one oxygen atom with a double bond (\(P=O\)), two hydroxyl groups (\(P-OH\)), and one hydrogen atom directly attached to phosphorus (\(P-H\)). Only the hydrogen atoms in the \(P-OH\) groups are ionizable. Therefore, phosphorous acid has a basicity of 2.

Hypophosphorous Acid \((H_3PO_2)\):

\(H_3PO_2\) has the structure:

In this case, phosphorus is bonded to one oxygen atom with a double bond (\(P=O\)), one hydroxyl group (\(P-OH\)), and two hydrogen atoms directly attached to phosphorus (\(P-H\)). Only one \(P-OH\) group is present, so the basicity of hypophosphorous acid is 1.

Why \(P-OH\) Bonds Determine Basicity

Ionizable Hydrogens: The \(P-OH\) groups in these acids contain hydrogen atoms that can dissociate in water to produce \(H^+\) ions. The ability of these \(H^+\) ions to dissociate determines the acid's basicity.

Non-Ionizable \(P-H\) Bonds: The \(P-H\) bonds in these acids are not ionizable under normal conditions, meaning the hydrogen atoms attached directly to phosphorus do not contribute to the basicity. Therefore, the presence of \(P-H\) bonds does not affect the basicity.

Count of \(P-OH\) Groups: The more \(P-OH\) groups present, the higher the basicity of the acid, as each \(P-OH\) group corresponds to one ionizable proton.

Conclusion

The basicity of oxoacids of phosphorus is primarily determined by the number of \(P-OH\) bonds in the molecule because these bonds contain ionizable hydrogen atoms that can dissociate as protons \((H^+)\). Each \(P-OH\) bond corresponds to one unit of basicity. In contrast, \(P-H\) bonds do not contribute to basicity as they do not dissociate into protons in aqueous solutions.

Thus, the main cause of basicity in oxoacids of phosphorus is the presence and number of \(P-OH\) bonds, making option 2 the correct choice.