Account for the following observations: (a) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is a diprotic acid. (b) Nitric acid is a strong acid, whereas phos- (c) Phosphate rock is ineffective as a phoric acid is weak. phosphate fertilizer. (d) Phosphorus does not exist at room temperature as diatomic molecules, but nitrogen does. (e) Solutions of \(\mathrm{Na}_{3} \mathrm{PO}_{4}\) are quite basic.

H3PO3, or phosphorous acid, is a diprotic acid because it has two ionizable hydrogen protons. In aqueous solution, it can lose two protons (H+) at different steps, creating two separate dissociation reactions: \(1^{st} \: dissociation: H_{3}PO_{3} \rightarrow H_{2}O \: + \: H_{2}PO_{3}^{-}\) \(2^{nd} \: dissociation: H_{2}PO_{3}^{-}\rightarrow H_{2}O \: + \: HPO_{3}^{2-}\) The third hydrogen atom in H3PO3 is bonded to the central phosphorus atom in a phosphorus-hydrogen (P-H) bond, which is much more difficult to ionize and thus does not contribute to the acid behavior.

Nitric acid (HNO3) is a strong acid because it donates protons (H+) very easily and almost completely dissociates in water: \(HNO_{3} \rightarrow H^{+} + NO_{3}^{-}\) Phosphoric acid (H3PO4), on the other hand, is a weak acid because it does not dissociate completely in water. It ionizes in steps, similar to H3PO3, but each dissociation step is characterized by a different equilibrium constant, meaning that it does not readily lose its protons (H+): \(1^{st} \: dissociation: H_{3}PO_{4} \rightarrow H^{+} + H_{2}PO_{4}^{-}\) \(2^{nd} \: dissociation: H_{2}PO_{4}^{-}\rightarrow H^{+} + HPO_{4}^{2-}\) \(3^{rd} \: dissociation: HPO_{4}^{2-}\rightarrow H^{+} + PO_{4}^{3-}\) Strong acids like nitric acid cause a larger increase in the concentration of H+ ions in a solution, making them more acidic.

Phosphate rock is a naturally occurring mineral deposit composed largely of calcium phosphate (Ca3(PO4)2). It is an ineffective phosphate fertilizer because the calcium phosphate present in the rock is not readily soluble in water, making it difficult for plants to absorb the necessary nutrients. For phosphate rock to be effective, it must be converted into more soluble forms of phosphate, like superphosphate, by treatment with strong acids like sulfuric acid. This process releases soluble phosphate ions that can be absorbed by plants.

Phosphorus exists in several allotropes, but unlike nitrogen (N2), it does not form diatomic molecules at room temperature. Phosphorus atoms have larger atomic radii and lower electronegativity compared to nitrogen atoms, making the attractive forces between two phosphorus atoms much weaker than those between two nitrogen atoms. As a result, phosphorus forms multi-atomic molecules in its most stable form as white phosphorus (P4) or red phosphorus. Nitrogen, on the other hand, forms very stable diatomic molecules (N2) due to its strong triple covalent bond, which requires a high amount of energy to break.

Na3PO4, or trisodium phosphate, is a soluble salt derived from a weak acid (phosphoric acid, H3PO4) and a strong base (sodium hydroxide, NaOH). When Na3PO4 dissolves in water, it dissociates into Na+ and PO4(3-) ions: \(Na_{3}PO_{4} \rightarrow 3Na^{+} + PO_{4}^{3-}\) The PO4(3-) ions can react with water molecules, accepting protons (H+) and generating excess hydroxide ions (OH-) in the solution: \(PO_{4}^{3-} + H_{2}O \rightarrow HPO_{4}^{2-} + OH^{-}\) Since there is an increase in OH- concentration, the solution becomes quite basic.