The basicity of tetraoxophosphate(V) acid is

Analyze the question: Tetraoxophosphate(V) acid is H₃PO₄ (phosphoric acid). Basicity means the number of replaceable (ionizable) hydrogen atoms per molecule of the acid, that is, the number of H⁺ ions it can give out in solution.

Explain the concept: Basicity is not the total number of hydrogen atoms in every case. It is the number of hydrogens that can actually be replaced by a metal. For example, HCl has a basicity of 1, H₂SO₄ has 2, and acetic acid (CH₃COOH) has only 1, even though it has 4 hydrogens.

Step-by-step solution:

1. Name to formula: "tetraoxo" means 4 oxygen atoms, "phosphate(V)" means phosphorus has an oxidation state of +5. The formula is H₃PO₄.

2. Structure: the phosphorus is bonded to one P=O group and three –OH groups. All three hydrogens are attached to oxygen, so all three are ionizable.

3. Stepwise ionization: • H₃PO₄ ⇌ H⁺ + H₂PO₄⁻ • H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻ • HPO₄²⁻ ⇌ H⁺ + PO₄³⁻

4. Three H⁺ ions can be released, so the basicity is 3. It is a tribasic acid.

Answer: The correct option is D. 3.

Exam tip: Do not confuse this with phosphonic (phosphorous) acid, H₃PO₃, which has only 2 replaceable hydrogens because one H is bonded directly to P. For oxyacids, count the –OH groups: that gives the basicity.