Understanding Basicity of Oxyacids

Basicity of an oxyacid refers to the number of ionizable hydrogen atoms (H⁺ ions) it can donate. To determine which oxyacid has the maximum basicity, we need to examine their molecular formulas and count the number of acidic protons.

Step 1: Write the Molecular Formulas

Let's recall the molecular formulas of the given oxyacids:

• Orthosilicic acid: H₄SiO₄
• Orthoboric acid: H₃BO₃
• Orthophosphoric acid: H₃PO₄
• Pyrosulphuric acid: H₂S₂O₇

Step 2: Count the Acidic Protons (Basicity)

Now, count the number of hydrogen atoms that can be donated as H⁺:

• Orthosilicic acid (H₄SiO₄): Basicity = 4
• Orthoboric acid (H₃BO₃): Basicity = 3 (Note: Boric acid is a weak monobasic acid, but structurally it has 3 H; however, it acts as a Lewis acid, not by donating H⁺ directly. For counting acidic protons, it is considered to have 1, but here we are asked about basicity which is number of H atoms, so we count 3, but typically it is not highly acidic)
• Orthophosphoric acid (H₃PO₄): Basicity = 3
• Pyrosulphuric acid (H₂S₂O₇): Basicity = 2

Note: Orthoboric acid (H₃BO₃) is not a typical protonic acid; it accepts OH⁻ rather than donating H⁺. So, its effective basicity is 1. But based on hydrogen count, it has 3.

Step 3: Compare the Basicity

Comparing the number of acidic hydrogens:

• Orthosilicic acid: 4 H
• Orthoboric acid: 3 H (but effectively 1)
• Orthophosphoric acid: 3 H
• Pyrosulphuric acid: 2 H

Thus, orthosilicic acid has the highest number of ionizable hydrogen atoms.

Final Answer

Orthosilicic acid (H₄SiO₄) has the maximum basicity (4).

Related Topics

• Acid-Base Chemistry
• Oxyacids and Their Strength
• Ionization of Acids
• Lewis Acids and Bases (e.g., Boric acid)

Formulae and Theory

Basicity of an acid: Number of replaceable hydrogen atoms per molecule.

For oxyacids, basicity is often equal to the number of hydroxyl groups (-OH) attached to the central atom.

Example: Orthophosphoric acid (H₃PO₄) has 3 OH groups, so basicity is 3.

Orthosilicic acid (H₄SiO₄) has 4 OH groups, so basicity is 4.

Note: Not all hydrogen atoms in an oxyacid are acidic; only those attached to oxygen are ionizable.