Question

Which of the following pairs of oxoacids has basicity as 2?

• A. \( \text{H}_3\text{PO}_3, \text{H}_2\text{SO}_4 \)
• B. \( \text{H}_3\text{PO}_2, \text{H}_2\text{SO}_3 \)
• C. \( \text{H}_3\text{PO}_4, \text{H}_3\text{PO}_2 \)
• D. \( \text{H}_2\text{S}_2\text{O}_8, \text{H}_3\text{PO}_2 \)

Hint:

The basicity of Phosphorus oxoacids is equal to the number of Oxygen atoms minus 1. \( \text{H}_3\text{PO}_4 \to 4-1 = 3 \). \( \text{H}_3\text{PO}_3 \to 3-1 = 2 \). \( \text{H}_3\text{PO}_2 \to 2-1 = 1 \).

Answer 1

The Correct Option is A

Step 1: Definition of Basicity:

Basicity of an acid is the number of ionizable hydrogen atoms. - For Sulfur oxoacids, H atoms attached to Oxygen are ionizable. - For Phosphorus oxoacids, only H atoms attached to Oxygen (P-OH) are ionizable. H attached directly to P (P-H) is not acidic.
Step 2: Analyzing Structures:

• \( \text{H}_3\text{PO}_3 \) (Orthophosphorous acid):
  P is bonded to one oxygen (P=O), one hydrogen (P-H), and two hydroxyl groups (P-OH). Basicity = 2.
• \( \text{H}_2\text{SO}_4 \) (Sulphuric acid):
  Contains two -OH groups attached to Sulfur. Basicity = 2.
• \( \text{H}_3\text{PO}_2 \) (Hypophosphorous acid):
  Contains only one -OH group (and two P-H bonds). Basicity = 1.
• \( \text{H}_3\text{PO}_4 \) (Orthophosphoric acid):
  Contains three -OH groups. Basicity = 3.

Step 3: Checking Options:

- Option (A): \( \text{H}_3\text{PO}_3 \) (Basicity 2) and \( \text{H}_2\text{SO}_4 \) (Basicity 2). Correct. - Option (B): \( \text{H}_3\text{PO}_2 \) (1) and \( \text{H}_2\text{SO}_3 \) (2). Incorrect. - Option (C): \( \text{H}_3\text{PO}_4 \) (3) and \( \text{H}_3\text{PO}_2 \) (1). Incorrect. - Option (D): \( \text{H}_2\text{S}_2\text{O}_8 \) (2) and \( \text{H}_3\text{PO}_2 \) (1). Incorrect.
Step 4: Final Answer:

The pair is \( \text{H}_3\text{PO}_3, \text{H}_2\text{SO}_4 \).